Introduction
Modern pinnipeds include three different distinct families: the lone walrus in the family Odobenidae, the true seals in the family Phocidae (also known as the 'hair seals' or the earless seals), and lastly, the fur seals and sea lions in the family Otariidae - also known as the eared seals. These pinnipeds live primarily in the North Pacific and the southern ocean. There are about 14 species, ranging in size from tiny fur seals like the Galapagos fur seal to the truly gigantic Steller's sea lion.* The Otariidae have a surprisingly good fossil record, and unlike the true seals, are known from many complete skulls and partial to complete skeletons - though they are quite conservative, anatomically speaking - to quote my PhD adviser Ewan Fordyce, "much of a muchness" (though he was referring to delphinids, but the same applies here).
*NOT the 'Stellar' sea lion - it's named after Georg Steller, the first western scientist to encounter and document the species - same guy who discovered Steller's sea cow, Steller's sea eagle, and Steller's jay.
Two immature male New Zealand sea lions (Phocarctos hookeri) sparring at Sandfly Bay, Otago Peninsula, NZ. Photo: R.W. Boessenecker.
Fur seals and sea lions are admittedly my favorite of all pinnipeds: in my opinion, they are the most fun to watch. Here in California, we're used to seeing harbor seals and California sea lions. Harbor seals are plump, cute little angel blubber torpedos that are eternally sleepy; they wriggle around a bit, and mostly just sleep when they're hauled out. They're also very quiet, and tend not to smell very bad. It's like seeing a couple of kittens sleeping together. Sea lions, on the other hand, are a veritable assault on the senses: every visit to a sea lion haulout, or to Pier 39 in San Francisco - is an instant reminder why sea lions are used in the circus. Hell, a visit to one of these feels like the circus: a deafening cacophony of barks, growls, sneezes, and snorts. Sea lions tend to sleep in large groups while hauled out - all piled on top of each other, but wet sea lions who are trying to find a sleeping spot walk right over the sleeping, now completely dried off sea lions - triggering a series of barks and growls from successive sea lions like your hand sliding down piano keys. Sea lions always seem ready to fight - whether over a sleeping spot or getting wet from a new arrival - or play fighting, whether in the water, on the haulout spot, or even on top of sleeping (or formerly sleeping) sea lions. And then, lastly, the smell: the smell of a sea lion haulout is a spectacular mixture of body odor, poop, and a sort of rotting fish smell. It is quite literally stunning. Make sure you observe a sea lion haulout from upwind! You'll only make that mistake once. This isn't to say that some true seals aren't similarly chaotic - any visit to an elephant seal rookery challenges this - but elephant seals are quite an extreme case within the true seals, many of which are fairly solitary. On another note, and this is admittedly very subjective, but I also think that otariid pups are by far and away the cutest of all baby pinnipeds - especially the fur seals, with hilariously large flippers.
So, how did fur seals and sea lions evolve? What is their fossil record like? What does the fossil record tell us about their evolutionary history? Where did they originate, and how did they get to their current distribution? This blog post is a deep dive into one of my favorite groups of marine mammals - and, one of the more challenging groups to work on.
Anatomy and Adaptations of Fur Seals and Sea Lions
Otariids can be relatively small - such as the dainty Galapagos fur seal (Arctocephalus galapagoensis), the males of which max out at 1.5 meters and about 60-65 kg (~140 lbs). They can also attain quite large sizes, with the bear-like Steller's sea lion (Eumetopias jubatus) attaining lengths of over 3 m and 500-1100 kg (~1,000 to 2,500 lbs). However, these are still less extreme than the smallest and largest phocid seals (Baikal and elephant seals).
Sexual dimorphism in the New Zealand sea lion (Phocarctos hookeri) on the beach at Cannibal Bay, NZ. The males are one third longer and weigh approximately double or triple the mass of females. Photos: R.W. Boessenecker.
Otariids exhibit sexual dimorphism - the males being much larger than the females; in some species, the males might only be slightly longer than the females, but double the body mass (e.g. Steller's sea lion); in others, the males can be 1/3 longer than the females and weigh in at four or five times the body mass of females (Northern fur seal). Female skulls are typically about 70-80% of the length of male skulls, and with lower muscle attachment crests, smaller supraorbital shelves, and less robust features in general; the mandibles are similarly about 3/4 the length, and narrower. The upper and lower canines of otariids are consistently larger and more proportionally robust than in females; in absolute terms, the canines of California sea lions, are, for example, consistently 160-200% larger than in females (not corrected by skull size). This is related to their breeding system - 'polygyny', meaning that males compete with others for mates and successful males might breed with numerous females in a breeding season. The most sexually dimorphic of all pinnipeds, the elephant seals, also share this sort of breeding. More on this below.
The skin and hair of these pinnipeds is interesting as it is the most primitive amongst pinnipeds. Like the walrus and true seals, fur seals and sea lions chiefly rely on blubber for insulation in cold ocean water. Sea lions have hair, as do all modern pinnipeds, but unlike the fur seals, they do not have much fur. Fur seals, on the other hand, get their name from primitively possessing copious underfur. Their fur is arguably luxurious, but still nowhere as dense as sea otters. In concert with underfur, fur seals have less blubber than sea lions. However, fur only insulates so long as it has trapped air; that same air escapes (as bubbles) as a fur seal (or otter) dives to a certain depth.
A young California sea lion (Zalophus californianus) surveys the rookery for a place to nap, showing off its external ear pinnae (flaps) - unique to otariids amongst modern pinnipeds. You can also make out the long vibrissae (whiskers) and impossibly large eyes. La Jolla, San Diego, California. Photo: R.W. Boessenecker.
The heads of fur seals and sea lions are unique amongst pinnipeds for retaining external ear flaps - these are tiny little triangular flaps that likely don't have much function. In fact, this goofy little ear gives the family its name - Otarion is Greek for "little ear". Aside from the external ears, the skulls and teeth of otariids are also quite distinctive. These pinnipeds have large eyes, but they are not quite as stupendously large as true seals. Yet, their orbits (eye sockets) are unique in possessing large bony shelves over the orbit; these vary in shape between species, most often being rectangular or triangular. There is additionally a large bump or flange called the antorbital process just at the anterior edge of the orbit that is unique to the family. The suture between the frontal bone (makes up the "forehead") and the nasal bones and maxilla is always shaped like an M or W, depending upon which way you're looking at the skull; this is because there are three small triangular "prongs" of the frontal that stick between the nasals at the midline of the skull, and between the posterior end of the nasal and the maxilla. All pinnipeds have large fenestrae or "vacuities" in the eye socket, but in otariids, this is positioned far forward, and is quite large; in many phocid seals, the vacuity is smaller, though positioned in roughly the same spot - and in tusked walruses, the vacuity is positioned posteriorly. There is also a pyramid, conical, or fin-shaped bump on the snout just above the incisors and anterior to the narial fossa (bony "nostril") called the prenarial process; this is absent in most phocid seals and reduced in the modern walrus, but quite well-developed in otariids. The zygomatic arch - the ring of bone encircling the temporal fossa - is quite delicate, with a long, tapering zygomatic process, strongly differing from the short stubby process in walruses and the expanded process seen in most phocids (and the extinct desmatophocid seals).
The skull of an adult male California sea lion, Zalophus californianus, in the collections of MVZ at Berkeley. Photos: R.W. Boessenecker.
Otariids are unique amongst modern pinnipeds in possessing a long sagittal crest - in fur seals and some sea lions (Arctocephalus, Neophoca, Eumetopias) it is confined to the braincase, but in some extinct otariids (Thalassoleon, Hydrarctos) and some sea lions (Zalophus, Phocarctos, Otaria) the sagittal crest extends forward nearly to the level of the orbits. The nuchal crests at the back of the braincase are also quite large, only similarly developed in some monk seals and some extinct "imagotariine" and dusignathine walruses.
An adult female California sea lion (Zalophus californianus) at Sea World showing off its unusually stained teeth. Photo: R.W. Boessenecker.
The lower teeth of a subadult male California sea lion (Zalophus californianus) showing initial staining of the enamel - from California Academy of Sciences collections. Photo: R.W. Boessenecker.
The mandible with teeth removed of a freshly cleaned juvenile female New Zealand fur seal (Arctocephalus forsteri) processed during my doctoral studies at U. Otago. Photo: R.W. Boessenecker.
The teeth of otariids are quite distinctive as they possess larger canines than most true seals (but, obviously not walruses!) and cheek teeth that are all single rooted and often only have one conical crown, occasionally with small cuspules on either side. In contrast, the cheek teeth of most phocid seals are double rooted and have three or four cusps. Many species of fur seals and sea lions also occasionally retain the upper second molar (Otaria byronia, Phocarctos hookeri, Callorhinus ursinus, Arctocephalus spp.) - which has been lost in all true seals. In some species, the very last molar is shifted far posteriorly to the rest of the cheek teeth, including in Arctocephalus spp., but especially in the Steller's sea lion, Eumetopias jubatus.
The skeleton of a Steller's sea lion (Eumetopias jubatus) at a former exhibit at California Academy of Sciences in San Francisco; there is a much more modestly sized harbor seal (Phoca vitulina) for comparison. Photo: R.W. Boessenecker.
Skeleton of a Northern Fur Seal (Callorhinus ursinus) in the same exhibit. This is a female skeleton and is quite small. Photo: R.W. Boessenecker.
Comparison of the forelimb skeleton of a California sea lion and a harbor seal; gt = greater tuberosity; dp = deltopectoral crest; op = olecranon process; rt = radial tuberosity. From Howell (1929).
Fur seals and sea lions are forelimb-dominated swimmers, using a rowing motion that looks superficially similar to the underwater flight of penguins. As a result, their forelimb skeletons are rather enormous and dwarf the hindlimbs. The humerus looks quite a bit like it does in the walrus, with robust proportions and a large and long deltopectoral crest on the 'front' of the bone. The radius and ulna are both considerably expanded and flattened from side to side, anchoring in many of the muscles of the forearm. The forelimbs of otariids notably have a much more strongly developed muscles including the pectoralis, deltoideus, supinator longus, teres major, external carpi radialis, triceps brachii and flexor digitorum. The attachment for these muscles in phocid seals is much smaller - and these seals only use their forelimbs for steering. The pelvis of otariids has a rather narrow ilium - the bone that makes up the large flattened blade in the pelvis of humans - and differs from phocid seals, which have a fan-shaped ilium. This is a land mammal like feature in fur seals and sea lions, and in phocid seals, is an adaptation for hindlimb dominated swimming.
The ankle of otariids is quite different from the true seals - the ankle bones look much like those of a bear or dog, and allow the 'plantar' surface (bottom) of the foot - or, rather, flipper - to be placed on the ground. This land mammal like configuration of the ankle bones (calcaneum and astragalus) allows fur seals and sea lions to walk, or even gallop. In some species, especially fur seals in the genera Arctocephalus and Callorhinus, the hindlimbs are bound together in soft tissue and are only separated around the ankles. As a result, they walk like a tripod: both hindflippers are brought forward together at the same time. In most sea lions, however, there is greater separation between the hindlimbs, and instead the hindflippers alternate steps - more extremely in some species. The walrus can also walk quadrupedally like otariids - but true seals have a triangular process on the 'front' of the calcaneum (the heel bone) which prevents the flipper from being pulled forward; as a result, the flippers of true seals are permanently pointing backwards (posteriorly) and they cannot walk - and must wriggle around.
Lastly, fur seals and sea lions have unusually long flippers, and have evolved a set of rod-like cartilaginous extensions originating from the tips of each of the very last phalanges on each digit of the fore and hindflippers. In phocid seals and the walrus, the claws (set into the last phalanx bone of each digit) are at the tips of the flippers - but in fur seals and sea lions, the claws are only 3/4 of the way out to the tips.
Two subadult New Zealand sea lions (Phocarctos hookeri) play fighting on a typical midsummer day in New Zealand. Sandfly Bay, Otago Peninsula, NZ. Photo: R.W. Boessenecker.
Otariid Biogeography
Modern otariids live chiefly in the North Pacific and the Southern Ocean, mostly in temperate and cold temperate waters. The northern species inhabit the coastline from Baja California to Alaska, through to the Aleutians, and south to Japan. The southern species inhabit the coastline of South America from Ecuador to Tierra del Fuego and north to Uruguay, South Africa, the south coast of Australia, New Zealand, and islands in the high latitudes of the southern ocean between New Zealand and South Africa.
The southern species, including all southern sea lions (Otaria, Neophoca, and Phocarctos) along with Arctocephalus form a monophyletic group in molecular studies, indicating that the Galapagos and Guadalupe fur seals immigrated north at some point during the Pleistocene and crossed the equator.
Aside from the Galapagos sea lion and fur seal, all otariids tend to inhabit temperate or cold temperate coastlines - being simultaneously averse to icy coastlines and the tropics. In our 2014 paper (Churchill et al.), Morgan conducted this interesting analysis by compiling sea surface temperature ranges for breeding areas for each species, representing the warmest temperatures tolerated by each species of otariid - and then reconstructed the ancestral temperature tolerance for otariid lineages based on this. Essentially, most of the branches within the Otariidae suggest an ancestral tolerance of up to 18-24˚C - about 64.5-75˚F. A few lineages have adapted to cold temperatures, including the northern fur seal (~15˚C), the northern sea lion (12-14˚C), and the antarctic fur seal (8-10˚C) - but these are likely to be independent adaptations to high latitudes. This means that otariids likely were unable to cross the equator during certain periods like the mid Pliocene warm period, where equatorial waters were warmer than reconstructed temperature tolerances for southern otariid lineages (~25-27˚C, as opposed to 20-22˚C).
Lastly, otariids seem to be distributed preferentially along coastlines with robust upwelling regimes. Upwelling is the flowing of deep, cold, nutrient-rich water from the deep sea floor up onto the continental shelf; nutrients like phosphorus allow booms in plankton biomass, which in turn drive booms in krill and fish that support marine mammal populations. Otariids generally forage close to shore (e.g. within a couple hundred kilometers) and over the continental shelf. The southern ocean, with its unique circumpolar current, is a different story, and permits otariid populations to inhabit dispersed islands and archipelagoes.
Are pinnipeds monophyletic (all evolving from a single ancestor) or diphyletic (two separate origins within the 'arctoid' carnivorans)? Molecular phylogeny, and the vast majority of morphological studies support monophyly, but a handful of extremely cherry picked analyses of limited morphological data (read: 'curated') by one working group support diphyly. From Berta et al. (2013).
Pinniped Relationships
The higher phylogeny of pinnipeds has been controversial for years - both the relations of pinnipeds to other Carnivora, or the relationship of different family level clades within the Pinnipedia. First off, there has been a long standing debate about whether pinnipeds are even monophyletic - some "traditionalists" have argued, for years, that the eared seals and walruses formed a single clade related to bears and that the phocid seals evolved in parallel, but from mustelids. I've always found this hypothesis to be spurious even on its own merits, but it has continued to fail to address overwhelming molecular evidence that strongly supports a single origin of pinnipeds. Additionally, copious morphological evidence also now exists that strongly supports monophyly. I won't discuss diphyly v. monophyly any further, as many of the arguments for diphyly have not exactly been rigorous in methodology and it has zero molecular support and only the barest, cherry-picked support from morphology.
Within the pinnipeds, there are two major hypotheses as to relationships between family level clades: the traditional Otarioidea hypothesis, which places otariids and the walruses in one clade, and the Phocomorpha hypothesis, which places the walruses instead with the Phocidae. The Otarioidea is strongly supported by molecular analyses, whereas many anatomical phylogenetic studies (especially Berta and Wyss, 1994) support the Phocomorpha. In the 1990s and 2000s, morphology based cladistic analyses tended to strongly support the Phocomorpha - at odds with the molecular data. I have a feeling that the Otarioidea is probably the right phylogeny - however, no matter how much additional character evidence Morgan and I added, we never quite got to a point where the Otarioidea became well-supported, though statistical support for the Phocomorpha weakened as we added more, previously unused characters. The work continues - and, I don't really have an answer, other than, some of the anatomical features supporting the Phocomorpha might simply be "symplesiomorphic" features - primitive features that walruses have, that are missing in the otariids. Key to answering these questions will be figuring out if the desmatophocid seals like Allodesmus are otarioids as well, or if they fall within the Phocoidea. Additionally, there have been very, very few pinniped-wide analyses of fossil and modern pinnipeds based on anatomical data; virtually all studies in the past 20 years have focused on the relationships within one of the family level clades (and this includes no fewer than three of my own published phylogenetic analyses on pinnipeds).
Phylogenetic analysis of the pinnipeds - with weak support for a monophyletic walrus + sea lion clade, the Otarioidea. From Paterson et al. (2020).
Few phylogenetic analyses of the higher relationships among pinnipeds have been attempted, but one recent study deserves a brief outline. Ryan Paterson et al. (2020) conducted a large phylogenetic analysis to reevaluate the position of the purported 'protoseal' Puijila darwini (which I covered here many years ago). These authors did a great job incorporating many purported fossil pinniped relatives like the otter-like Potamotherium and the "beach bear" Kolponomos, in addition to the enaliarctines. While there are a few funky results (Kolponomos sister to Enaliarctos emlongi, for example), the cladogram is otehrwise a very plausible one. They did recover a monophyletic Otarioidea - though it's quite weakly supported. Further exploration of pinniped anatomy and characters is sorely needed - but it's an excellent first step.
A pre-cladistic hypothesis of "otaroid" relationships. Surprisingly, few relationships depicted here have been challenged by cladistic analyses, except for otarioid monophyly (but, see above). Charles Repenning ("Rep") in particular sussed out the otariid part of the tree quite well. From Repenning and Tedford (1977).
The first cladogram of modern and fossil otariids, and showcasing a monophyletic fur seal clade, the "Arctocephalinae" - now known to be paraphyletic. From Berta and Deméré (1986).
Relationships within the Otariidae
Fur seals and sea lions were traditionally placed into their own subfamilies: the Arctocephalinae, and the Otariinae. The arctocephalines were always considered to be more 'primitive' than the otariines given that they primitively retained fur, and were also smaller - though this taxonomic framework barely survived even early attempts to perform cladistic analyses of otariids. Neither clade seems to be supported by many cladistic analyses, with the Northern fur seal (Callorhinus ursinus) typically being the earliest diverging otariid, followed by the species in Zalophus, and the Steller's sea lion (Eumetopias jubatus). The southern sea lions then branch off, and, in a complete reversal from the traditional taxonomic approach, the most crownward grouping of otariids are the southern fur seals in the genus Arctocephalus. Only one computer-aided phylogenetic analysis has ever supported the monophyly of either clade, and that was an early attempt in the initial 1986 description of Callorhinus gilmorei.
Dispersal to the Southern Hemisphere
Otariids are pretty conclusively found to have had a Northern Hemisphere origin, specifically in the North Pacific - though most species that are alive today (about 2/3 of all otariids) inhabit the southern hemisphere. This begs the question: when did otariids cross the equator? And, where? The second question is easiest to answer: given the absence of otariids from the tropical West Pacific, and the lack of strong upwelling regimes there, dispersal south from the coast of California and Mexico along the southward California current along the richly productive and cold waters of the tropical East Pacific seems most likely. And, in fact, we have otariids distributed nearly continuously throughout this region, with a few species off southern California and Baja, two species in the Galapagos, and two species along the west coast of South America from Ecuador/Peru to Chile.
"When" is a different matter. The oldest known otariid fossils from the Southern Hemisphere are not very old, and also not very far south: Hydrarctos lomasiensis was discovered in uppermost Pliocene strata overlying the more famous Pisco Formation of Peru, and recently named the Caracoles Formation. More recent fieldwork has recovered fossils of Arctocephalus as well from these rocks, which date to 2.7-1.9 Ma - straddling the Pliocene-Pleistocene boundary. In sum, we don't have any fossil evidence older than this for a southward dispersal of otariids across the equator. A single femur identified as Arctocephalus has been reported from the early Pliocene of South Africa, and interpreted as a new species - which may be as old as 5 Ma. However, the authors note that it might also be as young as 2.7 Ma - similar in age with Hydractos - and hardly a ringing endorsement of its stratigraphic provenience. Since being preliminarily reported in 2011, the specimen is still not formally published and the jury is out.
As it turns out, the equator is a formidable thermal 'barrier' to otariids, along with many other marine mammals. Marine mammals spend most of their time in the water, and have evolved all sorts of incredible adaptations for keeping warm - simple ones like blubber, and more complicated adaptations like counter-current heat exchanges, where veins with cold blood returning from the extremities pass right next to arteries with hot blood, warming the cold blood before it reaches the heart. All of these adaptations keep marine mammals warm in the water - and also largely prevents many species from inhabiting warmer waters. On land, many pinnipeds will cover themselves in cool sand to keep the suns rays from baking them a little too much. Within the Otariidae, few species typically tolerate sea surface temperatures warmer than about 24˚C for long periods - only the Galapagos sea lion (Zalophus wollebaeki) and fur seal (Arctocephalus galapagoensis) are truly tropical species, tolerating up to about 30˚C for a couple of months (roughly 85-87˚F).
The results of Morgan's analysis reconstructing the dispersal of fur seals and sea lions to the southern hemisphere along the eastern Pacific coastline, around 6 Ma. From Churchill et al. (2014).
In our study of otariid phylogeny, Morgan undertook a pretty neat analysis where he reconstructed the ancestral temperature tolerance for the base of the southern otariid clade, and compared it with sea surface temperature curves for the last ten million years. With a reconstructed maximum temperature tolerance of about 20-21˚C, there is one period during the latest Miocene - about 6 Ma - where sea surface temperatures were depressed along upwelling margins, and also corresponding to a spike in primary productivity. Additionally, this does correspond quite well to at least some molecular clock divergence dates for the origin of the southern hemisphere otariid clade (5.8 Ma, from Yonezawa et al., 2009). After about 5 Ma or so temperature picks up quite a bit during the middle Pliocene warm period - and conditions do not become favorable again until about 3.5 Ma. Admittedly I politely argued with Morgan about this date while we were writing this paper, since it seems to pre-date the southern hemisphere fossil record by about three million years. You can call this a bit of a 'gentleman's bet' between us; I favor a post-warm period dispersal, as it is in closer accord with the fossil record; Morgan favors the pre-warm period dispersal as it makes more sense with the phylogenetic pattern and molecular clock dating.
The geochronology of various marine vertebrates from the Pisco Formation and overlying rocks. Only phocid seals are known from the Pisco Formation (sedimentation ended ~4.5 Ma), and otariids do not appear until sedimentation of the Caracoles Formation (light gray band) started about 2.7 Ma (e.g. Hydrarctos lomasiensis). There is no fossil record from about 4.5-2.7 Ma owing to widespread erosion and nondeposition. From Ochoa et al. (2021).
Frustratingly, there is a big erosional gap in the Peruvian rock record staring about 4.5 Ma, and lasting for about two million years before sedimentation of the Caracoles Formation began. So, this *could* account for the missing early record of otariids. However, the uppermost Pisco Formation is richly fossiliferous, and from ~7-4.5 Ma, during the latest Miocene and earliest Pliocene, it has produced only phocid seals and not a single scrap of otariid. I understand that absence of evidence is not evidence of absence, blah blah, but we are talking about the most richly fossiliferous and well-sampled marine mammal bearing rocks on earth. After forty years of field crews searching for them, I think a single scrap would have been found. Then again, fossils of Eotaria and Pithanotaria are not very common. So I'll keep an open mind.
Anagenesis in the Northern Fur Seal Lineage - Callorhinus
The Pliocene "Gilmore fur seal", Callorhinus gilmorei, was reported from the San Diego Formation in 1986 by Annalisa Berta and Tom Deméré, and proposed as a "neospecies" (to borrow a paleornithological term) and possible ancestor of the modern Northern fur seal, Callorhinus ursinus. No other otariids are certainly known from the San Diego Formation or equivalent rocks - and all finds are consistent with identification as Callorhinus gilmorei. It is slightly smaller than modern Callorhinus ursinus (5% smaller), and considerably more gracile - evidence that could indicate it was less sexually dimorphic than the modern species. Fossils of this species are also known from Japan, and I've even found some fragments in the Purisima Formation - and Wayne Thompson has recently discovered a beautiful mandible in the Purisima near Santa Cruz that very likely represents this taxon as well.
Ron Bushell's spectacular concretion with associated left and right mandibles of Callorhinus gilmorei from the Rio Dell Formation, Scotia Bluffs, California. Photo: R.W. Boessenecker.
In the early 2000s, amateur fossil collector extraordinaire Ron Bushell discovered a pair of fur seal mandibles in a concretion along the Eel River at Scotia Bluffs in Humboldt County, California, where fossil marine mammals had not really yet been discovered (aside from by Ron, of course). A year or two later he agreed to donate the specimen for study, and Sarah and I took a summer road trip from Montana State out to Eureka to meet Ron and collect the specimen. At that year's (2006) SVP, I met Tom Deméré for the first time, and he suggested the specimen could be Callorhinus gilmorei given that most of the section there is Pliocene in age and equivalent to the San Diego Formation. I had initially thought the somewhat older genus Thalassoleon - but Tom indicated that I probably ought to get the stratigraphy pinned down. After some back and forth with Ron we determined that the specimen was probably from the middle part of the Rio Dell Formation. After further reading, I pinned the age down to about 2.0-3.0 Ma - which now would be latest Pliocene to earliest Pleistocene, and around the age of the youngest specimens from the San Diego Formation. Further, the specimen is quite small - about 13 cm, similar to Callorhinus gilmorei, but much smaller than adult male Thalassoleon or Callorhinus ursinus. Further, the second premolar is single rooted - a derived feature shared with Callorhinus gilmorei, and different from the condition in Thalassoleon spp. where this and all other premolars and molars (aside from the first premolar) are all double rooted. However, unlike the completely single rooted dentition in modern Callorhinus ursinus, the rest of the cheek teeth were strongly double rooted - also shared with Callorhinus gilmorei. Over the course of evolution, double rooted teeth fused together into single rooted teeth, and within otariids, this started at the front of the dentition (P2) and moved posteriorly to the back (M1).
Various fossil specimens of Callorhinus and a graphic highlighting the dental transition from an ancestor (probably a species of Thalassoleon, with T. mexicanus shown here for comparison) with completely doube rooted teeth, the extant species with completely single rooted cheek teeth, and intermediate conditions in Callorhinus gilmorei and Callorhinus sp. Modified from Boessenecker (2011); photos: R.W. Boessenecker.
Ron told me about another mandible from the Rio Dell Formation that he had collected - and, fortunately, donated to Sierra College Natural History Museum in Rocklin. I visited Sierra College for the first time, met Dick Hilton (recently retired) - who I would later go on to do a bunch of fieldwork with in Northern California and Oregon over the next decade. The mandible had a broken canine and a single molar that was strongly double rooted; the other teeth were lost, and their condition was not yet known, since siltstone still filled in the sockets. Fortunately, I was able to take the specimen on loan to Montana State and get the specimen prepared. I was further given permission at Museum of the Rockies to run it through their in-house X-ray machine. I did, and when I picked up the developed film a couple of days later, I was flabbergasted: all of the other tooth sockets were single rooted! I had assumed this to be a slightly larger male individual of Callorhinus gilmorei. "Where the hell is this specimen from in the section?" I thought, as it was suddenly much closer in morphology to the modern species. I raced back home with the X-ray in hand, and double checked the notes: it was collected from near the mouth of Nanning Creek, which was near the very top of the Rio Dell Formation. After some further reading, I determined that the specimen was quite a bit younger than Ron's beautiful specimen of Callorhinus gilmorei, and likely about 2.1-1.2 Ma, or early Pleistocene in age - entirely younger than his concretion specimen.
Based on this, I concluded that Callorhinus is possibly a good example of anagenetic evolution. These fossils are all within the modern range of the species, and we have no evidence yet of two species in the fossil record with overlapping age ranges - in other words, all fossils at any time period seem to represent the same taxon. One of the only specimens from the early Pleistocene is exactly intermediate in dental morphology between Callorhinus ursinus and the Pliocene species Callorhinus gilmorei. Anagenetic evolution is anatomical change within a non-branching lineage. It's more challenging to demonstrate than the opposite pattern - cladogenesis, or species branching - because you can always publish an article with a shitty cladogram and poorly coded phylogenetic matrix and split specimens into different species, whether or not it's warranted. Demonstrating anagenesis - the lack of branching events - on the other hand, ironically, needs more data. Many paleontologists unfortunately treat cladogenesis as the 'null' hypothesis, which is admittedly a bit silly, because we don't have infinitely branching species; species don't always branch, and they simply must go through long periods without branching. Bringing it back to Callorhinus, for example - 1) there's only one extant species, 2) fossils of this lineage indicate it's been around for at least 4 million years, and 3) just like today, there only seems to be one species at any point in the past four million years. Further evaluation of macroevolutionary patterns like this is possible if more otariid fossils are discovered and published, and there is certainly more work to be done as the fossil record of Callorhinus and Callorhinus-like fossils continues to improve.
A New Zealand fur seal (Arctocephalus forsteri) on the shore at Tairoa Head, Otago Peninsula, New Zealand. You can get an idea of how dense the fur is - a substrate suiable for only the louse genus Proechinophthirus. The lice that inhabit seals, sea lions, and the walrus are not adapted for living in dense, fine fur. Photo: R.W. Boessenecker.
Sucking Lice and Otariid Phylogeny
Prior to the advent of molecular phylogenetics, one unusual tool used to infer marine mammal relationships was the relationships and taxonomy of parasites - whale barnacles are the most famous marine mammal parasites - but these do not occur on pinnipeds. Pinnipeds, just like the dogs and cats we have as pets, get infested with different arthropod parasites - lice. Whales also get infested with whale lice, though whale lice are technically a type of skeleton shrimp (caprellids) and are therefore crustaceans. Seal lice, on the other hand, are actually a type of insect - and in an incredible but sort of gross evolutionary story, they evidently have been restricted to pinnipeds since they first became aquatic, and having no other hosts, were forced to become the world's only truly marine insects. These species of lice are present in a few different closely related genera, all within the family Echinophthiriidae (admittedly a mouthful).
Table of seal lice species and their pinniped hosts. From Kim et al. (1975).
Many of these lice are host-specific - in other words, each species of louse tends to colonize the skin and hair of a single species, or several closely related ones. There are two species of Proechinophthirus - one species (P. fluctus) only inhabits the Northern fur seal (Callorhinus ursinus) and the other (P. zumpti) inhabits Arctocephalus spp. The genus Antarctophthirus is more speciose, and includes species that inhabit all extant sea lions (A. microchir), the walrus (A. trichechi), several on true seals, and another (A. callorhini) that also inhabits the northern fur seal (Callorhinus ursinus). The northern fur seal, interestingly, is the only pinniped with two species of lice; A. callorhini - originating from fur-less sea lions and true seals - lives on the bare skin of the flippers, while P. fluctus lives within the fur. The fact that all sea lions are infested with one species may speak to a recent divergence between extant sea lion species - especially considering that, aside from the Steller's and California sea lions (Eumetopias and Zalophus) - none of these species are sympatric (overlapping).
The phylogeny of seal lice (right) as compared to the phylogeny of their host species (left) - the blue arrow indicates a jump from otariids to phocids by lice. From Leonardi et al. (2019).
Likewise, the fact that two sister species of lice infest the northern fur seal (Callorhinus) and southern fur seals (Arctocephalus) led support to the initial division of the Otariidae into the Otariinae (sea lions) and Arctocephalinae (fur seals). Given that neither clade seems to be monophyletic, this poses an interesting problem: how did we get two closely related species of fur-inhabiting lice on distantly related fur seals? My suspicion is that Proechinophthirus may be an ancient lineage of louse that formerly inhabited sea lions, and there was likely additional species inhabiting the furry ancestral population of sea lions - which went extinct as sea lions lost their fur. Sea lions then likely picked up Antarctophthirus from true seals. Alternatively, it is possible that Arctocephalus townsendi picked up Proechinophthirus from Callorhinus in southern California where their ranges overlap - which, depending upon dispersal of these little fur seals, could have introduced it to other southern hemisphere populations.
The baculum of a leopard seal (Hydrurga leptonyx), showing the general location of the bone within pinnipeds; there aren't any good X-rays I can find showing the bone in situ, and this image is just fabulous. From Rule et al. (2023).
Comparison of the anatomy and size of the baculum in modern pinnipeds including the walrus (Odobenus), elephant seal (Mirounga), leopard seal (Hydrurga), crabeater seal (Lobodon), and a fur seal (Arctocephalus). From Rule et al. (2023).
The Baculum and Otariid Phylogeny
Some readers may be aware of the existence of the baculum or penis bone, or, alternatively, os penis - and for those who aren't, yes indeed! Many mammals have a bone inside of their johnson. Some even have an os clitoris as well, though, predictably, it has not received as much study. And I hate that this sounds like a joke, but the Os Clitoris is very small and easily lost, so it is not always preserved in extant osteological specimens. The baculum, on the other hand, is generally quite large and is a symmetrical, rib-shaped bone; raccoons are surprisingly well-endowed* with an S-shaped baculum about the same length as their mandible. Bears have surprisingly small baculi. Sea lions and fur seals have reasonably large baculi, as does the extinct desmatophocid seal Allodesmus - but the most impressive are the nightstick-sized baculi of the walrus, Odobenus rosmarus; you could quite literally bludgeon someone to death with one, as they are nearly two feet long, and heavy. Kids could probably use them to hit baseballs in Little League. But I digress. The baculum essentially functions to maintain 'stiffness' during sex, and is more robustly developed in species with prolonged copulation (e.g. longer sex), and in pinnipeds, baculum length corresponds directly with the mass of the testes (e.g. similar to the size of the tusk in narwhals). The large size of the baculum in pinnipeds is likely relating to polygny: all otariids, and the walrus, have large harems and mate with many females during a single breeding season. In otariids, the baculum length is similar in proportion to the rest of the skeleton between species (approximately 6% of body length), but increases in mass and density during growth, keeping in league with body mass - the densest, most massive baculi being in the Steller's sea lion (Eumetopias jubatus), the most massive species of otariid. The baculum triples in length from being one year old to eight years old, but increases by 30 times in mass (3,000%). The baculum forms within the penis dorsal to the urethra, and in terrestrial carnivores the baculum has a deep urethral groove; the groove is generally absent or shallow in pinnipeds and the sea otter.
*During my first few months in South Carolina I noticed one... "good ole boy" had a raccoon baculum with a sharpened end on it hanging from a necklace. I was told that the "hillbillies" (not my phrasing) would keep them and file them down and use it as a toothpick... and they call them "coon dick tooth picks".
Comparison of some fur seal baculi, including the extinct fur seal Thalassoleon mexicanus. Sea lion baculi are much larger and more robust. From Repenning and Tedford (1977).
The evolutionary history of fur seals and sea lions as inferred from seal lice and the anatomy of the baculum. From Kim et al. (1975). A very similar version of this figure appeared two years later in Repenning and Tedford (1977) - see above.
In otariids, the baculum is shaped like a skinny banana (this isn't getting any better, is it?) and has a thickened base (sorry) that is situated near the pubis, and thins towards the apex, which in fur seals (Callorhinus, Arctocephalus) and the California sea lion (Zalophus) is shaped in side view a bit like a capital T: there is a small dorsal process of bone and a slightly longer ventral process. The baculum of Zalophus is very similar to Arctocephalus, and simply with a wider apex - suggesting that it is the most generalized of sea lions - consistent with its smaller size, and fur seal-like skull. One early study suggested that Zalophus was a rather robust "fur seal without fur" and that it might reflect the earliest sea lion lineage to evolve - borne out by some following molecular analyses. These species share a relatively narrow apex of the baculum, whereas the remaining sea lions - Otaria, Eumetopias, Phocarctos, and Neophoca - all exhibit a rounded apex. Short dorsal and ventral processes, like Zalophus, are retained in Neophoca, Otaria, and Eumetopias, but lost in Phocarctos. Interestingly, the growth of the baculum in Eumetopias has been observed to begin as fur-seal like early in ontogeny, and pass through successive stages where it resembles the baculum of Zalophus, and then appears similar to Otaria and Neophoca, and then Phocarctos, before terminating in the distinctive shape of adult Eumetopias.
There is certainly a size component here, so it's also possible that this gradient simply reflects allometry (though baculum length itself is decoupled from body size across mammals). Lastly, it has even been hypothesized that the differing shape of the bacular apex may be correlated with the shape of the actual apex of the soft tissue penis itself - and therefore, the females may actually be selecting for penis shape in an example of sexual selection. In phocid seals, the apex is unornamented, similar between species, and doesn't extend as far towards the apex of the soft tissue penis, and therefore probably only functions for mechanical support. In otariids, though, it seems as though female choosiness might be driving this diversification of bacular anatomy.
While a valuable tool in early pinniped evolutionary work, the anatomy of the baculum is now simply a handful of anatomical characters in our phylogenetic analyses that may not necessarily be any more important than cranial or dental features. However, it's entirely possible, if not likely, that bacular anatomy deserves more attention in future phylogenetic analyses of otariid relationships.
Arctocephalus v. Arctophoca
In addition to the fur seals being paraphyletic (e.g. not originating from a single common ancestor), it's also been found that the genus Arctocephalus may not be monophyletic either. Arctocephalus pusillus - the South African or Cape fur seal - is the type species for the genus, and it occasionally plots out as sister to the South American sea lion, Otaria byronia, or diverging earlier than some sea lions (along with Arctocephalus tropicalis, the subantarctic fur seal) - with all the other species of southern fur seals placed in a clade excluding all sea lions. This other clade, including Arctocephalus forsteri, A. australis, A. townsendi, A. phillippii, A. galapagoensis, and A. gazella - have been placed into the genus Arctophoca, which was originally established for the Juan Fernandez fur seal (A. phillippii). This didn't last terribly long and by the end of the 2010s most papers had abandoned this recommendation and the Society for Marine Mammalogy's Committee on Taxonomy has, at present, continued to use Arctocephalus for all of these species. My colleagues Annalisa Berta and Morgan Churchill wrote a paper on pinniped taxonomy recommending the use of Arctophoca in 2012, and in 2014, Morgan, Mark Clementz, and I published a phylogenetic analysis of the Otariidae - and though we used Arctophoca, ironically, our cladograms had a monophyletic southern hemisphere fur seal clade, supporting usage of Arctocephalus.
Ecology and Behavior of Fur seals and Sea lions
Fur seals and sea lions are chiefly fish eaters - though many also eat cephalopods and some species, such as the Antarctic fur seal (Arctocephalus gazella) even specialize on krill. Large species, such as the Steller's sea lion (Eumetopias jubatus) and New Zealand sea lion (Phocarctos hookeri) mostly feed on fish, but supplement their diet with warm blooded prey such as other pinnipeds, sea otters, and sea birds.
Otariids are all polygynous, with males breeding with several females. All species, except for the Australian sea lion, breed annually with a nine month pregnancy after a three month long delayed implantation - similar to elephant seals. The Australian sea lion (Neophoca cinerea) breeds on an 18 month cycle. The males of some species organize and herd female harems (e.g. Northern fur seal, Callorhinus ursinus) whereas other species control and manage areas more in a 'lekking' type behavior, such as the Steller's sea lion (Eumetopias jubatus). The largest harems are maintained by male Northern fur seals, with up to a hundred females.
In most otariids, females arriving at a "rookery" (breeding site) give birth shortly after coming ashore, which is followed immediately by estrus. After a couple of weeks of lactation, the mother starts returning to the ocean to forage, returning to the rookery to continue to feed the pup extremely rich milk. Unlike elephant seals, male otariids tend to forage further from shore, hundreds of kilometers out to sea, while females tend to hug the continental shelf. After a few months (e.g. Callorhinus) or an entire year (e.g. Zalophus) the pup is fully weaned, and the female heads back to the sea.
Two subadult male New Zealand sea lions (Phocarctos hookeri) already mock-fighting at a year or two of age. Sandfly Bay, New Zealand. Photo: R.W. Boessenecker.
Otariid Evolutionary Patterns: Sexual Dimorphism and Body Size
All modern otariids are sexually dimorphic and all show evidence of polygynous breeding - males mating with multiple females. It seems likely that the gregariousness of otariids at their haulouts likely drove this: in species that like to haulout in large groups along a shoreline - space is at a premium. Pinnipeds aren't safe on land - except on small islands lacking predators - so they're already restricted to a narrow strip of land along the coast. The limited space and dense population is a tinder box for competition. On the other hand, pinnipeds with more dispersed mating systems - say for example, harbor seals - exhibit much less dimorphism, with males and females of nearly equal body size and generally quite difficult to visually identify, unlike otariids. Early 'pinnipedologists' like Bartholomew (1970) argued that such a system was nearly guaranteed by the terrestrial habits of some pinnipeds. What drove the origin of sexual dimorphism, though? It's arguably driven by the initial gregariousness of females on land - and likely a result of the factors listed above (Another possibility is that large aggregations of females is triggered by a change in habitat or environmental conditions, and the concentration of females led to male competition.
When there is competition among males - either directly for females, or for control of specific areas of the beach where females are likely to aggregate - combat is inevitable. As a result of this, males tend to be selected for how able they are to drive off other males either by sheer intimidation or by combat - so, males evolve larger body size, proportionally larger canines, skulls and mandibles with more robust proportions, and larger muscle attachments for the jaw closing muscles (and, more robust muscle attachment crests and bone proportions throughout the postcranial skeleton as well, no doubt reflecting the larger body mass requiring more powerful muscles). While it is tempting to consider this to be an example of runaway sexual selection, there is probably some upper limit imposed on these species, as, past a certain point, there isn't enough food to eat to get you any larger, and it becomes more difficult to walk around on land.
The degree of sexual selection is not static, however. Pinnipeds have been thought to follow what is called "Rensch's Rule" - smaller species tend to be less sexually dimorphic than larger ones, where it is frequently exaggerated. This observation applies to closely related groups of species affected by sexual selection, and was initially formulated in birds. Let's take a few examples: we'll use the largest otariid (Eumetopias jubatus), smallest (Arctocephalus galapagoensis), and a medium sized example (Arctocephalus pusillus). Eumetopias (Steller's sea lion) have an approximate maximum mass of 1100 kg in males and 350 kg in females, with males weighing about three times as much as females. In the smallest species, Arctocephalus galapagoensis (Galapagos fur seal), the males weigh up to 70 kg, whereas females are up to 40 kg. In our medium sized example, Arctocephalus pusillus (South African sea lion), males weigh up to 360 kg and females up to 120 kg. While males of the tiny Galapagos fur seal are only about 1.75 times more massive than females, male Steller's sea lions are 3.14 times as massive. However, the medium sized example, the South African fur seal, is similar to the Steller's, with males being 3.0 times larger than females. So, what gives?
The relationship between the maximum body mass of males and female otariids; the calculation is simply maximum male body mass divided by female body mass. In other words, males of Arctocephalus galapagoensis are 1.75 times larger than females, whereas male Eumetopias are just over three times as large as females. Data from Reeves et al., 2002, Guide to Marine Mammals of the World, published by the National Audubon Society.
As it happens, pure body size related exaggeration of dimorphism (e.g. Rensch's Rule) doesn't entirely explain the pattern of sexual dimorphism in pinnipeds. When I plot the ratio of maximum male body mass to females (rough numbers taken from the Audubon Society Guide to Marine Mammals of the World - not far off from the data in papers like Lindenfors et al., 2002), many sea lions and larger fur seals have similar ratios of maximum male:female body mass, between 2.5 and 3.5 times female mass. So, if it's not Rensch's rule, then what is it?
The relationship of body mass ratio (see above - maximum body mass of males divided by that of females) and the size of fur seal and sea lion harems (number of females a single reproductively competitive male mates with). Data from above, and from Lindenfors et al. (2002).
Instead, it may be more closely related to harem size: the larger the harem of a particular species, the larger a fur seal or sea lion is likely to be. This is a bit closer, and a classic study by Lindenfors et al. (2002) found statistical support for this idea over a purely 'allometric' exaggeration of dimorphism with increasing body mass (e.g. Rensch's Rule). This seems a little better - though the correlation is admittedly not perfect. A lot of the larger species do have larger harems - but some have surprisingly small harems, like the Australian and South American sea lions (Neophoca, Otaria). It's no surprise that the Northern fur seal has such pronounced sexual dimorphism, with males weighing over four times as much as females - it famously has the largest harems of any otariid, averaging over thirty females per season (and documented with as many as one hundred). Meanwhile, species with smaller harems like the Galapagos fur seal and even the much larger South American sea lion, have lesser degrees of dimorphism. In reality, this correlation isn't perfect either - it is better - but I wonder if the pattern in otariids isn't a combination of Rensch's rule and harem size.
Sexual dimorphism in one of the largest sea lions, the New Zealand sea lion (Phocarctos hookeri) - the dainty little female is on the left, and the rather monstrous, bear-like male is on the right, partially obscured by a rock. Photo: R.W. Boessenecker.
But those are probably not the only factors. For example: what the hell is going on with the Antarctic fur seal, Arctocephalus gazella? It has medium sized harems (10 females) but is the most sexually dimorphic otariid, with males being five times as massive as females (and, amongst pinnipeds, only the elephant seals are more dimorphic!). As it happens, this species has fairly extreme sexual segregation as the males travel much further south on foraging trips than the females; they very well might have completely different diets, as males can dive deeper as well. Elephant seals, for comparison, have extreme sexually segregated foraging areas, dive depths, and even diets. So, once a species of pinniped is already sexually dimorphic, sex-based differences in feeding behavior and ecology might further drive a wedge. In a way, this makes a lot of sense: extremely sexually dimorphic species of pinnipeds are also likely to be quite gregarious; at a certain point, it becomes advantageous to have within-species niche differentiation. In other words, if the males and females are behaving as if they were completely different species, feeding on completely different food types, it increases their potential population size in a region (called carrying capacity) and minimizes competition.
Sexual dimorphism in the extinct fur seal Thalassoleon mexicanus. Photo: R.W. Boessenecker.
Lastly, there is some evidence of sexual dimorphism in fossil otariids - and, before that, sexual dimorphism also characterizes the extant walrus (Odobenus rosmarus), elephant seals (Mirounga spp.), gray seals (Halichoerus grypus), and to a lesser extent, monk seals (Monachus/Neomonachus spp.). Several non-otariid fossils have large enough samples to showcase sexual dimorphism, including the Sharktooth Hill sample of the large seal-like Allodesmus kernensis, its smaller relative Desmatophoca oregonensis from the Astoria Formation of Oregon, and a bunch of fossil walruses (Valenictus chulavistensis, Osodobenus eodon, Pontolis barroni, Imagotaria downsi, Neotherium mirum, Proneotherium repenningi) and even some enaliarctine proto-seals (e.g. Enaliarctos emlongi, which I have written about previously). For example, skulls of presumed male specimens of Thalassoleon mexicanus from southern California and Baja are approximately 270-275 mm, whereas female skulls are somewhat shorter and more gracile, about 245 mm in length - or 90% the size; the female mandibles are about 75% the length of the males, and, similarly gracile. Few other samples of published fossil otariids are large enough to evaluate dimorphism - but additional remains of Thalassoleon macnallyae from the Purisima Formation, and Callorhinus gilmorei from the San Diego Formation that I have examined - seem to confirm this.
Interestingly, the proposal that the early fur seal Eotaria citrica represents a species separate from Eotaria crypta is based on a handful of morphological characters as well as an analysis of sexual dimorphism in otariids. The difference in mandibular proportions between these two seems to exceed the degree of sexual dimorphism seen in extant sea lions, especially with respect to the depth of the mandible. Statistical analysis, on the hand, shows that the two species do not differ from one another quite so much as would be expected - unless of course Eotaria was not very sexually dimorphic (like the slightly larger extant Galapagos fur seal, for example).
Three characteristic Pacific Coast otariids all hauled out at the same spot: a titanic male Steller's sea lion (Eumetopias jubatus, upper left), a smaller but still large California sea lion (Zalophus californianus, center left), and a Northern fur seal (Callorhinus ursinus, right) that looks like he's reevaluating recent choices. All three are adult males. Image credit: Race Rocks Reserve.
Modern otariids, like the phocid seals, have quite a variety of body sizes, with some truly small species like the Galapagos fur seal (Arctocephalus galapagoensis) - weighing in at just 60 kg - to the order of magnitude larger Steller's sea lion (Eumetopias jubatus), attaining a hefty 1,100 kg. Early enaliarctine pinnipeds (Enaliarctos, Pteronarctos, 'Pacificotaria', Pinnarctidion) were generally quite small, perhaps 1.3-1.8 meters long - the enaliarctine "proto seals", and most enaliarctine-like early walruses ("imagotariines") were not much larger (~2-2.5 meters). Early otariids, on the other hand, were some of the smallest pinnipeds ever - the oldest otariid, Eotaria, was likely around 1-1.2 meters in length, and the somewhat younger Pithanotaria was likely around the same size. By the latest Miocene and early Pliocene, the larger otariids in the genus Thalassoleon were distributed around the North Pacific - ranging in size from 1.8-2.3 meters. In the late Pliocene, the small-bodied Callorhinus gilmorei was a bit of a dwarf - only 1.6 meters in length - and surrounded by considerably larger walruses, being the only late Pliocene otariid in the eastern North Pacific. In the eastern South Pacific, the large fur seal Hydrarctos lomasiensis likely was around the size of a California sea lion at about 2.2-2.4 meters. In the Pleistocene, otariid fossils from Japan, the Pacific Coast, South America, and New Zealand indicate diverse faunas with body size diversity paralleling the modern fauna - if a few species are extinct or out of place (e.g. Neophoca in New Zealand). We didn't really have any 'diverse' (e.g. more than two species) otariid assemblages anywhere on earth, perhaps except Japan, until the Pleistocene. Curiously, this is the same time period when otariids became large - exceeding three meter body lengths. This does seem to be correlated with climate: larger body sizes are needed in colder waters, shifting the theoretical minimum body mass for a pinniped upwards - but, perhaps more critically, also resulting in greater primary productivity, and permitting larger body sizes. Perhaps it's no coincidence that the evolution of truly gigantic sea lions corresponds to the evolution of the elephant seals (Mirounga) as well as the explosion in body size in baleen whales (see here for an earlier blog post on the evolution of body size in mysticete whales).
Trends in skull size for California sea lions (Zalophus californianus) from the 1960s to 2000s. Note the steady size increase for males, but not females. From Valenzuela-Toro et al. (2020).
Body size in pinnipeds is affected by a number of factors. First and foremost, water temperature dictates minimum body size: warmer water permits smaller body sizes as you don't need as much mass to keep warm. The tiniest modern otariids live close to the tropics (e.g. Galapagos fur seal) and smaller extinct species like Eotaria crypta and Pithanotaria starri lived close to the Middle Miocene Climatic Optimum when sea surface temperatures were about 5-7˚C warmer than today. More abundant food permits larger body mass - though in some species, an overabundance of food can also drive miniaturization through paedomorphosis. Like other marine mammals, many otariid species have undergone sharp population declines as a result of hunting and overfishing - but, after certain protections were put in place, some species expanded in population. Northern fur seals (Callorhinus ursinus) and South American Sea Lions (Otaria byronia) have undergone slight body size decreases since their populations boomed after being awarded legal protections - presumably as a result of increased competition for limited prey (perhaps corresponding to local overfishing). On the other hand, a fascinating study using hundreds of skulls of California sea lions collected since the 1960s revealed a size increase in males since passage of the Marine Mammal Protection Act - females, on the other hand, remained the same size. This suggests that the population increase may have resulted in increased sexual selection as males were competing more intensely.
Body mass trends in the walruses (Odobenidae - diamonds), desmatophocids (e.g. Allodesmus, Desmatophoca - circles), and the fur seals and sea lions (Otariidae - squares). Note that the time axis is flipped from what I normally do: modern (recent) is to the left and early Miocene (Burdigalian stage) is to the right. For those unfamiliar with the international marine stages, the Burdigalian, or late early Miocene, is approx. 20-16 mya; Langhian-Serravallian are the middle Miocene, or 16-11.6 mya; Tortonian-Messinian are the late Miocene, or 11.6-5.3 mya, the Zanclean is the early Pliocene, or 5.3-3.6 mya, and the Piacenzian is the late Pliocene, or 3.6-2.5 mya. The vertical scale is a log-transformed value of body length; the higher the number, the greater the body length. From Churchill et al. (2015).
Morgan Churchill examined body size trends in pinnipeds during his doctoral studies and wanted to evaluate "Cope's Rule" in pinnipeds. "Cope's Rule" is the rough idea that body size evolution is "one way" and that groups tend to start out small and evolve larger body sizes through time - in essence, a form of directional evolution. In a previous study, he established some predictive measurements for estimating body length from skull and mandibular measurements for modern pinnipeds of known body length, and then applied these to fossils. He found that there were just as many trends towards increasing body size as there were towards decreasing body size, and therefore, that "Cope's Rule" did not really apply to pinnipeds. In fact, there seemed to be more lineages undergoing decreasing in body size within the Otariidae! Species like Callorhinus gilmorei, and the many small species of Arctocephalus, no doubt contributed to this trend within the Otariidae. Further to the point, he suggested that the evolution of small body sizes within Arctocephalus may have led to the re-evolution of fur as a necessary supplement for insulation with blubber - a feature otherwise misleading taxonomists for a century, as we now know that Callorhinus and Arctocephalus are not closely related. Prior to the Pleistocene, large to giant walruses (Imagotaria, Osodobenus, Pontolis, Gomphotaria, Dusignathus) likely occupied some of the niches now occupied by sea lions, and before them, large desmatophocids like Allodesmus.
Diet and Feeding Behavior of Fur Seals and Sea Lions
All otariids are carnivorous and most commonly feed on various types of fish. California sea lions (Zalophus californianus), for example, feed extensively on many of the same fish that fisherman do on the Pacific coast, including salmon, anchovies, herring, hake, rockfish, as well as lamprey. This species also feeds on octopus, as well as squid. Steller's sea lions (Eumetopias jubatus) have a similar diet but live at higher latitudes in the North Pacific, including walleye pollock, halibut, mackerel, herring, cod, rockfish, and of course salmon - much to the ire of fishermen in the Pacific Northwest.
Large species of sea lions occasionally resort to feeding on other pinnipeds and sea birds; Eumetopias will catch and kill northern fur seals, ringed and harbor seals, as well as sea otters. The South American sea lion (Otaria byronia) as well as the New Zealand sea lion (Phocarctos hookeri) are also known to eat sympatric fur seals, as well as seabirds - notably including penguins. While most sea lion species feed on cuttlefish, Australian sea lions (Neophoca cinerea) are also known to consume cuttlefish - and are probably some of the only otariids that actually overlap in range with cuttlefish (they are notably only along "old world" coastlines and surprisingly do not seem to have made it very far inside the Polynesian Triangle, and are restricted to the westernmost Pacific).
Some otariids feed chiefly on invertebrates. The Antarctic fur seal (Arctocephalus gazella) surprisingly mostly feeds on krill, as well as fish and squid - after all, that close to Antarctica, you might as well take advantage of the most highly productive prey resource on earth. New Zealand fur seals (Arctocephalus forsteri) mostly feed on cephalopods, especially arrow squid and New Zealand octopus. Multiple species are also known to occasionally feed on crabs, lobsters, and even mollusks.
Otariids are not as adept at diving as true seals are - California sea lions (Zalophus californianus) can dive down nearly 300 meters and up to ten minutes, but typically less than 100 meters and for only a couple of minutes. Steller's sea lions (Eumetopias jubatus) can dive only slightly deeper despite their greater mass - down to about 400 meters. Compare this, for example, to female elephant seals of similar mass, that routinely dive down to over 1,000 meters; phocid seals evidently have a physiological edge over their admittedly more 'primitive' eared cousins. At smaller body sizes there is less blood volume to store dissolved oxygen, so dive length - and, ultimately, dive depth - becomes less and less. As a result, the smallest otariids like the Galapagos and Juan Fernandez fur seals (Arctocephalus galapagoensis and A. phillippii, respectively) only dive down to about 100 meters.
Despite their shallow diving depth, several otariids do feed on deep sea lanternfish like elephant seals - but do so at night when lanternfish follow their planktonic prey upwards closer to the surface - called diel migration. Some species, like the Guadalupe fur seal (Arctocephalus townsendi), are exclusively nocturnal hunters - which is one way to maximize abundant food despite being so tiny and having such a limited dive length. Subantarctic fur seals (Arctocephalus tropicalis) and Juan Fernandez fur seals (Arctocephalus phillipii) have similar diets.
The Taphonomy and Ichnology of Fur Seals and Sea Lions
Fossil preservation is frequently under-valued by taxonomists, and given my background in taphonomy, I would be remiss if I didn't outline a few interesting studies on the preservation of fur seals and sea lions - and, shockingly, have some of the only trackways known for a marine mammal!
First off, we'll start with what we know about the decomposition of otariids. The majority of otariid fossils in museum collections are isolated bones - in some cases this is a result of the marine environment where scavenging and the skeletonization of a carcass can be quite rapid. Sharks and fish can consume a carcass floating at the water's surface, and after it sinks to the seafloor, crustaceans (especially crabs, squat lobsters, and isopods) and gastropods really tear into the carcass and separate flesh from the bones. But, even an intact but cleaned skeleton is not safe - in shallow marine settings, currents from waves or, in deeper middle shelf settings, storm waves, have the capacity to transport bones across the sea floor and separate them from one another. Floating carcasses might drop bones over many miles. In fact, we do know of one style of disarticulation strongly suggestive of this mode of bone loss: there are several faceless juvenile and subadult fossil fur seal skulls from the Purisima Formation; two good examples are figured above. The eye socket (orbit) of otariids (and phocids for that matter) has become so enlarged that the bony connections between the interorbital region of the skull ("forehead") and the rostrum (snout) have become rather minimal and tenuous, and in a young enough animal, they will simply fall apart once the surrounding soft tissue is gone. For comparison is an image of a New Zealand fur seal with a completely disarticulated rostrum we processed during my Ph.D. at Otago. Most carcasses I've seen have lost the rostrum as well, even in what appear to be adults; rolling around on a beach, impacts with the sand probably help dislodge the rostral bones. I've seen more faceless carcasses than ones with an intact skull.
Taphonomic pathways of disarticulation for a sea lion: disarticulation of a skeleton after sinking to the seafloor, or shedding of bones from a floating carcass. Artwork by R.W. Boessenecker; from my 2010 SVP poster.
Aside from this, pinnipeds also seem to follow "The Law of the Lower Jaw" - a curious taphonomic phenomenon where the mandibles seem to be the first bones lost from the skeleton, owing to their proximity to the mouth. The mandibles are already halfway exposed by a mucuous membrane, which is easily eaten through - and scavengers typically attack the mouth and other weak points (eyes, armpits, anus, genitals), so decomposition (specifically, lost off soft tissue) seems to really spread from these areas. The mouth is the largest orifice, so the mandibles are exposed, loosened, and lost early. However, in otariids, the rostrum seems to be lost even earlier as it requires even less decomposition. In fact, the carcass figured above still has each mandible hanging by literal threads of ligament - but who knows where the rostrum is; it is long gone.
Circular crushing bite marks from another carnivore on a radius and humerus of juvenile Thalassoleon macnallyae from the Purisima Formation at Santa Cruz, California. Photos by R.W. Boessenecker.
A circular puncture on a late Pleistocene juvenile skull of Eumetopias jubatus from British Columbia - a canine puncture from another carnivore, or perhaps a spear point? From Harington et al. (2004).
There is also some direct evidence of extinct fur seals being food. Two different juvenile postcranial bones of Thalassoleon macnallyae from the Purisima Formation at Santa Cruz have circular crushing bite marks on them. Unlike the linear gouges formed by shark bites, these were made by a conical tooth. I personally prepared both specimens, and there wasn't anything embedded in either bite mark other than sand and silt - so these were not formed from diagenesis. These bite marks are now known as the trace fossil Nihilichnus, and can be made by crocodylians, mammalian carnivores, and toothed whales (there are even some traces like this on juvenile basilosaurids). In this case, the traces are most likely to be from another pinniped - there are no other bite marks, so something like a large dolphin with closely spaced teeth could have hardly been responsible. I suspect it's a case of cannibalism by Thalassoleon macnallyae, or possibly predation by the larger sea lion like walrus Dusignathus santacruzensis. I published this research with Frank Perry in 2011. Lastly, predation by a terrestrial carnivore is likely - pinniped pups are often targeted by coyotes on the Pacific coast, and prior to their extirpation, likely grizzlies and wolves as well. Another specimen from the latest Pleistocene of British Columbia, a skull of a juvenile Eumetopias jubatus, has a similar circular puncture with a collapsed rim on the top of the braincase. Because this specimen has a radiocarbon date of 12,570 years, this could have been from either a canine or perhaps a paleoindian spear point.
And there are of course fossils of otariids with shark bite marks on them. Here's a little femur fragment I collected during my master's fieldwork (~circa 2009) that has some teeny tiny little bite marks from a small shark, or perhaps even from a fish. Whatever animal this was, its teeth must have been quite small. Further interesting is the fact that the bony pores of this bone are highly enlarged - suggesting that it has been partially digested, likely in the digestive tract of a shark. I assume it was the same individual shark - though to be honest, the tooth marks look pretty small for a shark large enough to have swallowed a femur whole. This is one of the only good examples of a Cenozoic marine fossil preserving evidence of acid etching.
In addition to being food, we also have evidence of other organisms using the skeletons of fur seals and sea lions as a home. One of my favorite sorts of fossils to find are those encrusted with marine invertebrates. I collected a pair of associated sea lion vertebrae (?Proterozetes ulysses) encrusted with barnacles from the middle Pleistocene Port Orford Formation of Oregon, and a couple years later, found myself a nice femur, also with barnacles. I didn't know any of the specimens had any barnacles until I started preparing them. I found the barnacles on the vertebrae when I was hosing them off in my parent's driveway, and on the femur when I started trying to prepare it with acetic acid. Spotting a single barnacle, and knowing how many were on the vertebrae, I pulled it out immediately and rinsed it off: acid would destroy all of the barnacles. So, I had to prepare all three bones with an air scribe and a microblaster. The vertebrae have barnacles on nearly all sides, indicating that they must have been rolling around regularly so as not to kill off any of the barnacles. The femur, on the other hand, only has barnacles on the posterior side, so the anterior side must have been stuck in the silty bottom. Barnacles often attach to any hard substrate they can, and on the highest points of the seafloor, in order to access faster currents and make filter feeding more successful. Evidently, the femur never flipped over. Altogether, I counted 1,477 barnacles on these three bones (I can't believe I counted all of them!). Further, I had California Academy of Sciences barnacle researcher Robert Van Syoc identify the barnacles; he concluded that they were the extant species Hesperibalanus hesperius. Based on the maximum size of barnacles and published growth rates for this species, I calculated that the bones must have been on the seafloor, unburied, for a minimum of 7 months (continuously). Additionally, on the vertebrae, there is a bimodal size distribution of barnacles, suggesting a second colonization event by settling barnacles. Therefore the vertebrae may have lied, unburied (but rolling around nonetheless), for perhaps a full year prior to final burial. I published these in 2013 in the journal Palaios.
Late Pleistocene flipper prints from an ancient fur seal from the coast of South Africa- the only trackway from a marine mammal! From Helm et al. (2022).
One of the most surprising parts of the otariid fossil record is the discovery of terrestrial trace fossils! Along the southern coast of South Africa (about 400 km east of Cape Town), there is an upper Pleistocene beach sand that has become cemented surprisingly quickly and erodes out in blocks as you might expect for much older rocks. The Cape South Coast Ichnology Project has documented a number of important and unusual trackways, including land mammals such as giraffes along the ancient coastline, tracks of early humans, crocodiles, unusually large cranes, herons, and flamingoes, and even trackways of turtle hatchlings! More recently, this team has identified repeated flipper prints likely produced by something like a South African fur seal (Arctocephalus pusillus).
Fossil Fur Seals and Sea Lions - A Brief Review
The utterly tiny holotype mandible of Eotaria crypta, the day I "discovered" it hiding in a museum cabinet at the Cooper Center in October 2013. Photo: R.W. Boessenecker.
The holotype mandible of Eotaria crypta in medial, lateral, and dorsal views - from Boessenecker and Churchill (2015).
Eotaria
I first discovered the holotype specimen of Eotaria crypta in a cabinet at the Cooper Center in Orange County when I was a Ph.D. student in 2013. The specimen was from the late early Miocene to early middle Miocene "Topanga" Formation of Orange County, California, approximately 16-17.5 Ma in age. Based on its tiny size and unusually cuspate teeth, it was misidentified as the early walrus Neotherium - though it was considerably smaller. The specimen had more highly denticulate teeth than any other otariids - and even possessed some slight heterodonty - the cheek teeth are all of different sizes, with the lower molar largest. This tooth also possesses the metaconid - a cusp that was quickly lost later in otariids, but is present in all phocids and most early walruses. Critically, Eotaria possesses a lower second molar - also lost in all later otariids. Eotaria would have been extremely tiny, perhaps only 1-1.5 meters in length and likely about no more than about 45 kg (~100 lb).
The holotype mandible of Eotaria citrica and a newly referred mandible of Eotaria crypta. From Velez-Juarbe (2017).
What was crucial about Eotaria was that it filled in an anatomical gap between enaliarctine 'protoseals' and late Miocene fur seals like Pithanotaria, and a nearly 10 million year long chronological gap between them. Unfortunately, the type specimen is a partial mandible. Morgan Churchill and I published Eotaria in 2015. Two years later, our colleague Jorge Velez-Juarbe published several additional specimens, including a complete mandible of Eotaria crypta, and also named a new species, Eotaria citrica, from the same deposits. This new species has a somewhat more robust mandible, some cusps that are sharper, a second lower molar with a bilobed rather than circular root, and a slightly longer intramandibular joint. Whether these differences will eventually be found to represent variation within a single species has yet to be determined, though I suspect there may only be a single species.
The holotype skeleton of Pithanotaria starri, recently rediscovered by California Academy of Sciences Geology Collections Manager Crystal Cortez - the specimen had been stored in this cardboard box and not seen for years. Photo: R.W. Boessenecker.
Pithanotaria
The original specimen of Pithanotaria is a spectacularly compete, but poorly preserved impression of a skeleton of a small fur seal from the Monterey Formation, and is likely 7-8 Ma in age. The slab was discovered in the Celite Company Number 9 quarry just a few miles south of Lompoc in Santa Barbara County, California. This skeleton is quite small - only about 75 cm as preserved - and would not have had a body length exceeding one meter. The skull is quite clearly of a pup, and the specimen lacks a baculum - which would be expected to be present given the excellent state of completeness - suggesting that the holotype is a juvenile female. Kellogg remarked that the postcranial skeleton of this specimen closely resembled modern otariids, and was clearly quite derived - surprising, given that for 90 years, this species was the oldest known otariid. Kellogg reported some additional specimens mostly consisting of fore and hind flippers, and some scattered material has been referred to the species since - a partial skull and an isolated mandible from the 9-10 Ma Santa Margarita Sandstone near Monterey and Santa Cruz, mandibles, skulls, and partial skeletons from the Monterey Formation of Orange County (though still not described). The digits of some of the other Pithanotaria specimens show widened and rectangular ungual phalanges - the last bones of the fingers and toes - with flattened ends, indicating that Pithanotaria already had evolved cartilaginous rods to expand the flippers.
Additional fossils from the Monterey Formation (LACM colletions) referred to Pithanotaria starri by Velez-Juarbe (2017), though most of these specimens are not yet described.
Pithanotaria starri is a relatively small pinniped, up to 1.2-1.5 meters in body length (possibly slightly larger), with simplified cheek teeth and relatively gracile features of the skull and mandible - as might be expected for a relatively tiny otariid, as the smallest extant otariids have the least robust males and the most minimal degree of sexual dimorphism. Pithanotaria seems to have been the only otariid present in the eastern North Pacific during the early late Miocene (Tortonian stage). Though in the past Pithanotaria's rarity in late Miocene deposits had been attributed to possible offshore foraging, analysis of carbon isotopes of specimens from the Monterey Formation indicate enriched carbon isotopic ratios relative to other pinnipeds from the same deposit (e.g. Allodesmus, Imagotaria), indicating that Pithanotaria was most likely to be an inshore, shallow marine predator. Shallow diving would be more consistent with the tiny size of Pithanotaria.
Lastly, a mandible from the late middle Miocene Aoki Formation of Japan closely resembles Pithanotaria, though it has not been formally identified past the family level. I believe this specimen to represent Pithanotaria as the teeth are essentially identical. The coronoid process, however, is unusually steeply elevated, so it can't be the admittedly gracile Pithanotaria starri.
A cast of the holotype skull of the late Miocene fur seal Thalassoleon mexicanus, in Smithsonian collections. Photo: R.W. Boessenecker.
The skull cast of Thalassoleon mexicanus, in side view. Photo: R.W. Boessenecker.
A complete female skull referred to Thalassoleon mexicanus from the uppermost Miocene to lowermost Pliocene Capistrano Formation of Orange County (SDSNH 68313). Photo: R.W. Boessenecker.
Thalassoleon
Three species in the genus Thalassoleon have been named, all from latest Miocene and Pliocene deposits of California, Baja California, and Japan. Thalassoleon mexicanus was originally named in 1977 from Isla Cedros in Baja, based on a series of rather well-preserved skulls, mandibles, and postcrania. The holotype skull itself is quite large, with a skull similar in proportions to Arctocephalus and female sea lions. It differs from all modern otariids in having double rooted cheek teeth, lacking accessory cusps, and a wide space between the tympanic bullae. The sagittal crest is also quite interesting - though not commented on or considered diagnostic by the original describers, the sagittal crest is quite long and increases in height towards the back of the skull, and is somewhat wavy from side to side. The snout is also quite wide - wider than most fur seals, and definitely wider than in the California sea lion (Zalophus). Additional fossils of this species were subsequently described from the Capistrano Formation in 2005. Thalassoleon mexicanus was probably about 2-2.5 meters in length.
Is Thalassoleon monophyletic? It's entirely possible that Thalassoleon macnallyae and Thalassoleon inouei do not belong in the genus, and may represent something more closely allied with Callorhinus. Ornamentation of the bulla certainly suggests this may be the case - however, extant Otaria byronia also has an ornamented bulla. Could Thalassoleon macnallyae represent the ancestor of Callorhinus gilmorei? Maybe, but unfortunately, there is no basicranium yet known for Callorhinus gilmorei.
Mandibles of the extinct Pliocene "Gilmore fur seal", Callorhinus gilmorei, from the San Diego Formation, in SDSNH and LACM collections. The lower left specimen is the holotype. Photos: R.W. Boessenecker.
Callorhinus
Fossils of Callorhinus have been reported from both California and Japan. The modern species has a distinctive skull with a 'truncated' snout - the snout is quite deep, and the upper canines are only slightly in front of the orbit, and the edge of the narial fossa is vertical rather than obliquely sloping; as a result, the face in lateral view appears rectangular rather than triangular. There are, as of yet, no published skulls in the fossil record that resemble this. The tympanic bulla has a crenulated ridge on it with many small conical projections on it - similar to Thalassoleon macnallyae. Though Thalassoleon macnallyae has a similar bulla, Thalassoleon mexicanus does not, and the skull lacks many of the other features of the skull and rostrum of Callorhinus ursinus.
Referred mandibles of Callorhinus gilmorei embedded in a siltstone concretion of the upper Pliocene Rio Dell Formation. Photo: R.W. Boessenecker.
One extinct species in the genus Callorhinus - a "neospecies" - has been named, Callorhinus gilmorei from the San Diego Formation of San Diego County, California. This species is quite small - perhaps half or two thirds the size of Callorhinus ursinus - and is distinguished by possessing a couple of single rooted premolars, and the rest being double rooted. In the holotype, only the first two premolars are single rooted, but in others, three or four are; additionally, there is at least one specimen with all double rooted teeth. The type specimen is a subadult female skeleton, published in 1986. The mandible and teeth share a few features with the modern species, including a mandibular foramen on the lower jaw that is oriented dorsally. Additional fossils have been reported from the Pliocene of Japan, and (by yours truly) from the upper Pliocene Rio Dell Formation of Humboldt County, California. Callorhinus gilmorei was quite a bit smaller than extant Callorhinus ursinus, perhaps about 1.6 meters or so (compared with about 2 meters in extant C. ursinus).
Additional specimens of Callorhinus have been reported from Pleistocene rocks of the North Pacific. The most important of these is a robust (male) mandible from Nanning Creek at the top of the Rio Dell Formation, dating to the early Pleistocene. This specimen preserves a canine and only the first lower molar, and is nearly the size of extant Callorhinus ursinus. When I was an undergraduate student I had the opportunity to get the mandible X-rayed at Museum of the Rockies, prior to preparation of matrix in the tooth sockets. All four premolars had single rooted sockets, and the lower molar still had widely separated root lobes. This was critically intermediate in dental morphology between the Pliocene Callorhinus gilmorei, and the modern Callorhinus ursinus - and also intermediate in geologic age, being about 2.1-1.2 Ma in age.
The mandible of Callorhinus sp. and its position within the Callorhinus lineage. From Boessenecker (2011).
On a further note, Callorhinus is further interesting as it is one of the only marine mammals to persist through faunal changes that occurred in the eastern North Pacific during the Pliocene-Pleistocene interval. Most Pliocene marine mammals from California are clearly not belonging to extant (or recent) lineages, and some of the only "modern" groups in the region that have a continuous fossil record back to the Pliocene are some porpoises (Phocoena), right whales (Eubalaena), some rorquals (Balaenoptera), the recently extinct giant sea cow (Hydrodamalis), and of course, the northern fur seal lineage (Callorhinus). For more on this, see the section on Anagenesis in the Northern Fur Seal (above).
Hydrarctos
Hydrarctos lomasiensis was named in 1978 from Pliocene rocks in the Pisco Basin of Peru that were, at the time, unnamed. The type specimen consists of a complete skull resembling the fur seal Arctocephalus - in fact, it was originally placed in Arctocephalus with Hydrarctos as a subgenus. Later on, it was elevated to the genus level. The skull is relatively long with a narrow snout, and the skull is quite deep, with a long and low sagittal crest that reaches the posterior orbit. The cheek teeth are all single rooted with tall conical crowns. Another skull was referred to this species in 1989; Later on, the rock unit was named as the Caracoles Formation; it is now known to be 2.7-1.9 Ma in age (earliest Pleistocene). All known specimens of Hydrarctos are from this locality.
Arctocephalus
With Hydrarctos elevated to the genus level and removed from Arctocephalus, there aren't very many genuine fossils assignable to Arctocephalus. One of the few possible examples is a femur from the Pliocene Varswater Formation of South Africa. Other fossilized postcrania of Arctocephalus have been collected loose on beaches in southernmost Brazil, eroded from a barrier island deposit dating to the late Pleistocene.
Zalophus
A few scattered specimens from Japan and California have been identified as Zalophus. Several scattered, fragmentary remains from the early and middle Pleistocene of Japan have been reported. The oldest remains of Zalophus, some postcrania from the early Pleistocene of Japan, may be as old as 1.2 Ma; mandibles, crania, and partial skeletons of Zalophus are more commonly reported from the latest early Pleistocene and middle Pleistocene of Japan. Zalophus fossils from the eastern North Pacific are much more rare. A scapula identified in 2006 as Zalophus was collected from the same locality (Port Orford Formation at Cape Blanco, Oregon) as Proterozetes (see below); this specimen is complete, and indistinguishable from extant Zalophus. It's also covered in barnacles, and described and figured it in a 2013 taphonomy article. Though common today and dispersed across most of the temperate North Pacific (including the recently extinct Japanese sea lion), Zalophus has a disappointing fossil record.
Proterozetes
The extinct sea lion Proterozetes ulysses was named in 2006 from the middle Pleistocene (~500,000 years) Port Orford Formation of coastal Oregon. The holotype skull was discovered by the collector extraordinaire Doug Emlong, who also collected a complete (but not associated) mandible. A few years later, in 2009, I visited the type locality with Lee Hall and Ash Poust, and Ash discovered a partial mandible of a juvenile male. The skull of Proterozetes is quite interesting as it has a mix of features typical of Zalophus (including a large, elongate sagittal crest with a convex edge, narrow rostrum, narrow intertemporal constriction) and Eumetopias (rectangular supraorbital process, large gap between P4 and M1, domed interorbital region, large size). Proterozetes was likely about 3 meters long.
The referred mandible of the middle Pleistocene sea lion Proterozetes ulysses. Photo: R.W. Boessenecker.
A mandible of Proterozetes ulysses found by Ash Poust on a spring break road trip in 2009 in coastal Oregon. I found nothing of interest that day, but Ash found this on the riverbank.I was able to acid prepare the specimen later that year at Montana State University, and we published it in 2017. Photo: R.W. Boessenecker.
In most molecular phylogenetic analyses, Zalophus and Eumetopias form a clade, and in most analyses of morphological data, Proterozetes is nested right in there - usually as sister to Eumetopias, and with Zalophus as sister to that grouping. In this context, the mix of California sea lion like features and Steller's sea lion like features strongly suggests that 1) Proterozetes is either the common ancestor of these two modern sea lions, 2) Eumetopias evolved from a Zalophus-like ancestor, and 3) that this clade has its biogeographic origin in the eastern North Pacific.
An early Pleistocene mandible of Eumetopias jubatus from the Omma Formation of Japan. From Tsuzuku and Kohno (2020).
Eumetopias
The Steller's sea lion, Eumetopias, is one of the only sea lions with a decent fossil record - though the majority of fossils are from Japan. These include the fragmentary type specimen of Eumetopias ojiyaensis, which is mostly known from postcrania and a few teeth, from the earliest Pleistocene (1.1-2 Ma) Uonuma Group of Japan; later work suggests this species to be a nomen dubium, but the bones are rather large - too large to represent Zalophus, I think - and, whether or not it is truly diagnostic, I interpret as good evidence of Eumetopias. Other fossils that are reliably attributed to Eumetopias include various Pliocene specimens, and a relatively nice mandible of Eumetopias jubatus from the early to middle Pleistocene Omma Formation of Japan, initially described in 1953, and recently re-described. This specimen has been convincingly identified as the extant species, and dates to 1.3 Ma to 830,000 years in age. The cheek teeth of this specimen have bilobate or double rooted teeth, and is thus somewhat more 'primitive' (dentally speaking) than the extant species. Another species from the early Pleistocene of Japan, Eumetopias kishidai, is a partial rostrum which could in theory represent Zalophus (but is admittedly large), and may represent a female Eumetopias; it is certainly not a male Eumetopias.
A large mandible of a subadult male Eumetopias jubatus, originally published in 1953, was recently redescribed. The specimen dates to about 1.36 Ma to 830,000 years, or early Pleistocene, and has a couple of teeth with bilobate roots and a lower molar with partial double roots - a much more 'primitive' state of root fusion than seen in the extant species, and paralleling the trend I've documented in Callorhinus (see 'Anagenesis', above). The specimen was initially cautiously identified as Eumetopias sp. and more recently reidentified as Eumetopias jubatus.
Additionally, there is a latest Pleistocene skull of a juvenile Eumetopias from British Columbia with a puncture, likely from another carnivore, or perhaps a spear point or harpoon (see above). The specimen is radiocarbon dated to 12,570 years.
Lastly, future study is necessary to determine whether Proterozetes might better be considered a plesiomorphic species of Eumetopias.
A comparison of a cast of the skull of Neophoca palatina, the holotype skull itself, and a New Zealand fur seal (Arctocephalus forsteri). Photo: R.W. Boessenecker.
Yours truly coating the partially reassembled holotype skull of Neophoca palatina in ammonium chloride for photography in the U. Otago Geology Museum laboratory. Photo: Sophie White.
The partially reassembled holotype skull of Neophoca palatina (top; from Churchill and Boessenecker, 2016) compared with when it was originally photographed in King (1983).
Neophoca
There are plenty of Holocene age subfossils of Neophoca in Australia, but there is only a single pre-Holocene fossil that has ever been discovered - a partial skull from the middle Pleistocene of New Zealand. This fossil was named in 1981 as Neophoca palatina, indicating a broader geographic range for the genus in the past.
The specimen was originally named by British-Australian pinnipedologist Judith King in 1983; the specimen was collected quite early, in 1937, from a cliff at Ohope Beach on the North Island of New Zealand, and can now be summarized as 780,000-524,000 years in age (middle Pleistocene). A fossil sea lion from New Zealand would naturally be predicted to be an example of Phocarctos, the New Zealand sea lion - however, the specimen was originally named in the genus Neophoca, which today includes only the Australian sea lion. This isn't too surprising, to be honest - after all, New Zealand and Australia are quite close, and share a couple species of pinnipeds and penguins. The "forehead" of the skull is wide, like in Neophoca, and unlike the narrow region in Phocarctos; further, the back of the skull is narrow, like Neophoca.
The specimen was originally in a calcareous siltstone concretion, and was acid prepared. Unfortunately, the matrix from the nasal cavity and ventral part of the braincase was dissolved away. While the ventral part of the skull probably looked pretty good - at some point, the palate broke away from the rest of the skull, and much of the delicate bones in this area became completely pulverized. We took the specimen on loan from the Auckland War Memorial Museum in 2013 ahead of Morgan Churchill's EAPSI grant funded trip to New Zealand and Australia, and began reassembling the specimen. We managed to piece back together about 3/4 of the hundred or so fragments, enough to reassemble the specimen into two major elements: the palate and sides of the rostrum, and the heavier braincase fragment. We re-described the skull and published our reevaluation of the species in 2016.
Otariids often don't have many phylogenetic characters to be used, and so to test the taxonomic placement of this species, Morgan and I included the specimen in an analysis called a Discriminant Function Analysis. We (mostly Morgan) measured a bunch of Australasian otariid skulls of known identity, and then took the same measurements off of the skull of Neophoca palatina. The DFA effectively permits a prediction to be made about which species the fossil specimen is most close in anatomy to. Our analysis confirmed assignment to Neophoca, and in a similar analysis, the fossil plotted within the same 'morphospace' as Neophoca cinerea.
Late Pleistocene fossils of South American sea lion (Otaria byronia) from Caldera, Chile. From Valenzuela-Toro et al. (2013).
Otaria
Scattered remains of Otaria have been reported from Chile and Brazil. These include skull fragments found along the southernmost coastline of Brazil, which had likely eroded out from a barrier island deposit dating to the late Pleistocene - though imprecisely dated. Additional remains were reported from uppermost Pleistocene terrace deposits near Caldera in Chile, called the "Estratos de Caldera", dating to about 100,000 years. These include two mandibles, one of which is from an adult male and is quite large and robust, with a deep symphysis and large digastric flange - indicating that Otaria had already evolved some of its secondary sex characteristics. This is perhaps not too surprising, because the specimen is quite young at only 100,000 years, and these features are also found in Phocarctos, and therefore it is possible that these features could have been more widely distributed in the past (though unlikely; I suspect, given that Phocarctos and Otaria are not sister taxa, that these features are likely to be convergent).
Phocarctos
There are no known fossils of Phocarctos and its evolutionary history remains uncertainly known.
The largest otariid ever to live - or, at least the one with the largest canines - is not alive right now. This canine is even larger than extant Eumetopias jubatus. From Kohno and Tomida (1993).
Unidentified Japanese Giant Sea Lion
Tantalizing fragments of otariids abound from places like Japan and the west coast of the Americas. One such example is this gigantic canine from the early Pleistocene Setana Formation of Japan. It looks very similar to the extant Steller's sea lion, Eumetopias jubatus, but is about 25% larger. This strongly suggests that a truly gigantic, as yet unidentified sea lion awaits discovery in the Pliocene-Pleistocene rocks of Japan.
If you've made it this far, congratulations! This family has long been an obsession of mine. You now understand my pain, and also why I look so damn happy in this picture with the famous kissing sea lion (Zalophus californianus) at the Oregon Coast Aquarium. Photo by S.J. Boessenecker (2012).
Unanswered Paleontological Questions and Unsolved Problems
Question 1) Is Thalassoleon mexicanus more derived than we have previously assumed? It's possible, in my opinion, that Thalassoleon mexicanus might not be an ancestor of Callorhinus ursinus, but it is large enough and robust enough to perhaps represent an ancestral "stock" from which some of the earlier diverging southern hemisphere sea lions could have originated from.
Question 2) We know virtually nothing about the origin of southern hemisphere sea lions. Yes, we have the skull of Neophoca palatina, but it's quite young, and very, very similar to Neophoca cinerea. How and when did Otaria, Neophoca, and Phocarctos diverge from one another, and from Arctocephalus? What does Hydrarctos lomasiensis have to do with these lineages?
Question 3) When did northern hemisphere sea lions evolve - and where? There are tantalizing fragments from the Plio-Pleistocene of Japan suggesting that Zalophus and Eumetopias - and likely Proterozetes - originated in the western North Pacific, perhaps excluded from the eastern North Pacific by dusignathine walruses. However, few of these specimens have been formally written up, and our early and middle Pleistocene record of otariids is still quite poor - I think it's possible, if not likely, that these sea lions have an earlier fossil record in Japan, but the California/Oregon record is still so sketchy that I wouldn't be surprised if a different pattern might emerge.
Question 4) When did otariids cross the equator? Several studies (including one I have coauthored) have proposed a latest Miocene crossing of the equator in the east Pacific - yet otariids do not appear in the Peruvian fossil record until the late Pliocene (Hydrarctos). If otariids dispersed to the Pacific coast of South America during the late Miocene, why haven't we found their fossils in the uppermost parts of the Pisco Formation? Or, did they have a somewhat higher temperature tolerance than we currently think, and disperse during the mid Pliocene warm period?
Question 5) How ancient is the northern fur seal (Callorhinus) lineage? Do Pithanotaria, Thalassoleon macnallyae, or Thalassoleon inouei have anything to do with the ancestry of Callorhinus? The bullae of the latter suggest yes. Unfortunately there just aren't many characters we can use as leverage, and few specimens preserve the rostrum, and so it's unclear if any of these have the truncated rostrum of Callorhinus.
Problem 1) Sea lions and fur seals are very generalized. In other words, otariids all kind of look quite similar and are variations on a theme as opposed to having highly disparate skull and tooth shapes like you see in true seals (Phocidae) or extinct walruses (Odobenidae) for that matter. The list of phylogenetic characters for the otariidae is quite short, and often just a couple of character coding differences might result in very different looking phylogenetic trees - the trees we've developed are that unstable. This might reflect a relatively recent (e.g. entirely Pliocene) diversification of all modern lineages - so, it might reflect shallow phylogenetic history as opposed to these species just being a pain in the ass for no reason.
Problem 2) Otariids are highly sexually dimorphic. Male specimens tend to have secondary sex characteristics, and females do not - and so the skulls of females of different species tend to look much more alike than they do with the males of the same species. This makes interpreting isolated fossil skulls of females a very difficult endeavor. In practice, this means that an inexperienced paleontologist might misinterpret a female sea lion as a male fur seal, owing to some of the differences in allometric growth and more extreme secondary sex characteristics present in sea lions. Or, someone naming a new species based on a complete female skull might not be able to find many diagnostic traits that set that species aside - or, such a specimen might result in collapsing branches in a cladistic analysis. Barnes et al. (2006) even went so far as to propose that only adult male specimens ought to be designated as holotypes within the otariidae - a sad state of affairs, but from a pragmatic perspective, I'm inclined to agree. Even with adult male specimens at hand, there are perilously few anatomical characters available (see Problem 1).
Problem 3) Most of the fossil record of the otariids is from one coastline. There are some decent specimens from the Pliocene and Pleistocene of Japan, but, I would estimate that, by volume/number, perhaps 90% of the published fossil record of otariids is from California, Baja California, and to a lesser extent, Oregon. And there are perhaps hundreds of unpublished specimens from the Pacific coast including at least a half dozen skulls, mandibles, and even partial skeletons. Don't get me wrong - I love this coastline - but few of these unpublished fossils seem to represent new species. Point being - we've mostly sampled the North Pacific, and generally the areas with the largest population centers (California, Japan) and areas where marine mammal fossils are historically common (Peru, Chile). There are probably fossil sites awaiting discovery in New Zealand, Australia, Indonesia, perhaps Taiwan, and almost certainly the Aleutians and Alaska that might yield additional Neogene fossil marine mammals.
Lastly, a word of caution! Don't approach or pet sleeping fur seals and sea lions - they are wild animals, and not only have sharp teeth, but can move surprisingly quickly - faster than you can on beach sand - and also harbor many diseases. Give them plenty of room - fifty feet is required in the USA with the Marine Mammal Protection Act. Also, don't touch their carcasses; they are often covered in necrotizing bacteria that can give you a rather nasty infection called "sealer's thumb" or "seal finger". For your enjoyment, here is a set of photos documenting a young teenager in New Zealand approaching a fur seal (Arctocephalus forsteri) and changing his mind very, very quickly (still waiting on my Pulitzer for these photos). I get it! Aside from the largest males, most otariids are very very cute! But they are a nasty combination of fierce, sharp, infected, and testosterone. Photo: R.W. Boessenecker.
References/Further Reading
Avery, G. and Klein, R.G. 2011. Review of fossil phocid and otariid seals from the southern and western coasts of South Africa. Transactions of the Royal Society of South Africa 66: 14-24.
Barnes, L.G., Ray, C.E., and Koretsky, I.A. 2006. A new Pliocene sea lion, Proterozetes ulysses (Mammalia: Otariidae) from Oregon, U.S.A. In: Z. Csiki (ed.), Mesozoic and Cenozoic Vertebrates and Paleoenvironments: Tributes to the Career of Prof. Dan Grigorescu, 57-77. Bucharest, Romania.
Berta, A. and Churchill, M. 2012. Pinniped taxonomy: review of currently recognized species and subspecies, and evidence used for their description. Mammal Review 42: 207-234.
Berta, A. and Deméré, T.A. 1986. Callorhinus gilmorei n. sp., (Carnivora: Otariidae) from the San Diego Formation (Blancan) and its implications for otariid phylogeny. Transactions of the San Diego Society of Natural History 21 (7): 111-126.
Berta, A. and Wyss, A.R. 1994. Pinniped phylogeny. Proceedings of the San Diego Society of Natural History 29: 33-56.
Boessenecker, R.W. 2011. New records of the fur seal Callorhinus (Carnivora: Otariidae) from the Plio-Pleistocene Rio Dell Formation of Northern California and comments on otariid dental evolution. Journal of Vertebrate Paleontology 31 (2): 454-467.
Boessenecker, R.W. 2013a. A new marine vertebrate assemblage from the Late Neogene Purisima Formation in Central California, Part II: Pinnipeds and cetaceans. Geodiversitas 35: 815-940.
Boessenecker, R.W. 2013b. Taphonomic implications of barnacle encrusted sea lion bones from the Middle Pleistocene Port Orford Formation, coastal Oregon. Journal of Paleontology 87: 657-663.
Boessenecker, R.W. and Churchill, M. 2015. The oldest known fur seal. Biology Letters 11: 21040835.
Boessenecker, R.W. and Perry, F.A. 2011. Mammalian bite marks on juvenile fur seal bones from the late Neogene Purisima Formation of Central California. Palaios 26 (2): 115-120.
Boessenecker, R.W., Perry, F.A., and Schmitt, J.G. 2014. Comparative taphonomy, taphofacies, and bonebeds of the Mio-Pliocene Purisima Formation, Central California: strong physical control on marine vertebrate preservation in shallow marine settings. PLoS One 9: e91419.
Brunner, S. 2004. Fur seals and sea lions (Otariidae): identification of species and taxonomic review. Systematics and Biodiversity 1 (3): 339-439.
Churchill, M.C. and Boessenecker, R.W. 2016. Taxonomy and biogeography of the Pleistocene New Zealand sea lion Neophoca palatina (Carnivora: Otariidae). Journal of Paleontology 90: 375-388.
Churchill, M., Boessenecker, R.W., and Clementz, M.T. 2014. Colonization of the Southern Hemisphere by fur seals and sea lions (Carnivora: Otariidae), revealed by combined evidence phylogenetic and Bayesian biogeographic analysis. Zoological Journal of the Linnaean Society 172: 200-225.
Churchill, M., Clementz, M.T., and Kohno, N. 2014a. Cope's rule and the evolution of body size in Pinnipedimorpha (Mammalia: Carnivora). Evolution 69: 201-215.
Churchill, M.C., Clementz, M.T., and Kohno, N. 2014b. Predictive equations for the estimation of body size in seals and sea lions (Carnivora: Pinnipedia). Journal of Anatomy 225: 232-245.
Deméré, T.A. and Berta, A. 2005. New skeletal material of Thalassoleon (Otariidae:Pinnipedia) from the Late Miocene–Early Pliocene (Hemphillian) of California. Bulletin of the Florida Museum of Natural History 45: 379-411.
Drehmer, C.J. and Ribeiro, A.M. 1998. A temporal bone of an Otariidae (Mammalia: Pinnipedia) from the late Pleistocene of Rio Grande do Sul State, Brazil. Geociencias 3: 39-44.
Harington, C.R., Ross, R.L., Mathewes, R.W., Stewart, K.M., and Beattie, O. 2004. A late Pleistocene Steller sea lion (Eumetopias jubatus) from Courtenay, British Columbia: its death, associated biota, and paleoenvironment. Canadian Journal of Earth Sciences 41:1285-1297.
Helm, C.W., Carr, A.S., Cawthra, H.C., De Vynck, J.C., Dixon, M., Stear, W., Stuart, C., Stuart, M., and Venter, J.A. 2022. Possible Pleistocene pinniped ichnofossils on South Africa's Cape South coast. Journal of Coastal Research 38: 735-749.
Kaseno, Y. 1951. Pliocene pinniped remains from Kanazawa, Ishakawa Prefecture, Japan. Transactions and Proceedings of the Palaeontological Society of Japan (57-64).
Kim, K.C., Repenning, C.A., and Morejohn, G.V. 1975. Specific antiquity of the sucking lice and evolution of otariid seals. Rapports Procès Verbaux des Réunions 169: 544-549.
Kohno, N. 1992. A new Pliocene fur seal (Carnivora: Otariidae) from the Senhata Formation on the Boso Peninsula, Japan. Pp. 15-28. Natural History Research.
Kohno, N., Koike, H., and Narita, K. 2007. Outline of fossil marine mammals from the middle Miocene Bessho and Aoki Formations, Nagano Prefecture, Japan. Research Reports of the Shinshushinmachi Fossil Museum 10: 1-45.
Kohno, N. and Yanagisawa, Y. 1997. The first record of the Pliocene Gilmore fur seal in the Western North Pacific Ocean. Bulletin of the National Science Museum, Tokyo Series C (Geology & Paleontology) 23: 119-130.
Leonardi, M.S., Herrera, S.V., Sweet, A., Negrete, J., and Johnson, K.P. 2019. Phylogenetic analysis of seal lice reveals codivergence with their hosts. Systematic Entomology 44:699-708.
Miyazaki, S., Horikawa, H., Kohno, N., Hirota, K., Kimura, M., Hasegawa, Y., Tomida, Y., Barnes, L.G., and Ray, C.E. 1995. Summary of the fossil record of pinnipeds of Japan, and comparisons with that from the eastern North Pacific. The Island Arc (3): 361-372.
Morejohn, G.V. 1975. A phylogeny of otariid seals based on morpholgoy of the baculum. Rapports Procès Verbaux des Réunions 169: 49-56.
Muizon, C.d. 1978. Arctocephalus (Hydrarctos) lomasiensis, subgen. nov. et nov sp., un nouvel Otariidae du Mio-Pliocene de Sacaco. Bulletin de l'Institute Francais d'Etudes Andines 7: 169-189.
Ochoa, D., Salas-Gismondi, R., DeVries, T.J., Baby, P., Muizon, C.d., Altamirano, A., Barbosa-Espitia, A., Foster, D.A., Quispe, K., Cardich, J., Gutierrez, D., Perez, A., Valqui, J., Urbina, M., and Carré, M. 2021. Late Neogene evolution of the Peruvian margin and its ecosystems: a synthesis from the Sacaco record. International Journal of Earth Sciences 110: 995-1025.
Ochoa, D., DeVries, T.J., Quispe, K., Barbosa-Espitia, A., Salas-Gismondi, R., Foster, D.A., Gonzales, R., Revillon, S., Berrospi, R., Pairazamán, L., Cardich, J., Perez, A., Romero, P., Urbina, M., and Carré, M. 2022. Age and provenance of the Mio-Pleistocene sediments from the Sacaco area, Peruvian continental margin. Journal of South American Earth Sciences 116: 103799.
Paterson, R.S., Rybczynski, N., Kohno, N., and Maddin, H.C. 2020. A total evidence phylogenetic analysis of pinniped phylogeny and the possibility of parallel evolution within a monophyletic framework. Frontiers in Ecology and Evolution 7: 2019.00457.
Poust, A.W. and Boessenecker, R.W. 2017. Mandibles of the sea lion Proterozetes ulysses from the middle Pleistocene Port Orford Formation of Oregon. Journal of Vertebrate Paleontology 37: e1317637.
Rule, J.P., Richards, H.L., Pollock, T.I., Hocking, D.P., and Evans, A.R. 2024. Traditional and digital examination of the baculum of a leopard seal (Hydrurga leptonyx). Marine Mammal Science 40: 292-301.
Tzuzuku, N. and Kohno, N. 2020. The oldest record of the Steller sea lion Eumetopias jubatus (Schreber, 1776) from the early Pleistocene of the North Pacific. PeerJ 8: e9709.
Valenzuela-Toro, A.M., Gutstein, C.S., Varas-Malca, R.M., Suarez, M.E., and Pyenson, N.D. 2013. Pinniped turnover in the South Pacific Ocean: new evidence from the Plio-Pleistocene of the Atacama Desert, Chile. Journal of Vertebrate Paleontology 33: 216-223.
Valenzuela-Toro, A.M., Mehta, R.S., Pyenson, N.D., Costa, D.P., and Koch, P.L. 2023. Feeding morphology and body size shape resource partitioning in an eared seal community. Biology Letters 19: 20220534.
Valenzuela-Toro, A.M., Pyenson, N.D., Costa, D.P., Clementz, M.T., and Koch, P.L. 2024. Stable isotope evidence for resource parititioning in extinct marine carnivores. Palaeogeography, Palaeoclimatology, Palaeoecology 649: 112302.
Velez-Juarbe, J. 2017. Eotaria citrica, sp. nov., a new stem otariid from the "Topanga" formation of Southern California. PeerJ 5: 3022.
Yonezawa, T., Kohno, N., and Hasegawa, M. 2009. The monophyletic origin of sea lions and fur seals (Carnivora; Otariidae) in the Southern Hemisphere. Gene 441 (1-2): 89-99.
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