Ankylorhiza tiedemani, a giant dolphin from the Oligocene of South Carolina, part 1: bringing clarity to 170 years of confusion

Part 1 of a 2 part series on our new study describing the most complete skeleton of an early echolocating whale, Ankylorhiza tiedemani. This first post covers the background and introduction to the taxonomy of this ancient dolphin - the next one will dive deeper into feeding ecology, locomotion, and the evolution of early dolphins. Check out Part 2 here.

My first visit to College of Charleston in 2012 - checking out some Oligocene toothed and toothless mysticetes for my Ph.D. research on eomysticetid whales. I had no idea that, just a few years later, I would end up working on this tremendous collection at CCNHM!

A couple of weeks ago we published a new paper in the journal Current Biology on a very large dolphin from the Oligocene of South Carolina – Ankylorhiza tiedemani. I’ve been working on this study with my coauthors Morgan Churchill, Emily Buchholtz, Brian Beatty, and Jonathan Geisler on and off (but mostly on) since summer 2017. It’s based on a fossil collected in the 1990s by CofC alum Mark Havenstein, but serious research on this taxon – affectionately known as “Genus Y” for decades – actually began in the early 1970s with the discovery of specimen ChM PV 2764, a well-preserved skeleton from the Chandler Bridge Formation of South Carolina, discovered and excavated by the late Al Sanders. Al was the Natural History curator at Charleston Museum until 2012; I first met Al a few months later in October 2012 on my very first visit to Charleston.

Al Sanders (right) and Ewan Fordyce (left) at CCNHM in 2012 discussing CCNHM 108 on the table, which five years later would become the holotype specimen of Coronodon havensteini.

Al had no advanced degrees, but was an unparalleled expert in paleontology and natural history of the southeastern USA and his knowledge, discoveries, and body of published research commanded respect in spite of that. Al was a titan in marine mammal paleontology. He was also well-situated to inherit a flood of fossils, as Charleston had significant population growth after World War II, and with the baby boom in full swing, suburbs and subdivisions were being built on the outskirts of Charleston at a rapid rate. Now, here’s the interesting bit: there were suburbs built much earlier, from 1900 to 1940 or so, that are within about five miles of downtown: West Ashley (where I used to live), North Charleston, and Mount Pleasant – but all of these are inboard of I-526, which is a ‘ring’ highway that makes a halo about 5 miles radius from downtown Charleston. As it happens, within this region the Oligocene rocks are quite buried: 5-30 meters (or more) deep, with a healthy cover of Pleistocene overburden (chiefly the Wando Formation). In the areas where phosphate mining was most extreme – West Ashley outside 526 and the area of the North Charleston airport (Charleston International/Joint Base Charleston) – the rocks are quite shallowly buried here, only 2-3 meters down – 4 meters was about the maximum depth that the strip mines during the heyday of phosphate mining would go, past that it was too much work (recall they were stripping off overburden by hand, horse, ox, and plow). By the late 1960s, subdivisions were being constructed north of the airport in North Charleston, near another zone called the Fort Bull Bulge, where the Oligocene deposits are only 2-5 meters below the surface. In one of these subdivisions in Summerville, South Carolina, Al Sanders and company found a spectacular skeleton including a nearly complete skull and vertebral column of an obscenely large dolphin.

The spectacular cast skeleton of ChM PV 2764 - "Genus Y" - on display in the old Natural History hall at Charleston Museum, which has since been revamped thanks to the efforts of our colleague Matt Gibson, NH curator at Charleston Museum. This specimen is now best identified as Ankylorhiza sp. or Ankylorhiza sp. 2. 

This dolphin was dubbed the informal name “Genus Y” in a landmark paper by Frank Whitmore and Al Sanders in 1977 – which was able to review all published Oligocene cetaceans (worldwide) in about 16 pages. They also made reference to a “Genus X” and in notes and labels spread throughout Charleston Museum collections, there is also a Genus Z, Genus A, B, C, and so on. This was necessary as there was so little published at the time on these sorts of cetaceans, Al’s early research was truly groundbreaking. At the same time, Al faced an “embarrassment of riches” issue: there was almost too much to study. In about 15 years, Al and others collected about 10-20 lane cabinets worth of fossils, and many unopened jackets and unprepared fossils await attention or are currently being worked on by our good friend Matt Gibson, the current Natural History curator of Charleston Museum. Sadly, Al only published a few of his cetacean discoveries – these did, however, crucially included the publication of Eomysticetus whitmorei, which was the cornerstone paper that made my own Ph.D. research possible. Others included the naming of the ur-dolphin Ashleycetus planicapitis (one of the most plesiomorphic dolphins ever discovered), a redescription of Xenorophus sloanii (in the same paper, no doubt a lifelong goal of Al's that would unlock the taxonomy of the more diverse collection of South Carolina xenorophids), and the naming of Micromysticetus rothauseni (proposed to be a "cetotheriopsid", later strongly recovered within Eomysticetidae by my Ph.D. research). When visiting Charleston Museum in 2012, I got to see specimens of the “Charleston toothed mysticetes”, which Al and Larry Barnes were supposed to have published. An independently discovered skull was later published as Coronodon havensteini in 2017 by Jonathan Geisler, Brian Beatty, Mace Brown, and myself, and Al was fortunately alive to see a name applied to one of these toothed mysticetes – though Ankylorhiza is the “one that got away”, so to speak. On the other hand, it’s critical to remember that Al published on a wide variety of fossils and geology and had a long, remarkable, and successful career – this included naming the Chandler Bridge Formation, revising the entire record of Plio-Pleistocene mammals from the state, and the entire record of dinosaurs and other Mesozoic reptiles from South Carolina. While this study did not describe Al’s Genus Y skeleton (ChM PV 2764), it would not have been possible were it not for that discovery, so thanks to Al.

The presumed holotype tooth of Saurocetus gibbesi, from Allen (1924), along with a lower postcanine of CCNHM 103, Ankylorhiza tiedemani.

The story *really* begins in 1848 with the discovery of an isolated tooth from the Ashley Phosphate Beds, which THE Louis Agassiz named Saurocetus gibbesii – a big triangular molar with some serrations, longitudinally fluted enamel, and possibly double-roots. The specimen was never figured, and the specimen number was not cited by Agassiz, and then lost for about 50 years (because of course that happened to an unfigured specimen of unknown catalog number…). Some three decades later, a partial rostrum (snout) of a skull of a very large dolphin – though not containing any teeth resembling Saurocetus – or, any well-preserved teeth for that matter – was dredged from the Wando River on a phosphate barge, and presented to the American Museum of Natural History by a Mr. I.B. Tiedeman, and published in 1887 by J.A. Allen as Squalodon tiedemani. Of course it was placed in Squalodon; it had a similar rostrum with some European specimens of Squalodon. Generally, any large odontocetes with heterodont teeth were placed into Squalodon, and Remington Kellogg attempted to sort through this quagmire in 1924 when he named Squalodon calvertensis – an actual species of Squalodon. On Squalodon tiedemani, Kellogg wrote that it was much larger than any known “squalodont”, and was perhaps the first to cast doubt on squalodontid affinities, noting “A careful comparison of the types of Squalodon atlanticus and Squalodon tiedemani has failed to convince the writer that these two cetaceans are closely related to one another.” Kellogg did mention that the incompleteness of the specimen meant that “considerable uncertainty exists as to whether or not Squalodon tiedemani should be placed nearest the squalodonts or the zeuglodonts [=basilosaurids]”.

The holotype rostrum of "Squalodon"(now Ankylorhiza) tiedemani - from Allen (1887).

Sadly, the age of the “S.” tiedemani holotype was completely unknown, and for a long time, widely assumed to be Miocene. By the 1970s, fossils of superbly large squalodontids from the Chesapeake Group (Pungo River Formation and Calvert Formation) were being identified at the Smithsonian as S. tiedemani, first and foremost a phosphatized skull with clam borings from the mid Miocene Pungo at the Lee Creek Mine (Boreske et al., 1972), identified by Frank Whitmore. Sometime later, a giant Squalodon skeleton from the mid Miocene Calvert Formation of Virginia was identified in Alton Dooley’s Ph.D. thesis as S. tiedemani – but he later changed his mind, concluding later that “S.” tiedemani has something to do with the “Genus Y” skeleton of Al Sanders. Dooley named this new skeleton from Virginia as Squalodon whitmorei in 2003. A following study by Frank Whitmore accordingly reversed course (Whitmore and Kaltenbach, 2008) and assigned the reworked Boreske et al. skull to S. whitmorei. However, they also referred a couple of large odontocete specimens including a titanic dentary from the rivers of Charleston to S. whitmorei. Not to skip ahead, but in our supplementary info, we concluded it was probably actually Ankylorhiza, reidentifying the Charleston mandible as Ankylorhiza sp.

The absolutely monstrous holotype skull of Squalodon whitmorei, which is a bit more derived and a bit larger (S. whitmorei likely had a skull at least 15-20 cm longer than CCNHM 103, and is roughly the same size as the larger, undescribed species of Ankylorhiza represented by ChM PV 2764).

My first look at CCNHM 103, in October 2012 after the Society of Vertebrate Paleontology meeting in Raleigh, NC. 

In the late 1990s Mark Havenstein and colleagues discovered an enormous dolphin skeleton during construction of the Crowfield Plantation subdivision, sandwiched between Summerville, Ladson, and Goose Creek in South Carolina – about a 15 minute drive from my house. The skeleton needed to be removed quickly, so as not to interfere with construction activities; considerable piecing was needed in order to put it back together. The benefactor of our Mace Brown, acquired the specimen sometime later and spent years piecing the skull and skeleton back together – and in 2010, the skeleton was donated and became one of the first specimens to be catalogued into our collection as CCNHM 103* (our catalog begins at CCNHM 100). When I first visited CCNHM in October 2012, I met Mace and saw this specimen on display – on the same hanging platform it occupies right now. I thought “sweet jesus that’s a big dolphin” but was too preoccupied with examining eomysticetid remains and the fossil I would eventually help publish as Coronodon havensteini five years later (unbeknownst to me at the time!) to consider that specimen any further. That same week, I didn’t even examine Genus Y at Charleston Museum; I wouldn’t see it in person until 2015. Nor would I examine CCNHM 103 in detail until 2016.

*CCNHM is the original acronym, after “College of Charleston Natural History Museum”, and appears in the Geisler et al. (2014) publication on Cotylocara (CCNHM 101). Shortly thereafter, the museum was renamed (in early 2015 I believe) the Mace Brown Museum of Natural History, after Mace’s voluminous contributions to the collection and well-being of the museum.

Coauthor Jonathan Geisler examining the teeth of CCNHM 103 back in 2017. The braincase and rostrum of the larger, undescribed species, CCNHM 1075, sits off to the right hand side. The orange specimen is our referred skull from the Ashley Formation, CCNHM 220.

We went back and forth on the naming for a while, we were uncertain if there was one or two species of “Genus Y”, and CCNHM 103 had been nicknamed “Genus Y not” or Genus Y°” by Brian. I tend to be a lumper rather than a splitter (I did most of my paleontological education at Montana State University after all), so it took a bit of convincing and lots and lots hand wringing between the initial members of the team. Alternative hypotheses were: 1) Genus Y and Genus Y not were different species; 2) different sexes of the same species; and 3) different growth stages. Some specimens do have a significantly wider vertex than others – it’s quite narrow in CCNHM 103, somewhat narrow in ChM PV 2764, and surprisingly wide in the largest specimen, CCNHM 1075. Since the occipital shield on the back of the skull anchors in the neck muscles that stabilize the head, it’s perhaps not surprising that this might scale with body size (bigger head, after all). However, we finally settled on a few consistent differences: in CCNHM 103 and the holotype of “S.” tiedemani, the rostrum is turned up a bit and dorsoventrally thickened, with the first incisor positioned dorsal to the second; secondly, the cheek teeth of CCNHM 103 differ from Genus Y proper (ChM PV 2764 and CCNHM 1075) in lacking large accessory cusps – they are there, but instead take the form of tiny, 1-2 millimeter wide “beads” of enamel on the cutting edges. So we settled eventually on the two species idea, and sure enough, our phylogenetic analysis grouped the specimens precisely how we hypothesized – which was a satisfying vindication. At this stage, however, we were full steam ahead on naming a new genus and species.

CCNHM 220, our less spectacular specimen of Ankylorhiza tiedemani from the Ashley Formation - still a very important specimen as it documents this same species as being present in the early Oligocene.

At some point I couldn’t shake the idea that we very likely were not dealing with a completely new species, even though Al Sanders had concluded otherwise. I had extensive discussions on lumping v. sinking with my Ph.D. adviser Ewan Fordyce – and Ewan is very much in favor of stabilizing old names with newer, better preserved and more anatomically informative fossils that improve the diagnosis. After all, he had done this for many taxa, and declaring others nomina dubia or cetaceans of uncertain affinities (incertae sedis). This was necessary given his arrival on scene in the late 1970s, where the New Zealand cetacean fossil record had been almost criminally abused by various workers, and much of his dissertation through U. Canterbury consisted of mopping up this enormous mess. Ewan on occasion cited the extreme example of Zarhachis flagellator – originally based on a caudal vertebra (doubtfully diagnostic), later stabilized with the referral of a skeleton by Remington Kellogg. The name is now indelibly associated with the nicely preserved skeleton that Kellogg published, and so if someone really wanted to declare it a nomen dubium and commit some opportunistic taxonomic piracy and name a new genus and species off the skeleton (as is frequently done in the frankly overcrowded realm of archosaur paleontology) – there’s actually enough of a case to ‘preserve’ Zarhachis under ICZN rules. After all, this sort of thing played out recently with Zygorhiza kochii – the holotype of which is a piece of junk braincase without any redeeming diagnostic features other than size, and meanwhile everyone mentally pictures Kellogg’s beautiful skeleton (USNM 11962) when they think of Zygorhiza. The taxonomic viewpoints of Gingerich v. Uhen on Eocene basilosaurids would make for a great post… later. While I would not go so far as to did what Kellogg did with Zarhachis (and I imagine even the most conservative paleocetologists today would not either), I was determined to find if we truly had something new or something old. In the end, it was a bit of both.

A slide from an updated SVP presentation showing the features linking CCNHM 103 with the holotype of Ankylorhiza tiedemani, AMNH FM475. Also, these are both shown to scale: CCNHM 103 is a bit smaller and slightly less robust - this is a BIG dolphin!

After trawling through all the papers on early large odontocetes I could, I re-read Allen (1887) and remarked upon the similarity between CCNHM 103 and the “S.” tiedemani holotype: the “double decker” incisors, the expanded premaxillae, the upturned rostrum, and very large size. Jonathan Geisler visited the AMNH and identified another similarity: clusters of foramina within the embrasure pits that the lower teeth fit into when the mouth was closed. He also determined that there is some adhering matrix – indicating that the holotype of “S.” tiedemani originated from the Ashley Formation, which is early Oligocene – 28-30 myo, a little older than our skeleton CCNHM 103 from the 23-24 myo Chandler Bridge Formation. So, that settled it for me: we agreed that CCNHM 103 was referable to “S.” tiedemani, necessitating a new genus name, because there’s no way this thing was related to Squalodon – tacitly acknowledged 96 years ago by Kellogg. So we assigned it to the new genus Ankylorhiza, and referred CCNHM 103 to that species. The other species of Ankylorhiza, represented by Al Sanders’ Genus Y skeleton ChM PV 2764 and our specimen CCNHM 1075, is not yet named.

Further Reading

Allen, 1887. http://digitallibrary.amnh.org/handle/2246/1611

Allen, 1924. https://academic.oup.com/jmammal/article-abstract/5/2/120/837661?redirectedFrom=fulltext

Boessenecker et al., 2020. https://www.cell.com/current-biology/fulltext/S0960-9822(20)30828-9?_returnURL=https%3A%2F%2Flinkinghub.elsevier.com%2Fretrieve%2Fpii%2FS0960982220308289%3Fshowall%3Dtrue

Boreske et al., 1972. https://www.jstor.org/stable/1302923

Dooley, A. C. 2004. A new species of Squalodon (Mammalia, Cetacea) from the middle Miocene of Virginia. Virginia Museum of Natural History Special Publication 8:1-43.

Whitmore, F.C., and J.A. Kaltenbach. 2008. Neogene Cetacea of the Lee Creek Phosphate Mine, North Carolina. Virginia Museum of Natural History Special Publication 14: 181–269.

Whitmore and Sanders, 1977. https://academic.oup.com/sysbio/article-abstract/25/4/304/1653287?redirectedFrom=fulltext

Whale, Dolphin, or Porpoise? A meaningful question about meaningless terms

Disclaimer 1: I’ve made no attempt to dive into any literature regarding the definition of any of these terms; this is instead a “gestalt” approach calling on my admittedly shallow experience in whale paleontology and successful and (especially) unsuccessful attempts to communicate my science to the public. To be honest, I don’t know if some essay exists about the common names of marine mammals with recommendations, and I don’t care: if it exists, it’s hardly done its job, has it? Nevertheless, I’d like to hear about it.

Disclaimer 2: The common names I use are all from the List of Marine Mammal Species and Subspecies published by the Society for Marine Mammalogy; these common names are generally not debated, though SMM writes: “common names are arbitrary and change with time and place.” Nevertheless, common names for individual species seem to be quite stable and meaningful to some degree; it is the bigger labels – and common names of higher taxa – being dissected here. You can view the list here. 

Cetaceans that stretch the definitions of names (top) and those that don't (bottom). Top left: the Risso's dolphin, despite being quite large and lacking a beak; Top right, a Sowerby's beaked whale, a whale despite having a beak. Bottom left: a spinner dolphin; bottom right: humpback whales. Photo credit: Endlessocean.fandom.com; Pierre Jaquet via Flickr; NationalGeographic.com; Tony Wu, NPL.

Whale, Dolphin, Porpoise: what do these words mean? 

I’d rather be talking about our new study on the large extinct killer dolphin Ankylorhiza tiedemani, but after speaking with several journalists about it I’m instead motivated to have a brief rant discussion of the vernacular taxonomy of cetaceans. I get asked (or see) these sorts of questions/statements frequently:

            “So is it a whale or is it a dolphin?”

            “That’s a whale tooth, not a dolphin tooth”

            “The killer whale is actually a dolphin”.

These words are used with such imagined precision and it breaks my little heart every time because, well, they don’t mean a whole lot. So many arguments are had over the identification of cetacean fossils, for example, when in fact different completely defensible alternative definitions exist and so many are comparing apples and oranges. And, I’ll note right now: I’ve never heard an academic in mammalogy or paleontology ever get hung up on what the definition of these mean, because we all A) tacitly acknowledge that they’re not very well-defined and a bit meaningless in practice and B) use scientific terms that have extremely precise meanings. So, a quick break down of the terms.

Whale – from the Old English hwael, Old Norse hvalr, Dutch & German wal, etc.

Definition according to Merriam Webster Dictionary: “any of various very large, aquatic, marine mammals (order Cetacea) that have a torpedo-shaped body with a thick layer of blubber, paddle-shaped forelimbs but no hind limbs, a horizontally flattened tail, and nostrils that open externally at the top of the head”

Dolphin – from the Greek delphin, Latin delphinus, old French dauphin.

Definition according to Merriam Webster Dictionary: “A) any of various small marine toothed whales (family Delphinidae) with the snout more or less elongated into a beak* and the neck vertebrae partially fused. B) any of several related chiefly freshwater toothed whales (as of the families Platanistidae and Iniidae)”

*beak, of course, is the cetological term for the snout in a cetacean that protrudes beyond the melon – think of the bottlenose in a bottlenose dolphin.

Porpoise – from the old French porpais, translating to “pig fish” – originally from Latin porcus + piscus, porc + peis in French.

Definition according to Merriam Webster Dictionary: “a blunt-snouted usually dark gray whale (Phocoena phocoena) of the North Atlantic and North Pacific that typically ranges from 5 to 6 feet (1.5 to 1.8 meters) in length.”

The definition of “whale” above perhaps is the most broad, and characterizes dolphins and porpoises as well. The definition of “dolphin” at first seems to only include the Delphinidae, but then admits that there are some other dolphins – mostly riverine – that are similar-ish in external body form that can be called dolphins. The definition of porpoise offered by Merriam Webster highlights one species, the harbor porpoise. Dolphins v. porpoises seems rather clear here: dolphins have beaks, and porpoises do not. It is from these definitions that many accept a quasi-taxonomic use of the words: dolphin means “Delphinidae” and porpoise means “Phocoenidae” – the families containing the bottlenose dolphin and harbor porpoise (respectively).

What about other definitions? I’ve read elsewhere that a whale could be defined as any cetacean lacking a beak but being larger than 9 feet in length, and that porpoises are any cetacean lacking a beak smaller than that, and in many variants, possessing a triangular rather than hooked (falcate) dorsal fin. This at first seems fine: phocoenids mostly have triangular dorsal fins and none that live today possess a beak, clearly differentiating them from delphinid species like the bottlenose dolphin. However… what about the genus Cephalorhynchus? These are a fascinating group of southern hemisphere delphinids with a “ring species” biogeographic pattern of dispersal around the southern ocean – and none have a beak or a falcate dorsal fin. Externally, they are so similar that if a stranded individual were so decomposed that the teeth had fallen out and identification was not possible based on the coloration, you would probably need to remove the skull in order to tell which family it belonged to. On top of this, there has been rampant use in the past of the word porpoise for delphinids with beaks (spinner porpoise, bottlenose porpoise, etc.), as recently as the 1970s – in peer reviewed papers by marine mammalogists like William Perrin!*

*I have also read this may be specific to Hawaii, where local fisherman call all small cetaceans porpoises because they instead use the term “dolphin” exclusively for the dolphinfish, which really chaps my ass because there’s already a &#$ing local indigenous name for that species – Mahi-mahi, that is now used more frequently than dolphinfish outside of Hawaii.

Phylogenetic distribution of common names for cetaceans. Phylogeny from McGowen et al., 2011. Only the term porpoise, and only in its modern usage, appears to define an actual clade.

The distinction between “dolphin” and “porpoise” is sadly, in my view, the clearest, and matters only become worse when you take a closer look at “whale”. The first and biggest problem is that whale doesn’t even attempt to mean anything biological: it includes both baleen whales and large odontocetes, which, granted, are all cetaceans, but some whales (large odontocetes) are obviously much, much more closely related to whatever dolphins and porpoise are than others (e.g. mysticetes). There are two aspects to the difference between whales and dolphins I’ve seen in most definitions or uses of the words: 1) whales lack a beak and 2) whales are larger than dolphins. At first glance, this works: baleen whales are all larger than dolphins, and sperm whales, and killer whales are as well – and none of these have a beak. But, there are, as before, exceptions.

The first exception are the beaked whales: family Ziphiidae, all of which are larger than dolphins but… paradoxically have beaks. One is even called the bottlenose whale! Another, Berardius, is the second largest odontocete after the sperm whale Physeter, measuring in at 11 meters or more – larger than minke whales, but smaller than the largest bryde’s whales. Berardius, for the record, most certainly has a beak.*

*I was lucky enough to participate in a necropsy of a stranded female B. arnuxii on a frigid, foggy beach on the southern tip of the south island of NZ during my Ph.D.

The second exception are some of the smaller species of the “blackfish” – the Globicephalinae. These are a highly derived group of darkly pigmented blunt-snouted delphinids with some suction feeding adaptations. Despite being large bodied, darkly pigmented, and lacking a beak, the killer whale is not actually a member of the Globicephalinae, and is instead allied with a number of robust Tursiops-like fossils in the Orcininae, the earliest diverging lineage within extant Delphinidae. Three of these are the same size as most dolphins – 6-10 feet in length (2-3 meters) – these are the melon-headed whale Peponocephala electra, pygmy killer whale, Feresa attenuata, and the snubfin dolphins, Orcaella spp. (Irrawaddy river dolphin, Australian snubfin dolphin). Peponocephala (8-9 feet, 2.6-3 meters) and Feresa (6-7 feet, 2-2.3 meters) illustrate that not all “whales” are large-bodied. Orcaella, the snubfin dolphins, are small (6-9 feet, 2-3 meters), and called dolphins yet lack a beak! If your brain wasn’t already close enough to melting out through your ears, let us also consider the relatively large delphinid Grampus griseus – the Risso’s Dolphin – which is a large globicephaline, smaller than a pilot whale or the false killer whale, but much larger than the rest of the globicephalines. It is large, and lacks a beak – externally it looks the same as a pilot whale aside from coloration.

These terms are old with origins in the whaling industry, pre-dating modern biology.

Whale also is frequently used as an umbrella term to refer to cetaceans in general, thereby tacitly including dolphins and porpoises rather than always being distinguished from them. The term “toothed whales” is the broadly accepted vernacular name for the Odontoceti, and likewise, “baleen whales” for the Mysticeti. In older literature (prior to World War II) you will find frequent references to the amusingly named “whalebone whales”, and after cursory reading, you realize (just quickly enough to avert a brain aneurysm, in my case) that all whalebone whales are baleen whales. As it turns out, “whalebone” is an old whaling term for baleen, which would be cut into strips and sold on shore to be used for the boning in ladies’ corsets.

Another term you may hear is the “Great whales” – which is defined by the International Whaling commission as the most economically viable whales for the purposes of whaling – most baleen whales (except for Eden’s and Omura’s whales), and the sperm whale. This is obviously not a taxonomically informative moniker.

In summary, just looking at the distribution of modern cetacean common names indicates that “whale”, “dolphin”, and “porpoise” have no consistently used, biologically meaningful definitions. They are informal terms, with multiple accepted definitions.

Testing the water: an informal twitter poll

I started writing this post, and then realized as an afterthought, that I should post a poll on twitter asking folks to vote on what they think a particular cetacean was – the intention was to ask my twitter followers “should the common name of this species have whale, dolphin, or porpoise in it?” not “is this a cetacean?” whereby whale would have been correct every time. So, who knows how useful the results are – regardless, I used the exact imprecise language I’m railing against in the rest of this post, so confused results do speak for themselves. It’s also worth noting that at present I have perhaps 5,000 followers give or take, and while many are scientists, most are not, and even many scientists are not guaranteed to be familiar with obscure cetacean species I’ve intentionally chosen to prove a point. [Admittedly, I have jumped the gun an hour or so, as the polls are not yet finished, but I don't expect them to change much. If there is any change I will update them accordingly].

The first poll had an unlabeled photograph of a dolphin-like animal leaping from the water – without scale. Most respondents (~49%) called this a dolphin (with the remaining votes split (25% each) between porpoise and whale) – but it is not a dolphin. This is a female Sowerby’s beaked whale (Mesoplodon bidens) – difficult to tell, but the pectoral fins are pretty far from the head. Answer: WHALE. [photo credit: azoreswhalewatch.com]

The second poll has a similar looking animal leaping out of the water – and most (70%) responded correctly. This is a spinner dolphin (Stenella longirostris) known for its acrobatic spinning breaches. This was formerly frequently called the spinner porpoise, so you get a free pass if you wrote porpoise. Answer: DOLPHIN. (or porpoise?) [photo credit: Getty Images TV via youtube]

The third poll is just a cute little face poking out of the water – a face that is dark and lacks a beak. This one was clearly confusing, as only a slight majority – 41% - identified it as a porpoise, and about 30% said whale or dolphin. This is not a porpoise, however – it is a Heaviside’s dolphin (Cephalorhynchus heavisidii), one of the smallest dolphins, and a beakless one at that – in the porpoise-mimic genus Cephalorhynchus. Answer: DOLPHIN. [photo credit: S. Elwen, Namibian Dolphin Project]

The fourth poll shows a sadly dead, strikingly countershaded animal with an absurdly large dorsal fin, a white eye ring, and black lipstick. Most respondents (~70%) correctly said this was a porpoise. This is indeed the spectacled porpoise, Phocoena dioptrica. That’s actually my Ph.D. adviser R.E. Fordyce (U. Otago, Geology dept.) on the left*, and Steve Dawson (U. Otago, Marine Sciences dept.). Like the Heaviside’s dolphin above, it does not have much of a beak. Answer: PORPOISE. [photo credit: 1 News, NZ]

*This beautiful porpoise stranded on the Otago Peninsula while I was in the Fordyce lab - but it was September 2014 and I was in the throes of Ph.D. writing, and desperately attempting to complete my Ph.D. thesis before January 10, my arbitrarily set deadline (which I made). I naturally had to decline the invitation, as necropsies followed by detailed dissections (this is R.E. Fordyce we're talking about; detail is everything) eat up several consecutive days.

The fifth poll, just for fun, was my illustration of the recently re-named Ankylorhiza tiedemani – which I call a ‘killer dolphin’ or ‘giant dolphin’. Respondents were pretty split between calling this a whale (~43%) or a dolphin (~45%), with a few for porpoise. Answer: ???

What’s the moral of the story from these polls? These terms are helpful in some cases but otherwise prove my point that they do not reflect biological groups. Reliance upon these terms can lead to confusion.

Is the Eocene archaeocete Ambulocetus a whale, dolphin, or porpoise? Artwork by Carl Buell. 

Fossil whales, dolphins, and porpoises – a whaleontologist’s perspective

Paleontologists have a very laissez faire approach, because we are constantly befuddled by strange cetaceans that further blur the (admittedly very crookedly drawin) lines between these terms. Are four legged Eocene semiaquatic archaeocetes - like Pakicetus - whales, dolphins, or porpoises? They look rather close to a dog to be called any (but are typically referred to as whales). What about some early sperm whales that had relatively long rostra and may have had a beak, like the Miocene Zygophyseter? If we stick to the strict Dolphin= Delphinidae, and Porpoise = Phocoenidae taxonomy – what about dolphin-like phocoenids with beaks (e.g. Piscolithax), or extinct delphinoids, like Kentriodon, that are not delphinids, but also small-bodied with a beak? What about large kentriodontids the size of pilot whales, also with a beak? What about our newly named Ankylorhiza – is it a whale or a dolphin?

A somewhat outdated but nicely illustrated tree of modern and extinct cetaceans, highlighting the number of wholly extinct groups discussed by paleontologists without incident; modified from Barnes et al. (1985: Marine Mammal Science).

The strict dolphin v. porpoise dichotomy is perhaps somewhat useful for modern cetaceans (delphinids including porpoise-like species, and others labeled as whales, notwithstanding), but completely falls apart thanks to a slew of extinct dolphin-like species that are not delphinids. This is not universal, but from 15 years in the field, I can say that whaleontologists do the following things: 1) whale broadly is used by us to mean cetaceans, and all archaeocetes are generally referred to as “whales”. 2) Dolphin is broadly used for any small odontocete, including those with short rostra (e.g. some fossil globicephalines, the extinct xenorophid Inermorostrum), but also for larger species with beaks. Large dolphins, like Ankylorhiza or the Miocene Hadrodelphis, are simply called large dolphins or giant dolphins. Squalodon is called a shark toothed dolphin by most. In general, with the exception of some modern species, we generally use the term dolphin to indicate any extinct odontocete that is not a sperm whale or a beaked whale. 3) Porpoise is always meant to refer to the family Phocoenidae. 4) River dolphin may be convenient for modern species, but most extinct relatives of the four modern genera – Pontoporia, Inia, Lipotes, and Platanista – were fully marine. River dolphins are three or four separate clades representing a minimum of four riverine invasions.

One thing that seems obvious from my look back is that the terms dolphin and whale are frequently used for many extinct clades – but always with a family name in tow: e.g. eomysticetid whales; kentriodontid dolphins, etc.

Coronodon havensteini, a baleen whale with teeth - a common source of confusion in our museum. Photo by me.

Toothed whales, or Echolocating whales

Now that we’ve clarified paleontological usage of whale, dolphin, and porpoise, it’s time to take a closer look at the two major groups, and terms that at first glance *appear* to be fine. Odontocetes are the toothed whales, and Mysticetes are the baleen whales. But, these vernacular names become problematic and confusing when communicating about fossils to a lay audience. Some modern odontocetes don’t have any erupted teeth or only have some tusks (e.g. narwhal, most beaked whales, risso’s dolphin), and many extinct odontocetes similary had only tusks or were completely toothless (Inermorostrum, Odobenocetops, Australodelphis, Vanbreenia, Dolgopolis). This is not too much of a problem, to be honest.

Instead, the real problem is the toothed mysticetes – baleen whales with teeth. Toothed baleen whales are the earliest lineages of mysticetes that still had teeth – like Coronodon from South Carolina, the Aetiocetidae (north Pacific flat-snouted teeth + baleen bearing mysticetes), the Mammalodontidae (big-eyed Australasian toothed mysticetes), and the Llanocetidae (southern hemisphere large bodied toothed mysticetes with large gaps between teeth). We recently had a comment on a social media post about Coronodon exclaiming “baleen whales don’t have teeth!”

While I have no idea how to rebrand “baleen whales”, I have found a bit of a helpful compromise in the past few years: rather than used the far more problematic term “toothed whales”, which relies upon modern species and rapidly falls apart after cursory consideration of fossils, I’ve started calling odontocetes “echolocating whales” since discoveries like Cotylocara macei suggest the origin of echolocation to be very, very early within the Odontoceti, quite possibly at the base of the clade itself. A major exception to this is a paper that Dr. Rachel Racicot published (I was a coauthor) on an Olympicetus-like dolphin from the Oligocene of Washington state that apparently could not echolocate, thereby implying that echolocation evolved twice within Odontoceti – or that this dolphin lost the ability to echolocate. This probably doesn’t matter much, to be honest, when you consider that virtually all modern and extinct odontocetes have structures otherwise indicative of some facial structures associated with sound production – and that it’s possible that our Olympicetus-like dolphin may have lost the ability to echolocate. Such a case is clear with Odobenocetops, which either had a tiny melon (O. leptodon) or probably did not have one at all (O. peruvianus). To clarify: there are far fewer odontocetes that cannot/could not echolocate than there are whales with teeth that are not odontocetes. Therefore, I think the term “echolocating whales” is probably a better common name than “toothed whales”. 

So, what the hell is an orca? A whale or a dolphin?

Now we’ve come full circle. Is an orca a killer whale, or a killer dolphin? It’s both, really. Because it’s a member of the family Delphinidae, it’s totally cool to call it a dolphin. If found as a fossil, whaleontologists probably would’ve just called it a giant dolphin. At the same time, whale doesn’t really have a specific biological meaning, so it’s also just as fine to call it a whale. If you’re going to raise the point, you may as well indicate that the Delphinidae are an ecomorphologically disparate clade of cetaceans including dolphins, some that look and behave exactly like true porpoises, and some that are called whales. So, don’t argue with people because when you get down in the weeds about dolphin v. whale, none of it makes any goddamn sense.

A possible compromise? A whaleontologist’s glossary of vernacular terms

The semiformal vernacular nomenclature used by paleocetologists suggests that a slightly more precise way of talking about cetaceans is easy and can substantially clarify communication with the public. Here is a brief synonymy list of terms I’ve heard used in paleocetology and marine mammalogy (or used personally without precedent). It avoids for the sake of useless repetition names including the family (e.g. xenorophid dolphins). If you have more suggestions, please let me know in the comments and I'll consider adding them.

Whale = any cetacean, archaeocete or neocete alike

Ancient whales**** = archaeocetes

Baleen whales = Mysticeti

Toothed baleen whales** = toothed Mysticeti

True baleen whales or baleen-bearing whales** = Chaeomysticeti

Right whales = Balaenidae

Rorquals* = Balaenopteridae

Gray whales = Eschrichtiidae

Pygmy right whales = Neobalaenidae

Toothed whales or echolocating whales = Odontoceti

Dolphin**** = 1) for extant taxa: any odontocete without whale or porpoise in the common name; 2) for extinct taxa: any odontocete that’s not a phocoenid, physeteroid, ziphiid, or globicephaline

Shark-toothed dolphins**** = “Squalodontidae”

Spear-toothed dolphins*** = Waipatiidae

Swordfish dolphins*** = Eurhinodelphinidae

Porpoises = Phocoenidae

Oceanic dolphin = Delphinidae

Blackfish**** = Globicephalinae + killer whales

River dolphin**** = Platanista, Lipotes, Inia, Pontoporia & extinct allies (marine or otherwise)

White whales = Monodontidae

Sperm whales = Physeteroidea

Walrus faced whales = Odobenocetops

*names used by mammalogists but are probably useless for science communication

**extinct subdivisions/subtaxa of these could benefit from better common names, thus far the family name is used exclusively

***names I use that may or may not ever catch on

****names that in no way refer to monophyletic groups

The terrible fossil record of sea otters, part 3: The oldest sea otter in the Pacific, revision of their biochronology, and future directions in otter paleontology and evolution

Make sure to read Part 1 and Part 2 of this blog series!

This is the finale to my three-part blog series on the evolution and fossil record of Enhydra - I hope that you leave with 1) a sense of clarity about what we *do* know after all and 2) a sense of longing for finding out more - there is simply a lot that we don't know, even about the history of rocky shore faunas in general. It is my profound hope that someone interested in otter evolution will read this and get inspired to look into one of these new directions. This series will be updated in the future as more on our fossil otter research comes to fruition - our studies of the Gubik Enhydra and expanded hypodigm of E. macrodonta are just starting - so there are no preliminary insights from either project I feel comfortable with sharing yet. Go forth and discover some fossil otters!

The Thornton Beach otter - the oldest Pacific basin Enhydra

When I was a Ph.D. student in New Zealand I was nearly as far away as you can get on earth from these sea otter bearing localities, and had already grown so frustrated with my lack of results that I gave up trying. Earlier in my career I had tried looking for Plio-Pleistocene marine mammal fossils in the Merced Formation, much, much closer to home – the Merced Formation is exposed on the San Mateo Peninsula between Pacifica and Ocean Beach in San Francisco. However, localities of this unit require very long hikes up and down cliffs that are several hundred feet tall, and at least a couple of areas where I have not exactly felt safe. My first visit had crime scene tape at one – which I ignored – and upon returning home, my mother, who is a judge, recalled “oh yeah, there was a murder down there last week.” No thanks!

The rugged type section of the Merced Formation - a critical stratigraphic section for studying invertebrate biostratigraphy and climate across the Pliocene-Pleistocene transition. This is the view north from Mussel Rock; Fort Funston is at that far point, and north of there, Thornton Beach and Ocean beach. The Marin Headlands are the far hills in the distance across the Golden Gate (not visible). Photo from californiabeaches.com

Marine mammal fossils do occur in the Merced Formation, but they are extremely rare relative to the Purisima Formation further south that I was more used to: the Merced Formation was deposited about 5-10 times as rapidly, so the bones are dispersed and do not occur in easily accessed phosphatic bonebeds. I did about a half dozen trips between 2004 and 2006 and didn’t find a single bone. Needless to say, I knew intuitively that if I did not find any fossils, someone else might- but I might be an old man before that happened, since the last marine mammals were collected in the 60s!

The femur after preparation was perhaps halfway complete - thanks to RE Fordyce, and Sophie White for letting me prepare this in the Otago Geology Museum preparation lab!

Needless to say, I was pleased, excited, and a little in shock when I received some photos of a bone in a concretion in an email from my buddy Chris Pirrone – a civil attorney in the bay area who is an avid, ethical, and very generous fossil collector. Chris has also helped me with several excavations of whale and dolphin skulls! This specimen appeared to be a five inch long sea otter femur, but about 3/4 of it was still embedded and there is no shortage of land mammal bones from the upper parts of the Merced Fm. Nevertheless, from the little that was exposed I was confident and so I asked Chris if he wouldn’t mind donating the fossil to UCMP – and he generously offered to mail the specimen to me in New Zealand for preparation. I spent about two weeks mechanically preparing the specimen and, in the process, developed a neck ache that refused to go away for about 5 months (I could only afford very cheap pillows in New Zealand, which did not permit my neck to recover quickly).

The published illustration of the Merced Fm. otter, complete with ammonium chloride coating! From Boessenecker (2018).

The excellent stratigraphic control of the Thornton Beach sea otter - thanks to high resolution strontium dates from Ingram and Ingle (1998).

After preparation, the femur appeared to be longer than modern Enhydra lutris (reported also for a couple Enhydra sp. femora from the Pleistocene of Oregon), but still very clearly a specimen of Enhydra. After asking Chris details about the locality and stratum, it became clear that it was from a very well-dated horizon – where individual Strontium isotope dates were collected and reported every 5-10 meters (Ingram and Ingle, 1998), which is unusually high resolution for what I am used to! So I waited to publish anything (also, so I could finish my thesis on eomysticetids) until I was able to visit the locality myself with Chris and he could point to the individual rock layer. We visited the spot together in Summer 2015 after Sarah and I had returned from New Zealand, and I was able to pinpoint the stratigraphic interval to a horizon bracketed by dates of 620,000 to 670,000 years. After exhaustive reading of the stratigraphic literature, this was clearly the oldest known specimen of Enhydra from the Pacific basin.

Possible hypotheses for sea otter evolution & biogeography

There are two major hypotheses used in the evolution of sea otters from riverine ancestors.

Hypothesis 1) Enhydra evolved in North America, from something like Enhydritherium – the only other Enhydra-like otter from the north American fossil record.

Hypothesis 2) Enhydra evolved somewhere in the old world and had a more recent dispersal to the Pacific – either westward through the Central American seaway prior to Panama uplift about 3 Ma or through the arctic afterwards, or perhaps from east Asia along the north Pacific coast.

Unfortunately, most of the fossils are super fragmentary, as discussed in the prior blog post – virtually all specimens of Enhydra are isolated elements and not complete enough to code. There are also not a whole lot of characters available, and even when adding some to the Wang et al. 2017 matrix, I didn’t get much out of it. Nevertheless, the Wang et al. phylogeny does cast doubt on a close relationship between Enhydritherium and Enhydra – echoing the comparisons by Lambert (1997) with the more completely preserved skeleton of Enhydritherium from Florida. While we don’t have a reliable phylogeny, we DO have some pretty reliable characters to identify these things – Enhydra teeth are very distinctive, and we do have geochronology. This information cannot answer problems of phylogeny (e.g. which forms of extinct Enhydra are more closely related) but it most certainly can tell us where Enhydra was and when.

The two different results of the phylogeny of lutrines (otters) by Wang et al. (2017); the one on the left is better resolved (50% majority rules consensus tree), and shows a close relationship between Enhydra and Enhydriodon, which is not resolved in the second phylogeny under Bayesian methods; however, a close relationship between Enhydra and Enhydritherium is unlikely owing to the well-resolved sister taxon relationship between Paludolutra and Enhydritherium. 

Revision of the biochronology of sea otters in the North Pacific

Hypothesis 1 listed above, endorsed by Mitchell (1966) and Repenning (1976), has two requirements: firstly, that fossils of Enhydra truly pre-date Enhydra reevei from the UK, and secondly, that there is a close relationship between Enhydra lutris and Enhydritherium. Phylogenetic analysis by Wang et al. (2017) calls the latter into question. So what about the dates? My review of up-to-date stratigraphic research found that all Pacific basin specimens of Enhydra are from the late or middle Pleistocene. In particular, the Timms Point Silt tooth which was so critical to Mitchell’s dismissal of Enhydra reevei – is no older than 400,000-500,000 years in age. The Timms Point Silt specimen is therefore at least 1-1.5 million years younger than the British specimens, and 100,000-200,000 years younger than Chris Pirrone’s femur from San Francisco. The significance here is that updated geochronology tells us that once again, Enhydra reevei is the oldest bone fide example of a fossil Enhydra anywhere in the world. This on its own, along with the re-shuffling of Pacific coast specimen ages, knocks out one of the two requirements for the Mitchell hypothesis and is suggestive of a relatively recent invasion of the Pacific basin. One caveat is that while late Pliocene strata like the San Diego Formation are very well-sampled, there are virtually zero early Pleistocene marine mammal bearing localities where more than ten individual specimens have been discovered anywhere in the eastern Pacific. As for the second requirement, a close relationship between Enhydra and Enhydritherium does not seem to be likely, thanks to the Wang et al. (2017) phylogeny.

Revised biochronology of Enhydra fossils. All of them. From Boessenecker (2018).

What about those fossils from Alaska? At least one or two specimens from the Gubik Formation indicate the presence of Enhydra in the Arctic about 1.5-2 million years ago, slightly younger than the British fossils. Because of these, and the fact that the Central American Seaway would have already been closed after 3 Ma, I proposed in my 2018 paper that Enhydra evolved from Enhydriodon in western Europe during the Pliocene and dispersed through the Arctic, into the North Pacific, during the early Pleistocene. An earlier dispersal is not tenable at present owing to the well-sampled San Diego Formation, and derivation from Enhydritherium seems unlikely.

This is quite surprising as it means that the modern kelp forest ecosystem, seemingly dependent on sea otters to keep urchin numbers down, was very different only one million years ago. Who used to eat all the urchins? I honestly have no idea – walruses don’t really eat urchins, to my knowledge, and they’re some of the only durophagous predators from the Pliocene.

In conclusion: the oldest bona fide Pacific basin fossils of Enhydra are less than one million years old, and since older fossils exist in the Atlantic, sea otters probably evolved in the late Pliocene in western Europe and immigrated to the North Pacific very recently.

Lingering questions

Why are sea otter fossils so rare? Even during their established geochronologic range, otter fossils are quite rare. To be honest, all marine mammals are rare in Pleistocene deposits – but it’s much easier to find pinniped fossils, for some reason. One argument is that otters tend to be restricted to rocky shore environments – which are environments characterized by erosion rather than deposition, so they’re already biased. Sea otters are quite small – and small vertebrates have a lower preservation potential than larger vertebrates, right? For example, there’s the ‘colloquial knowledge’ about how rare it is to come across a bird carcass that isn’t just a pile of feathers. Could taphonomy explain the rarity of sea otters? Carcass drifting experiments using… car tires, in the 1990s, showed that they modeled the drifting behavior of sea otters quite well. Sea otters tend to float if they die at the surface, owing to large lungs. Sea otter carcasses typically float for up to six weeks – which, surprisingly, is just as long, if not slightly longer, than harbor seals and dolphins observed by Willhelm Schafer in the North Sea. What does this extreme floating v. body mass ratio mean? Probably several things:

1) Because sea otters don’t exhale when they dive, and do not dive deep, this could make the discovery of complete skeletons very rare, as most end up on shore rather than sinking.

2) Because sea otters live so close to shore, most carcasses are probably going to end up drifting towards the shore at some point. Indeed, carcass and dummy drift experiments have 2/3 of each washing up on shore. The shoreline environment is one of the highest energy marine environments, and preservation of marine vertebrates in any meaningful volume is rare: only a handful of vertebrate fossil sites have ever been demonstrated to really represent this environment, and most rich assemblages of marine mammals represent deposition in at least 10 meters water depth. Virtually all stranded carcasses will not enter the fossil record except as unrecognizable bone pebbles and pulverized sand-size particles. It is for this reason I am utterly skeptical of the utility of many studies of taphonomic patterns of stranded carcasses. Stranded carcasses also give us meaningless data on disarticulation, because continually floating carcasses never undergo drying of the tissues, but that’s another problem.

Sea otter carcass and car tire drift experiments - Young et al. 2019.

3) Floating carcasses have the opportunity to shed isolated elements, particularly from the extremities, as they disarticulate – if they have enough time to decompose enough.

4) The distance of transport and the likelihood of stranding depends entirely upon currents and wind direction, which varies seasonally and daily. This is likely why 1/3 of the carcasses from the experiment never stranded, and why some drifted 100-200 km, which is still quite far. This drift distance casts doubt on ever being able to infer habitat preference from the fossils, as one carcass can, in its post-mortem road trip, cross virtually ever environmental boundary on the steep, narrow California shelf. Any discoveries of sea otter bones and teeth in rocky shore environments may be completely accidental.

5) Lastly, I’m not sure if comparable carcass drift experiments with larger seals and sea lions exist – but I expect them to differ little. The larger a marine mammal is, the higher the capacity for long-distance carcass transport. Sea otter drift patterns mean that it’s possible for partial skeletons to sink after the ‘bag of bones’ stage of bloat and float down into middle shelf sediments, where they are less likely to be scoured and separated by bottom currents. So why are most of our sea otter fossils from shallow settings adjacent to rocky shore environments?

Rocky shore environments, like the Monterey headlands, are characterized by erosion rather than deposition - which heavily biases the rock record and fossil record against rocky shore faunas, and since most marine mammals from the eastern North Pacific are found in these scattered deposits - their remains are often quite poorly preserved. Photo by RWB - the famous Lone Cypress on 17 mile drive in Pebble Beach, CA.

There are precious few pre-Pleistocene rocky shore faunas on the west coast: this is because rocky shore fossil assemblages are deposited as geographically limited lenses around sea stacks or other rocky exposures. These rocky exposures indicate the long-term likelihood of erosional destruction. Therefore, most rocky shore faunas from California, for example, probably have a short shelf life of 1-2 million years. True marine deposits formed by basin subsidence, rather than resistant bathtub rings around bedrock, are frequently thicker and more laterally extensive – but also generally terminate about 2 million years ago, when the California coast ranges began to be uplifted and all of the large shallow marine embayments dried up (now the SF Bay area, the Salinas river valley, the LA basin, and others). Two notable exceptions are the Port Orford Formation of Oregon, which is middle Pleistocene, and the upper part of the Merced Formation near San Francisco, which is middle Pleistocene in age (lower Merced is late Pliocene). It is possible that fossils of Enhydra will be found in Pliocene shelf deposits in California or Japan – after all, we assumed the same thing about monachine seals, until my good friends Jorge Velez-Juarbe and Anita “Phocita” Valenzuela-Toro (2019) reported a couple of monachine seal teeth from the Monterey Formation of Orange County, CA. So, it’s possible – but the evidence just isn’t there yet.

So what makes fossil sea otters rare, in my opinion? I’m not sure genuine scarcity has anything to do with it, and I am always hesitant to ascribe any paleoecological reasons, as dead animals float around all over the place and cross these convenient ecological boundaries rapidly after expiring (and, relative to body mass, sea otters take this to an extreme). Do sea otters have high preservation potential? Certainly it’s not much lower than that of pinnipeds, as their bones are still large, but smaller and perhaps slightly more easily abraded than seal and sea lion bones. We also have no shortage of tiny pinniped and dolphin skeletons in the rock record. Their preservation potential has to be much higher than sea birds – however, sea birds actually have a surprisingly high preservation potential, and are remarkably common fossils in Neogene marine deposits (even at places like Moonstone Beach – bird bones outnumber marine mammal bones by 10:1). So, maybe sea otters are rare – but there’s so much in terms of unknown variables I will hesitate for a long time before ever suggesting that as a viable hypothesis. In sum, I do not think there is enough that is fundamentally different in terms of anatomy or ecology, either from numerically common sea birds or small fur seals, that might explain the rarity of fossil sea otters.

Rather, I think it is an artifact of rock bias: there is a fundamental disconnect between the rich open shelf deposits of the Miocene/Pliocene and the rocky shore bathtub rings of the Pleistocene: the Pleistocene deposits are small in volume, geographically disparate, with poor (and rapidly shrinking) exposures. They are also generally terrace deposits that do not benefit from the typical sedimentological processes that concentrate vertebrate fossils in open shelf environments. Because 1) there is this difference in the abundance of marine mammal fossils between Pliocene and Pleistocene deposits owing to sedimentological factors, 2) difference in the volume of rock and area of exposure between these two epochs, and 3) sea otters are Pleistocene-only in the Pacific, sea otter fossils are rare. If we had open shelf deposits with preservation/sedimentology more similar to Neogene deposits like the Purisima and San Diego Formation, there would probably be more fossils of Enhydra on the west coast.* In conclusion, while rock bias cannot on its own explain the lack of pre-Pleistocene sea otters (bloat and float would have introduced at least a few sea otters into units like the San Diego Formation), sea otters invaded at a time when virtually all relevant deposits are scattered and limited in volume. So there are scraps instead of skeletons (unlike the Pliocene record of marine carnivores).

*”But what about Enhydritherium?” I hear someone moaning somewhere in the distance. Enhydritherium is Pliocene and therefore should be found in Pliocene marine deposits more frequently. Do recall that this species is probably not open marine and is probably freshwater-estuarine in distribution and therefore is not a good analog.

When did tool use evolve? We don’t really know, since there aren’t really many skeletal adaptations other than tiny forelimb size that correspond to tool use. The modern clawless otters are very dexterous, but do not use tools, and have river otter like limb proportions – suggesting that the bizarrely tiny forelimbs of Enhydra are a reasonable skeletal correlate. Enhydritherium is one of the only proposed Enhydra relatives with good postcrania, but it has river otter-like fore/hindlimb proportions. We don’t have much in the way of postcrania of Enhydriodon that is informative. There are isolated humeri from southern California of middle and late Pleistocene age that are anatomically identical to modern Enhydra lutris, and which Mitchell (1966) referred to the extant species. This would at minimum suggest – if the humeri are identical in size (difficult to evaluate owing to the fact that these are isolated elements) – that tiny forelimbs have been present in the sea otter lineage for a few hundred thousand years.

Who filled the otter niche before the Pleistocene? The modern kelp forest ecosystem probably originated in the late Miocene (Estes and Steinberg, 1988) - and today, sea otters are a keystone species: without otters, sea urchin populations get out of control and raze most of the kelp forest to the ocean floor. These "urchin barrens" now dominate rocky coasts between Monterey and British Columbia. The purple urchin, Strongylocentrotus purpuratus, is one of the most distinctive west coast marine invertebrates, and has a voracious appetite for kelp. This species is known from the Pliocene San Diego Formation, and certainly pre-dates Enhydra. So who ate urchins and kept their populations down before the arrival of sea otters? Sheepshead wrasses (Semicossyphus pulchrer) eat them - and have a fossil record in California extending *probably* back to the late Miocene owing to abundant tooth plates found in the Santa Margarita Sandstone. Annarichthys - the horrifying but very gentle and shy wolf eel - feeds on urchins, and is known from the Purisima Formation. Some sea stars (with admittedly a very limited fossil record on the west coast) feed on them. Would some of the extinct walruses have filled the niche of sea otters? Modern walrus occasionally feed on urchins, but they are probably second rate food consumed when more favorable mollusks are not available (Sheffield, 1997).  My money is on durophagous fish - but more research is clearly needed on the evolution of rocky the shore fauna and flora!

*This species has extra relevance to my family. My mom told us of her first field trip as a grade school student (St. Gregory's in San Mateo) to the Fitzgerald Marine Preserve in Moss Beach CA - indeed my first tidepool I ever visited - which was cancelled about 10 minutes after arrival because class idiot "Patrick R" stuck his tongue into a purple urchin and it became so swollen he couldn't breathe. Patrick had earlier gotten his head stuck in a chair during class.

Otter tool use leaves distinctive patterns of breakage on mussel shells - from Haslam et al. 2019.

Will we ever find sea otter tools in the fossil record? My guess is, probably not. There's one paper out there documenting what sea otter anvil stones and accumulations around them look like - but the odds of finding one in a rocky shore deposit are staggeringly tiny in my opinion. What is far more likely is looking at the distinctive patterns of breakage of different species of mollusks, like Haslam et al. (2019) did, and looking for those same patterns of breakage in the fossil record. This might even turn up more of a record than actual sea otter bones, since there are many more Pleistocene rocky shore deposits that completely lack vertebrates than there are that preserve them. As for the rocks that sea otters bring along with them? As discussed in part 1, it's not clear that the whole "sea otters carry around their favorite rock!" claim is more than a popular factoid, and doesn't seem to have a basis in scientific observation. We don't know how long sea otters carry their hammer stones around, or if the banging leaves any observable traces on them; indeed, it's not even clear if the anvil stones would be recognizable either without observation of otters using them. So, I don't think we'll be able to have a sea otter technology 'parataxonomy' like the "Oldowan" technology in the east African rift valley for early humans just quite yet.

When exactly did otters arrive in the Pacific? The lack of well-sampled early Pleistocene deposits in the eastern North Pacific precludes a precise estimation. We know it was sometime before 600,000-700,000 years ago, unless the Thornton Beach sea otter is the very first individual. However, we’re pretty certain that it was after the Plio-Pleistocene boundary 2.5 million years ago, since no sea otters (Enhydra spp) have ever been found in densely sampled units like the Purisima and San Diego formations. I predict that by the time I retire, zero fossils of Enhydra will be discovered in Pliocene rocks of the Pacific coast, and will happily eat my hat if this prediction turns out to be wrong.

Which lineage of freshwater otters did Enhydra evolve from? This is going to require more skulls of Enhydriodon and a significantly expanded morphological matrix for otters, in concert with molecular data in a combined analysis. I’m not interested in doing this, as to be honest, I really only care about sea otters and aside from them being adorable, don’t really give two shits about the phylogeny of river otters. But, someone will have to: right now our only ‘good’ phylogeny has about two dozen species and only 40ish morphological characters, with lots of uncertainty (Wang et al., 2017). The Wang et al. matrix is a fantastic start, don’t get me wrong – and it does hint at Enhydriodon-affinities of Enhydra, and casts doubt on an origin from Enhydritherium in North America.

The Future

1) We really need more fossil otters. Future exploration of latest Pliocene and Pleistocene (early, middle, and late) units, especially near Los Angeles, the Channel Islands, the Merced Formation near San Francisco, the Humboldt County localities, southwestern Oregon, and the Gubik Formation of Alaska will produce more otter material. More fossils will help with further clarifying the geochronologic range of Enhydra, and hopefully add to the hypodigm of Enhydra macrodonta, and perhaps reveal the existence of other species in the Plio-Pleistocene transition. We also need to be creative and reach out more to amateur collectors in northern California and Oregon who may have already collected some fossil otter material; thus far I’ve been pretty secretive about my otter research for various reasons, but I think getting the word out about how little we know about fossil sea otters, and what we *think* is going on, is a better course of action. We also need to go out and explore more Pleistocene marine mammal localities; the San Pedro Sand and Palos Verdes Sand near LA have been explored *somewhat* but there is undoubtedly more that can be done, provided there are surviving outcrops. We’re down to one major locality left (Moonstone Beach) in Humboldt County since Crannell Junction was overgrown in the 1980s, and it’s smaller than it used to be. Another option further south needing more field exploration is the Santa Barbara Formation – which is very well-exposed near Coal Oil Point in Isla Vista, a site I haven’t visited since I was in High School. There are some marine mammal scraps from that locality, and I think seasonal visits by some of my colleagues in Southern California could turn up some precious Pleistocene marine mammal material.

2) A reevaluation of Enhydra macrodonta is needed, including description of referable specimens from the same locality. This is a planned study with Ash Poust for the near future.

3) An interesting partial skull of Enhydra (identified in 1983 by Repenning as E. lutris), from the Gubik Pliocene-Pleistocene Formation of Alaska, awaits description – and is currently under study by yours truly, with assistance from Ash Poust, Morgan Churchill, and invertebrate paleontologist Chuck Powell. I won’t spoil it, but some aspects of it are pretty exciting (for example, sea otters cannot inhabit the Arctic ocean today because of the sea ice).

4) A more exhaustive phylogenetic analysis of lutrines is desirable, but this may be too much of an ask.

5) Granting agencies really need to be more supportive of field-based paleontology that doesn’t involve looking for dinosaurs in places we’ve found them for over 100 years.

References

Boessenecker, 2018. https://link.springer.com/article/10.1007/s10914-016-9373-6

Estes and Steinberg, 1988. https://www.jstor.org/stable/2400895?seq=1#metadata_info_tab_contents

Haslam et al. 2019. https://www.nature.com/articles/s41598-019-39902-y

Ingram and Ingle, 1998. https://www.sciencedirect.com/science/article/abs/pii/S0033589498919901

Lambert, 1997. https://www.jstor.org/stable/4523861

Mitchell, 1966. https://www.nrcresearchpress.com/doi/abs/10.1139/f66-177?journalCode=jfrbc

Repenning, 1976. Repenning, 1976. C. A. Repenning. 1976. Enhydra and Enhydriodon from the Pacific Coast of North America. Journal of Research of the United States Geological Survey 4(3):305-315.

Sheffield, 1997. http://www.adfg.alaska.gov/static/home/library/pdfs/wildlife/research_pdfs/walrus_feeding_re_examination.pdf

Wang et al. 2017. https://www.tandfonline.com/doi/abs/10.1080/14772019.2016.1267666?journalCode=tjsp20

Young et al. 2019. https://onlinelibrary.wiley.com/doi/abs/10.1111/mms.12609