Wednesday, February 18, 2015

Introducing Eotaria crypta from the Miocene of Southern California - the oldest known otariid pinniped

Photos of the holotype specimen and life restoration of Eotaria crypta, with Allodesmus for scale (Allodesmus is roughly the size of an adult male Steller's sea lion). Artwork by yours truly.

Fur seals and sea lions are grouped into the family Otariidae, and are otherwise known as eared seals; each informal group used to be considered as clades, and were grouped into the subfamilies Arctocephalinae and Otariinae (respectively). Modern fur seals are the most diverse, and include the northern fur seal (Callorhinus ursinus) and many species of southern fur seal within the genus Arctocephalus (some of which have been grouped into the genus Arctophoca; more on that later). Sea lions include the Steller’s sea lion (Eumetopias), California, Japanese (recently extinct), and Galapagos sea lions (Zalophus spp.), South American sea lion (Otaria), and the New Zealand (Phocarctos) and Australian (Neophoca) sea lions. Sea lions are generally larger than fur seals, lack underfur, and have thicker blubber. The problem is, most of the features that have been used to consider fur seals as a monophyletic group are primitive or associated with small body size, and morphological analyses have had trouble recognizing these two subfamilies, and molecular analyses have completely failed to support monophyly of each group. In simple terms, according to more robust molecular studies, the fur seal “morphotype” evolved at least twice, and the sea lion “morphotype” may have evolved up to three times – there’s not really much distinction between, and most modern otariids appeared to have diverged rapidly and recently, within the last 3 million years – the sole exception is the northern fur seal, fossil evidence of which indicates it’s been around, perhaps as an unbranching (anagenetic) lineage since the Pliocene (~3-4.5 Ma).

The great thing about the fossil record is that it often helps sort out these biological dilemmas; fossils are the only evidence we have for biological events that took place before people started writing anything down (e.g., most of earth’s history). Molecular phylogeny is a powerful tool, but it’s a bit like trying to figure out how big a cave is or what it looks like inside from looking at the entrance. Unfortunately, in this case, there are a few problems with the otariid fossil record. So as not to appear too negative, I’ll start with what we do know. We know that “sea lions” have a sporadic fossil record restricted to the Pleistocene, almost all of which are identifiable to modern genera or were very similar to modern genera (Proterozetes ulysses from the middle Pleistocene of Oregon, for example, may be a species of Eumetopias). Some fossil sea lions were slightly outside their current range (Neophoca palatina, for example, is known from the Pleistocene of New Zealand instead of Australia). All published pre-Pleistocene otariids are what we would call fur seals: relatively small bodied, similar to Callorhinus and Arctocephalus. No known examples of co-occurring otariid species exist prior to the Pleistocene; all Miocene and Pliocene marine mammal assemblages appear to have only a single species of otariid, whereas Pleistocene assemblages will have one sea lion and one fur seal (and in at least one case – two sea lions and a fur seal). Pre-Pliocene otariids are known only from the North Pacific; Thalassoleon (9-4 Ma) is known from Japan, California, and Baja California, the earlier otariid Pithanotaria is known from California only, and a Pithanotaria-like otariid has been reported from the early late Miocene of Japan (~12.5-11 Ma). No older fossils existed, but the fossil record of true seals possibly extends back to 19 Ma, and the fossil record of walruses extends back to 17 Ma, indicating a ~5 My ghost lineage for otariids. A ghost lineage is where we know indirectly from other fossils that a group must have existed during a particular period.

The tiny holotype of Eotaria crypta - in my hand. I don't exactly have large hands.

What we didn’t know was when otariids diverged from other pinniped groups (which, according to molecular studies, the closest relatives are walruses), because the fossil record of early otariids just doesn’t exist. Those that appear in the late Miocene already look like modern fur seals, but with slightly different teeth. What’s worse is that the teeth are already hugely simplified, and lack all of the neat little cusps that could inform us of their evolutionary heritage – the teeth are so simple that they’re effectively useless for inferring relationships, and the rest of the skull is effectively identical to modern species. So, the fossil record of otariids could suggest that the otariid “morphotype” evolved quickly and very, very early – becoming one of the first really modernized groups of pinnipeds. In contrast, it wasn’t until about 5 million years ago that walruses really first started to look like walruses and not seals with goofy teeth. Most importantly, we also don’t know what the ancestral otariid looked like; did early otariids look like the ancient bearlike “enaliarctine” pinnipeds? Early walruses (Prototaria, Proneotherium) looked very similar to “enaliarctines”, so it’s reasonable to guess that early otariids did as well. Also… why are otariids absent in the middle Miocene? Where were they? Did they evolve in a different ocean, and we’re just looking in the wrong area? A North Pacific center of origin is implicated for all pinniped lineages aside from the phocids (earless or true seals), so it really is curious why there are no middle Miocene otariids. Furthermore, Sharktooth Hill is the right age, and is one of the most intensely sampled Cenozoic marine vertebrate localities in the world. Where are they?

The holotype mandible of Eotaria crypta - Figure 1 of Boessenecker and Churchill (2015).

Last week saw the publication of a short paper by Morgan Churchill and myself on the new otariid pinniped Eotaria crypta. The type specimen of Eotaria, a partial mandible with beautiful teeth, was collected in the early 1980’s from Mission Viejo in Orange County, California – from the early middle Miocene “Topanga” Formation. This is the same stratum where our Pelagiarctos sp. fossil originated, and is also where some fossils of the west coast sirenian Metaxytherium arctodites were reported from. The “Topanga” Formation has dated interbedded basalts and diatoms (fossil siliceous plankton) that date it to 17.1-14.9 Ma (updated from our estimated date of 17.5-15 Ma in the 2013 Pelagiarctos article in PLoS). This overlaps but is otherwise slightly older than the Sharktooth Hill Bonebed; the bonebed was deposited within the “Temblor Sea”, an inland embayment within the basin formed within the modern San Joaquin Valley. In contrast, the “Topanga” Formation was deposited along the continental shelf of California as opposed to a few hundred kilometers inland from the open ocean. What were pinniped assemblages like during the early and middle Miocene? Early Miocene pinnipeds are predominantly “enaliarctines” – small bodied, archaic pinnipeds with generalized skulls and bear-like dentitions; some species of Enaliarctos retained shearing carnassial teeth. The earliest large-bodied (e.g. sea lion sized) pinnipeds appeared during the early Miocene – the early relative of Allodesmus, Desmatophoca. The tiny seal Pinnarctidion – an “enaliarctine” and possible ancestor of true seals and desmatophocids – is also known from the early Miocene, and was the only early/middle Miocene pinniped as small (if not smaller) than Eotaria. During the middle Miocene, the first walruses show up – small bodied enaliarctine-like forms like Proneotherium and Neotherium and larger sea lion-like species like Pelagiarctos thomasi, and the largest desmatophocids (Allodesmus; some Allodesmus from Japan approached the size of a modern elephant seal).

 Comparison of mandibles of early and middle Miocene pinnipeds from Oregon and California - Eotaria crypta is the smallest, absolutely dwarfed by Pelagiarctos and Allodesmus.

Enough context; let’s talk about the fossil itself. First, it’s adorably tiny; it’s only the size of a juvenile NZ fur seal mandible, and would’ve been larger than a sea otter (1-1.5 m, 20-50 kg) but smaller than the smallest adult fur seals living today (Juan Fernandez/Guadalupe fur seals; ~1.5-2 meters, 50-150 kg). The entire fragment is 6 cm in length and fits comfortably in the palm of my hand; it’s about the size of, and weighs only slightly more than a stick of Wrigley’s chewing gum. There are no mandibular differences with modern fur seals; all the action is in the teeth. The fossil includes the P2, P3, P4, and M1; at first glance, the teeth are pretty similar to modern fur seal teeth. However, on closer inspection, the molar has a distinct – but small (and thus vestigial) metaconid cusp – this is an extra cusp present in ancient “enaliarctine” pinnipeds, but absent in all modern and younger fossil otariids. This condition shows that the premolars have already lost the metaconid – but the molar is slightly lagging behind. The molar is also slightly larger than the premolars, but not by much – this is a “ghost” of the carnassial tooth in “enaliarctines”. For the uninitiated, carnassials are the large shearing teeth that your pet dog or cat slices meat with – the carnassials include the upper fourth premolar and the lower first molar. Pinnipeds don’t chew (fish and other prey are just swallowed whole), so the carnassials were quickly lost amongst early pinnipeds. While the tooth isn’t preserved, a socket (alveolus) is present for a second molar – lost in all later fossil and all modern otariids, and hypothesized as a synapomorphy for the group. The socket is tiny, indicating that the tooth was reduced to a tiny little peg (maybe the root would have been 2 mm in diameter, at most). These dental features not only indicate that otariids evolved from an “enaliarctine” ancestor, but that the otariid “morphotype” evolved really, really early – something like 12 million years before the first tusked odobenine walrus ever started showing off its sexy teeth to attract mates. Here’s the take home message about critters like Eotaria: fur seals and sea lions, other than changes in body size, appeared to have changed very little since the middle Miocene. Eotaria is only ten million years younger than the oldest dates on “enaliarctine” pinnipeds, and to go from the ancestral pinniped to a modernized species in ten million years, is, I think, pretty fascinating. The putative proto-pinniped Puijila (too many ‘Ps’, sorry) is only ~4 million years older than Eotaria. That all being said, fur seals are typically regarded – on the basis of retaining underfur, external ear pinnae, and an ankle capable of forward rotation and galloping (like terrestrial carnivores) – as the most primitive modern pinnipeds. The discovery of Eotaria, and the similarity of most fossil otariids to modern forms, paints a picture of a relatively archaic group that found a niche early and had little reason to change for over 15 million years.

North and South Americans working during the 2013 Cooper Center visit, from left to right: David Rubilar Rogers (Museo Nacional de Historia Natural, Chile), Paulina Jimenez Huidobro (mosasaur specialist - U. Alberta), Roberto Yury Yáñez (fossil bird specialist, Laborotorio de Zoologia Vertebrados, Chile), Peter Kloess (marine birds - CSU Fullerton), and Sarah Rieboldt (trilobite specialist, mitigation paleontologist, and wife of CSU Fullerton paleontologist Jim Parham).

Lastly, I’ll end on the story of the discovery of the specimen. I didn’t collect it, so I can only claim to have “rediscovered” it. I found it in a cabinet at the Cooper Center in Orange County on a visit in October 2013 after the Society of Vertebrate Paleontology. Pinniped specialists hadn’t gone through their collections in some time, so Curator Meredith Rivin invited me to take a look and see if I could provide any updated identifications. I was looking through a drawer of fossils from the “Topanga” Formation and saw what at first assumed was a misplaced specimen – a cute little otariid mandible. I immediately thought “they’ve misplaced a younger otariid specimen in here” and then I looked at the label – collected from the “Topanga” Formation! I won’t lie, the first word out of my mouth was “Holy” and I won’t dare repeat the second. Upon examining the specimen I saw the vestigial metaconid and empty second molar alveolus, and realized what I had found. A bunch of colleagues from South America (mostly Argentina, Chile, and Brazil – all marine vertebrate specialists) were also visiting the Cooper Center, and my colleague Carolina Gutstein jokingly called the specimen “Microtaria”, which would’ve also been a good name (and, to my knowledge is available, should anyone find another dwarf otariid fossil!).

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