Pinnipeds (seals, sea lions, and walruses) are a group of marine carnivores related to dogs, bears, and weasels. Three extant and one extinct family of pinnipeds exist; these include eared seals (aka fur seals and sea lions; Otariidae), walruses (Odobenidae), true or hair seals (Phocidae), and the extinct pseudo-seals (Desmatophocidae). All three of these groups have extensive fossil records, particularly in regions like the west and east coasts of the USA, Japan, Europe, and South America. The oldest pinnipeds, a paraphyletic group called the “enaliarctines”, are known from Oligocene-lower Miocene rocks in the North Pacific, are relatively small and ecologically generalized species intermediate in size between a sea otter and a harbor seal. Fossils of pinnipeds are generally more rare and fragmentary than those of cetaceans, and researchers in the past have seriously confused pinniped paleontology by naming (or over-interpreting) isolated postcranial bones. Many spectacular pinniped specimens including skulls with associated skeletons from the Americas await study. Much of my own research has focused on fossil pinnipeds from the eastern North Pacific; in many cases, better preserved specimens of perhaps the same species exist but (until recently) have been locked away in cabinets and closely guarded – meaning that in some cases, isolated mandibles were all that were available to me as a student. Several of my projects with NYIT postdoctoral fellow Dr. Morgan Churchill have focused on such specimens, leading to our tongue-in-cheek nickname for our pinniped research program, the “pinniped mandible working group”.
One of my very first projects involved the study of some particularly rare specimens of the extant fur seal Callorhinus from Scotia Bluffs in Humboldt County, California. Scotia Bluffs is a famous locality for well-preserved Pliocene mollusks. In the early 2000s, local collector Ron Bushell found two important specimens: an isolated mandible, and a pair of mandibles in a concretion the size of a car tire. He generously donated these two specimens, and I started working on them when I was still an undergraduate student. The pair of mandibles originated from the lower part of the Pliocene Rio Dell Formation and turned out to be Callorhinus gilmorei, an extinct relative of the modern species Callorhinus ursinus, the Northern Fur Seal, known from the Pribilof Islands near Alaska. Callorhinus gilmorei was previously reported from the Pliocene of San Diego, California, and Japan, and noteworthy for its small size and presence of double-rooted cheek teeth – these are all single-rooted in the modern species. The larger mandible is more similar to the modern species, but given that it has a lower molar with double roots, it was only identifiable as Callorhinus sp. Further intriguing was that this specimen was recovered from the upper Rio Dell Formation and therefore early Pleistocene in age – the first good Pleistocene specimen of Callorhinus anywhere. This specimen crucially showed that Callorhinus remained stable within the North Pacific and surviving a period of marine mammal extinctions and faunal change, the only pinniped to do so during the Plio-Pleistocene interval. These fossils suggest that the Northern Fur Seal is a product of a 4-5 million year old anagenetic lineage in the North Pacific.
Boessenecker, R. W. 2011B. 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.
Walruses are a formerly diverse group of pinnipeds, and in the past were no more normal than the single surviving species. Extinct walruses included bizarre double-tusked walruses that chewed clams, “toothless” walruses that completely lacked teeth aside from the tusks, and relatively long-snouted giant walruses. A particualrly early sea lion-like walrus, Pelagiarctos thomasi, was named from the richly fossiliferous Sharktooth Hill Bonebed in central California, and based upon the rarity of the species, a fused “chin”, large body size, and hyena-like teeth, Pelagiarctos was interpreted as a “killer walrus” that fed on marine mammals and seabirds. Morgan and I were invited to study a new specimen of Pelagiarctos (from the Topanga Formation, Orange Co., CA) at the San Diego Natural History Museum by curator Tom Deméré, and we considered this an excellent opportunity to expand the known anatomy of the genus and also reevaluate the “killer walrus” hypothesis. We found that the teeth are not really much different from other, smaller bodied walruses like Neotherium, and that fusion of the mandibles at the chin is not really related to bite force amongst mammalian carnivores. Rarity may be related to other factors (e.g. time averaging, gender-segregated foraging, vagrancy) and the body size of Pelagiarctos was likely initially very exaggerated, and it is actually much smaller than the coeval desmatophocid seal Allodesmus.
A followup study I co-authored with Dr. Carolina Loch (U. Otago), Dr. Morgan Churchill (NYIT), and the late Dr. Jules Kieser (U. Otago) examined the enamel ultrastructure of an isolated premolar of Pelagiarctos thomasi (collected from Sharktooth Hill and generously donated by longtime friend J.P. Cavigelli, Tate Museum) using scanning electron microscopy (SEM). The analysis showed that enamel is equivalent in structural adaptations to that of other pinnipeds which are known to be generalist fish-eaters (piscivores) – hammering another nail in the coffin for the “killer walrus” hypothesis.
Loch, C., R. W. Boessenecker, M. Churchill, and J. Kieser. 2016. Enamel ultrastructure of fossil and modern pinnipeds: evaluating hypotheses of feeding adaptations in the extinct walrus Pelagiarctos. The Science of Nature 103:44:1-8.
Eared seals (Otariidae) are some of the more common modern pinnipeds on the west coast of North America, and are some of the more common pinniped fossils. The oldest described otariids are all from the eastern North Pacific (see Eotaria, below) but most are not terribly different from modern species (or from one another). The conservative skeletal anatomy of these pinnipeds has made studying their evolution and fossil record difficult. We attempted to mine as much phylogenetic information as we thought possible from the skeletal anatomy of modern and extinct otariids and executed one of the only cladistic analyses on this group. Our analysis led some support to recent molecular cladistic hypotheses including that both fur seals (Arctocephalinae) and sea lions (Otariinae) are paraphyletic. Other findings include 1) recognition of a northern sea lion clade including Zalophus (California sea lion), Eumetopias (Steller sea lion), and Proterozetes (a Eumetopias-like extinct species), 2) dispersal to the southern hemisphere across the equatorial eastern Pacific during the late Miocene, and 3) otariid distribution and evolutionary biogeography are tightly linked with productivity and sea surface temperature.
Churchill, M.M., Boessenecker, R. W., and Clementz, M. 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 Linnean Society 172:200-225.
Some of the best fossil discoveries are made in museum collections. In October 2012 I was visiting the Cooper Center in Orange County – a series of warehouses full of fossils collected by mitigation paleontologists from local construction sites since the 1970s. This program has been wildly successful and has uncovered literal mountains of fossils. Already numerous master’s theses have been written on small parts of this collection, and perhaps ten or more dissertation’s worth of research sits in the warehouses. I was rifling through their pinniped collection when I came upon a tiny fur seal jaw. I assumed it was a specimen of Pithanotaria from the late Miocene – an uncommon and poorly known species (still exciting). However, my jaw hit the floor when I saw the formation the specimen originated from – the lower Miocene Topanga Formation. I knew the Topanga was old, slightly older than Sharktooth Hill. This specimen would be 5-7 million years older than the oldest known otariids (e.g. Pithanotaria), which somehow had never turned up in the incredibly well-sampled Sharktooth Hill Bonebed. I quickly examined the specimen, and noted that it had an enaliarctine-like molar and a socket for a second lower molar. All modern and previously reported otariid fur seals and sea lions have cheek teeth that are identical and lack a second lower molar, whereas this specimen had just the vestiges of the ancestral carnivore-like carnassial teeth which characterize enaliarctines – archaic holdovers befitting of an early otariid. I knew within about five minutes that this was a new species, and the oldest discovered otariid specimen. Morgan and I named it Eotaria crypta – the genus name meaning “dawn sea lion”, and the species name referring to the ease at which this taxon “hid” from researchers, both in terms of overall rarity and the fact that it sat unrecognized in a museum collection for three decades.
On occasion particular species may be published and named decades before other researchers followup on it. In this case, the late Australian pinnipedologist Judith King studied a fossil skull which had been discovered early in the 20th century from the North Island of NZ but never published on. In 1983 she named it Neophoca palatina and gave a brief description of the skull with little discussion of the host stratigraphy and locality. She placed this species within the genus Neophoca, which today only includes the Australian sea lion, Neophoca cinerea. This would indicate that Neophoca formerly occupied a larger range. Exactly thirty years later, Morgan and I would both be in New Zealand at the same time – I was in the second year of my Ph.D. program, and Morgan secured an East Asia Pacific Summer Internship through NSF to study modern (and a few fossil) pinnipeds from Australasia. We took out the holotype specimen on loan from the Auckland War Memorial Museum, though we had been warned that the specimen had been seriously damaged. Upon arrival, we started reassembling the specimen, and glued together about as much of the fragments as anyone was likely to do. The end result was seeing the mostly complete skull as a whole for the first time in 30 years. Our study reevaluated the genus-level identification, which we confirmed both by detailed anatomical study and by including N. palatina within a principal components analysis (PCA) of skull measurements – since earlier attempts at cladistic analysis of otariid relationships proved problematic. This analysis supported King’s placement within Neophoca, indicating that during some part of the Pleistocene, Neophoca either 1) had a broader range throughout New Zealand and Australia, or 2) arrived in New Zealand (perhaps from a South American ancestor) prior to Australia. The absence of diagnostic fossils from Australia precludes testing these hypotheses, however. In either scenario, we also found that N. palatina likely had a greater tolerance for colder water temperatures than N. cinerea.