Wednesday, July 19, 2023

Fossil prep updates: beluga and fur seal specimens from the Purisima Formation of California

We had some great luck while on our May field trips in coastal California, as reviewed in my last couple of posts. Sarah and I have added a few more scientifically significant specimens to the faunal list for this locality. The fossil vertebrate assemblage from the Purisima Formation is certainly not the most numerous in terms of specimens, but is rapidly becoming the most completely published. In 2013 I published a hefty monograph on the marine mammal assemblage from the locality, following an earlier paper reporting all of the sharks, bony fish, and sea birds - and a followup that same year reporting the world's youngest well-dated bony-toothed bird fossil (Pelagornis). Localities like this are quite critical since they are the youngest diverse assemblages of marine vertebrate fossils from our coast: Pleistocene marine vertebrate assemblages are poorly known from the west coast and typically non-comparable in terms of diversity, given that they are typically 'singleton' records and not evenly sampled for both "lower" and "higher" vertebrates. The new records I'll be discussing in this post are marine mammals, though we've made some interesting new non-mammal additions which I'll cover in a later post. 

 This photo is unintentionally hilarious - I was *so* happy by this fossil! Sarah made no mention of the fact that you cannot tell what the hell I am actually holding, it just looks like a shitty little chunk of sandstone. Didn't matter, I was so happy, and this is far funnier than if it was this spectacular field glory photo.

The first noteworthy specimen is a partial mandible with teeth of a tiny little fur seal. Pinniped remains are quite rare, outnumbered by cetacean fossils in the Purisima Formation by at least 10 to 1 or 20 to 1 (they are over-represented in UCMP collections, but even then still outnumbered by cetaceans by 5 to 1). Most pinniped fossils from the Purisima Formation consist of isolated limb elements and teeth; skulls, mandibles, and fragments thereof are considerably more rare. Mandibles are frequently diagnostic within pinnipeds, and so I had always been on the look out for one at this locality. I had previously collected a partial radius of a dusignathine walrus (Dusignathus sp.) and a very nice calcaneum and upper third incisor from a small fur seal - along with specimens collected by Larry Oliveira (such as a nearly complete but very small humerus), I tentatively identified this fur seal as the extinct Callorhinus sp., cf. C. gilmorei.

The holotype (female) mandible of Callorhinus gilmorei from the Pliocene San Diego Formation.

Referred mandibles of Pithanotaria starri from the upper Miocene Monterey Formation of Orange County, CA. From Velez-Juarbe (2017: PeerJ 5:e3022) .

Callorhinus gilmorei just so happens to have a pretty distinctive mandible, even if the teeth aren't terribly distinctive. It is one of the few fossil otariids where only a couple of the premolars are single rooted, and the rest are double rooted; the original specimen is a fragmentary but nonetheless well-preserved female skeleton from the coeval San Diego Formation. Other mandibles show a bit of variation, and at least one mandible has all double-rooted teeth. You see, pinnipeds evolved from terrestrial mammals that had double rooted premolars and triple rooted molars, and this condition was retained in the earliest pinnipeds like Enaliarctos. Early fur seals and sea lions (Otariidae) from the middle and late Miocene, such as Eotaria, Pithanotaria, and Thalassoleon, mostly have double rooted teeth - though all modern fur seals and sea lions have single rooted teeth. 

 Partially prepared mandible fragment with in situ premolars collected back in May.

This new mandible I collected in May ended up having all double-rooted teeth, which alone would not prevent this specimen from representing Callorhinus gilmorei. However, unlike this Pliocene species, this specimen has relatively smooth enamel that lacks accessory cusps and completely lacks lingual cingulum: cingulum is a little ridge on the base of the tooth crown. Most modern fur seals and sea lions have cingulum along the tongue side (lingual = tongue side) and only a few sea lions have labial cingulum. This specimen, however, has neither labial nor lingual cingulum, and the one complete tooth is also a bit low crowned. These features make this specimen resemble Pithanotaria - which, so far, is only known from the late Miocene (Tortonian, ~9-11 Ma). This specimen is probably, but uncertainly early Pliocene in age.

Another such specimen I found ended up being the anterior (chin) end of a mandible of a relatively large fur seal, collected from a bonebed dating to the early Pliocene - and a bit further up-section. After a bit of preparation, it ended up having single-rooted first and second premolars, identifying the specimen as Callorhinus gilmorei, and likely confirming my earlier identification of specimens of Callorhinus sp., cf. C. gilmorei from these same layers.

 The beautiful beluga periotic I collected in May! My very first monodontid periotic. I've only been looking for one for 15 years. Even on the east coast where they're more common!

 The new associated periotic and bulla of a small monodontid, collected under permit from the Purisima Formation.

One of the other really interesting specimens that I collected was an associated periotic and tympanic bulla of a large odontocete - which I was immediately able to identify to the family level in the field, owing to its distinctive anatomy: it is from a white whale, family Monodontidae - closely related to modern belugas and narwhals (and probably more the former). If you're unfamiliar with the fossil record of monodontids - and, surprisingly, I've never ever covered them here before (I should change that) - they parallel the walrus fossil record in a few surprising ways. First, they show up during the late Miocene, and have a similar distribution in the temperate and even subtropical North Pacific and North Atlantic. Aside from a couple of fragmentary finds of fossil narwhals, most are attributable instead to "delphinapterines" - tuskless monodontids more closely resembling belugas (Delphinapterus) than the narwhal (Monodon). These include species like Denebola brachycephala from the latest Miocene of Baja California, Bohaskaia monodontoides from the early Pliocene of North Carolina, and the more recently named Haborodelphis japonicus from the early Pliocene of Japan. There is additionally an unnamed fragmentary monodontid from the Pliocene of Belgium briefly described by my colleague Olivier Lambert in 2007, and several unnamed species of monodontids in the late Miocene-Pliocene fossil assemblages of California known from partial to complete skulls, possibly representing two or more genera.


The fractured skull (left) and periotic (right) of the small pilot whale-convergent beluga Denebola brachycephala from the upper Miocene Almejas Formation of Baja California, Mexico. From Barnes (1984: PaleoBios 42:1-46).

The small beluga Haborodelphis japonicus from the lower Pliocene Embetsu Formation of Hokkaido, Japan. From Ichishima et al. (2019: Papers in Palaeontology 5: 323-342).

 The periotic and tympanic bulla of
Haborodelphis japonicus. From Ichishima et al. (2019: Papers in Palaeontology 5: 323-342).

Out of these three named fossil species, only Denebola and Haborodelphis have periotics preserved that can be directly compared with my new specimen. One unnamed monodontid from a different locality in the Purisima Formation a few miles away has periotics that are a bit dorsoventrally thicker than my new one. The new one seems to have a proportionally larger anterior process than Denebola, and more closely resembles Haborodelphis. The tympanic bulla, on the other hand, is perhaps a bit more informative: the thickened part of the bulla, the involucrum, is absurdly thick anteriorly in Denebola and bears a long anterior tip; the new specimen looks quite a bit more like Haborodelphis and many other delphinoids. The bulla of the new specimen is only 3/4 the size of Denebola, yet the periotic is somehow larger. These are cursory observations, but I'm inclined to identify the specimen as cf. Haborodelphis for the time being. Finding fossils of marine mammals in California belonging to species originally discovered in Japan should not be surprising: Desmostylus and Paleoparadoxia (now Neoparadoxia), Callorhinus and Eumetopias, Herpetocetus, Hydrodamalis, and many others have been found in both places.