Friday, October 19, 2018

New publications on middle Miocene walruses of southern California and Baja California Sur

One of the first things I did when I started working at the Natural History Museum of Los Angeles County (MNHLA) was getting familiarized with the collection of fossil marine mammals. Coming in I knew it was (and is) one of the best in the World, but I was still surprised with the extent and amount of fossil pinnipeds. Pinnipeds is the name given to the group that include walruses (Odobenidae), sea lions/fur seals (Otariidae), seals (Phocidae) and a few other related, but extinct groups (e.g. desmatophocids). My first thought was that I would eventually get around to work on pinnipeds after several years, thinking I would first focus more on the cetaceans and herbivorous marine mammals. But then the more I looked at pinnipeds, the more interested I became in their fossil record and what new information we could learn about their evolutionary history based on the specimens at NHMLA and other nearby collections. And so, my interest in this group grew and I found myself working on pinnipeds much sooner than I had originally planned. My first scientific publication on the subject was last year when I described the pan-otariid Eotaria citrica, based on a well-preserved mandible and lower dentition which was published in the open access journal PeerJ (you can download the paper here (Velez-Juarbe, 2017) and also read my post about it).
On the left, the type mandible of Eotaria citrica; on the right, a life reconstruction of how it might have looked like.

The dwarf walrus from Baja California Sur
More recently, my colleague Fernando Salinas-Márquez and I described a new species of walrus that we dubbed Nanodobenus arandai, aka Smallrus or Aranda's dwarf walrus if, you know, you're not into the whole brevity thing.
Myself, holding the type (name-bearing specimen) of Nanodobenus arandai; on the right, the skull of the living walrus, Odobenus rosmarus.
Nanodobenus comes from a very interesting locality in the Vizcaíno Península in Baja California Sur. It was collected from sediments that are part of the Tortugas Formation as exposed along Arroyo La Chiva (also known as Arroyo Tiburón) (Throughton, 1974; Applegate et al., 1979; Moreno-Ruiz and Carreño, 1994). The age range of this formation is somewhat uncertain, or more precisely, broader than I would prefer it to be, as it has been described as ranging from the middle to the late Miocene (Barnes, 1998). For this work, Fernando and I reviewed the scientific literature and were able to restrict its range to 15.7-9.2 Ma, which is admittedly still fairly broad, but we'll hopefully be able to get more precise dates in the not so distant future.  
A more detailed view of the holotype mandible of Nanodobenus arandai, in lateral (top), medial (middle) and occlusal (bottom), views. Modified from Velez-Juarbe and Salinas-Márquez (2018:fig. 2).
 One of the most striking features of Nanodobenus was its small body length, which, based on measurements of the toothrow, was estimated to be around 1.65 m. This places it as the smallest odobenid known so far (see body size outlines below), with a body length more similar to that of the early otariid Pithanotaria starri and around the range of the living fur seal Callorhinus ursinus (Velez-Juarbe, 2017). The small body length of Nanodobenus seems to break a pattern in odobenid evolution, where body size increase seemed to be the norm throughout the mid-late Miocene, culminating in the gigantic walrus Pontolis magnus (Velez-Juarbe, 2017; Boessenecker and Churchill, 2018). The absence of Nanodobenus from contemporaneous units farther north in California could have been due to a number of reasons, like habitat preference, local endemism, or simply that they were there, but we still haven't identified its remains in these other units. Interestingly, dwarfism in odobenids seems to parallel a pattern observed in the South Pacific and the North Sea and Paratethys, where dwarf seals were sympatric with larger species.
Relationships and body sizes of odobenids. Modified from Velez-Juarbe and Salinas-Márquez (2018:fig. 4).
Our paper describing Nanodobenus was published in the open access journal Royal Society Open Science, which means you can download it for free (Velez-Juarbe and Salinas-Márquez, 2018).

Pinnipeds of Sharktooth Hill
Just out today (10/19/2018) is my most recent publication. In it I describe in detail the lower jaw and dentition of the early odobenid Neotherium mirum - originally named by none other than Remington Kellogg in 1931 - in it, I also describe some other significant pinniped mandibles from the Sharktooth Hill Bonebed (STH). I've written about Sharktooth Hill before and I have to say it is one of the most amazing middle Miocene marine deposits in the World! This bone
bed was deposited between 15.9-15.2 million years ago near Bakersfield, CA, and is one of the densest accumulations of marine vertebrates that we know of, with a fairly large number of taxa described so far (e.g. Kellogg, 1931; Howard, 1966; Mitchell, 1966; Barnes, 1988; Lynch and Parham, 2003; Pyenson et al., 2009; Velez-Juarbe, 2018).
Pinniped remains abound in the Sharktooth Hill bonebed, with the second most common one being Neotherium mirum; however, relatively little is known of this species. The original description by Kellogg (1931) was based on postcranial elements, and since then a few papers have discussed and/or described some of its cranial and mandibular features (Mitchell and Tedford, 1973; Repenning and Tedford, 1977; Barnes, 1988; Deméré, 1994; Kohno et al., 1995). In my new publication I described the morphology of the mandible and lower teeth in detail, based on several specimens from the collections at the Natural History Museum of Los Angeles County and the University of California Museum of Paleontology (UCMP). One other aspect I set out to explore was wether Neotherium showed body size sexual dimorphism as previous authors had suggested (Repenning and Tedford, 1977; Barnes, 1988). The morphology of the lower jaw of Neotherium shows an interesting mix of characters, some which are shared with early pinnipedimorphs and others which it shares with more derived odobenids. Size difference amongst the several mandibles I studied support the hypothesis that the species is sexually dimorphic, with male mandibles being around 25% larger than the female mandibles. This was similar to the differences amongst male/female mandibles of Steller's sea lion (Eumetopias jubatus), but less extreme than those shown between males/females of California sea lion (Zalophus californianus).
Various mandible of Neotherium mirum, representing male (A-D, F, and H), and female (E, G, and I) individuals. Modified from Velez-Juarbe (2018:fig. 1).
Neotherium mirum is not the only pinniped known from the Sharktooth Hill Bonebed; two other walruses and two desmatophocids are also known from this unit. One of the other walruses is Pelagiarctos thomasi, which was a relatively large odobenid with fused mandibles and bulbous teeth; its morphology is still poorly known as only a few specimens are known, making it one of the rarest pinnipeds from this unit (Barnes, 1988; Boessenecker and Churchill, 2013). The second walrus is even rarer and was one of the other subjects of my paper. While browsing the collections I came across a mandible that did not look like any of the other STH pinnipeds, so I decided it would be an interesting addition and worthy of describing even if I couldn't pinpoint its precise id because of its incompleteness. This other mandible which I identified as belonging to some unknown odobenid was intermediate in size between male Neotherium mirum and Pelagiarctos thomasi, and showed some interesting characters regarding reduction of the postcanine dentition. Specifically, this unknown odobenid showed a bilobed first molar (m1) while still having double-rooted second through fourth premolars (p2-4). This pattern of root reduction departs form the usual pattern observed amongst odobenids where root simplification usually begins with the more anterior premolars and moves backwards towards the molars.
Phylogenetic analysis showing relationships amongst odobenids and other pinnipeds, highlighting reduction of the postcanine teeth across the different groups. Modified from Velez-Juarbe (2018:fig. 5).
Reduction and simplification of the postcanine dentition is a phenomenon observed across different pinniped groups (Repenning and Tedford, 1977; Barnes, 1989; Deméré, 1994; Barnes et al., 2006; Boessenecker, 2011). Based on the analysis in my paper it seems that simplification/reduction of the postcanine roots in odobenids has occurred several times throughout their evolutionary history (see phylogenetic tree above).
The other pinnipeds from STH are the desmatophocids Allodesmus kernensis and A. cf. A. sadoensis (or A. cf. A. sinanoensis according to Tonomori et al., 2018). The precise number of valid species of Allodesmus from STH has been a issue discussed in the literature for quite some time, with up to three species considered valid: Allodesmus kernensis, A. kelloggi, and A. gracilis (Kellogg, 1922; Mitchell, 1966; Barnes and Hirota, 1995). Fortunately this was resolved recently by Boessenecker and Churchill (2018) who determined that A. kelloggi and A. gracilis are junior synonyms of A. kernensis. In their study they also determined that the morphology of a fragmentary mandible that had been referred by Barnes (1972) as 'Desmatophocine B' was more consistent with Allodesmus sadoensis and referred to it as A. cf. A. sadoensis. The presence of this species in California, otherwise known from middle Miocene deposits in Japan, was further confirmed in my study, where I described and assigned a second mandible to that particular taxon. However, the mandible I assigned to this species was quite rare, with only four postcanine teeth, instead of the more normal five or six; did this represents a more extreme case of tooth reduction or a pathology? We can't tell for now, but hopefully additional specimens will turn up in other collections which will help clarify this. So at present there are up to five species of pinnipeds that were likely sympatric during the deposition of the Sharktooth Hill Bonebed, making it a fairly diverse pinniped assemblage, and unlike anything we see today, especially since there's only one living species of walrus.
Mandibles representing pinnipeds from the Sharktooth Hill Bonebed. Modified from Velez-Juarbe (2018:fig. 6).
This paper on Neotherium and other STH pinnipeds is out on Journal of Vertebrate Paleontology, if you are interested in a PDF feel free to email me and I'll send it your way!

The Titan walrus of Orange County
Out this month too was the publication another work with colleagues Isaac Magallanes, Jim Parham and Gabe Santos. In this work (Magallanes et al., 2018) we described a fossil odobenid which we dubbed Titanotaria orangensis from the late Miocene of Orange County, California. In addition to describing this interesting new species, we took a closer look at the fossil record of odobenids as well as revised the taxonomy of the group. I was honored to have been invited to participate in this project and I will be discussing it in more detail in a future post. Meanwhile, you can download the PDF and start reading it as it is open access. 

Even more marine mammal news coming soon, so stay tuned!!!!


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