Was Pelagiarctos a "killer" walrus? Part 5: life restoration

Now that I’ve published research that made the front page of the fox news website, I can consider myself satisfied as a scientist. The Pelagiarctos article has made some additional press, including Science World Report, the Orange County Register, the Otago Daily Times (our local newspaper), and my favorite popular article, titled "Ancient "killer walrus" cuter than originally thought." Also, take a look at Brian Switek's write up on Laelaps.

 

It's in there, in between the articles about the second amendment and how the government 

will be coming after you.

I thought I’d go ahead and explain in a post about the making of the life restoration of Pelagiarctos, and discuss what it may have looked like in life. As detailed in our phylogenetic analysis, Pelagiarctos is most closely related to the late Miocene walrus Imagotaria downsi. Imagotaria downsi is known from the Santa Margarita Sandstone and Sisquoc Formation of Northern and southern California (respectively), and is early late Miocene in age (Tortonian stage – 9-12 Ma). The type specimen of Imagotaria downsi is sort of a cruddy specimen, but a beautiful collection of well preserved fossils was reported by Charles Repenning and Richard Tedford in 1977 from the Santa Margarita Sandstone near Santa Cruz, California. Two skulls including an adult female (“Rep’s Girl” as some marine mammal paleontologists call it) and a subadult male are sea lion like in their morphology. They have long snouts, the skulls are flat-topped, and have low saggittal crests, large canines, and deep mandibles. Although superficially sea lion-like in general form, they lack the supraorbital shelves typical of otariids.

The female skull of Imagotaria downsi from Santa Cruz, affectionately known as "Rep's girl".

The lack of similarity between Pelagiarctos and the modern walrus is evident in its morphology. In the modern walrus, the canines are reduced in size to small pegs, no larger than the premolars (and are thus called ‘premolariform’); the incisors are totally absent, and the “chin” of the modern walrus mandible tapers to a triangular point that lacks teeth and instead has a longitudinal furrow. The transverse tapering of the mandible accommodes the hugely expanded tusks, which for the uninitiated, are the upper canines. The reduction in size of the lower canines presumably also permits enlarged upper tusks – while the complete loss of incisors probably allows an unobstructed pathway for suction into the oral cavity (see the section on odobenid dental and mandibular evolution in Odobenidae in our paper).

 

Rough line drawing of "Rep's girl".

Instead, Pelagiarctos – like Imagotaria – appears to be rather sea lion-like in overall morphology, perhaps something similar to one of the larger, more robust sea lions such as the New Zealand sea lion (Phocarctos hookeri), South American sea lion (Otaria byronia), or Steller’s sea lion (Eumetopias jubatus); the mandible of California sea lions is noticeably smaller and less ‘deep’ than Pelagiarctos. So, we have an overall idea of the shape of the head of Pelagiarctos. To start, I took a line drawing of the skull and mandible of Imagotaria downsi, and in adobe illustrator reduced them to the same mandible size. Then, I took the facial part of the skull, and shortened it to fit the short toothrow of Pelagiarctos. Figuring that the rostrum of Pelagiarctos would have probably been deeper and more robust like the mandible, I also made the facial region more dorsoventrally deep.

 

Cranial reconstruction of Pelagiarctos sp. based on the proportions of Imagotaria downsi.

Then, using this new skull reconstruction, I sketched it in an oblique view by using a reference photo of an Imagotaria skull in the same oblique view, while adding in the changes in proportions. I made sure to sketch it in an angle where I had a photograph of the new Pelagiarctos specimen, so that I could later on digitally overlay the photo of the mandible.

 

 

What did Pelagiarctos look like? More like a sea lion, or a fellow walrus? Due to its early position and early timing in pinniped evolution, it probably had some sort of external ears. It was not a gigantic cold-adapted pinniped like the walrus, so it may have primitively retained abundant fur or hair. It's long whiskers as I've reconstructed it are consistent with a piscivorous habit, rather than the molluskivorous feeding behavior of the modern walrus. My initial black and white graphite drawing is shown here.

So, we’ve got a general shape of the skull and head – but what would it look like? There are still a couple more considerations. For example – would it have had fur? Long vibrissae (whiskers), or short vibrissae? Would it have had thick blubber instead, like the modern walrus? And what about ears – modern walruses (and true seals) don’t have external ears, but sea lions and fur seals (Otariidae) have dinky little ear flaps. Before I continue with this discussion, I must stress that this is all highly speculative. Using the “extant phylogenetic bracket”, and assuming that molecular analyses have correctly identified sea lions and walruses as sister taxa, we can infer that it would look closest to a sea lion or a modern walrus. Okay, that basically includes all of the aforementioned features. The modern walrus is technically closer – but it is a highly derived animal, whereas Pelagiarctos is a very generalized sea lion-like pinniped. I reconstructed Pelagiarctos with external ear flaps to reflect the fact that most early pinnipeds probably had ear flaps or even large external ears (e.g. like an otter). After all, external ears are primitive, and it would be silly to assume that true seals and walruses have lacked ears throughout their evolutionary history. Considering the molecular support for a sea lion + walrus clade, it appears that external ear loss is convergent in the walrus and true seals anyway. What about fur, then? Only a few pinnipeds truly lack dense fur or hair – the walrus, and the elephant seals. The southern elephant seal and the walrus are both high latitude, cold water adapted - but they are also substantially larger than Pelagiarctos. Given the temperate latitude and similarity in size of Pelagiarctos with modern sea lions, which lack fur but have dense hair – it can be parsimoniously reconstructed as “fuzzy”. On that note, I really ought to talk about Heather Liwanag's awesome study on the evolution of marine carnivore fur/hair. Lastly, I reconstructed it with long whiskers because it’s a pelagic hunter – the short, stubby whiskers of the modern walrus are an adaptation for “feeling” benthic invertebrates and sediment.

And the final reconstruction, all colorized and everything. I had a lot of fun doing this reconstruction, and it seemed to do the trick.

Anyway, this concludes my series of posts on the new study by Morgan and I. Hope you enjoyed reading it (and hopefully the actual paper as well).

References-

Boessenecker, R.W. and M. Churchill. 2013. A reevaluation of the morphology, paleoecology, and phylogenetic relationships of the enigmatic walrus Pelagiarctos. PLoS One 8(1) e54311. doi:10.1371/journal.pone.0054311.

Epic fossil whale collage

I walked into my labmate Cheng-Hsiu Tsai's office recently and saw that he had a new desktop background - there were some specimens I didn't even recognize. It's a collage he made with nearly every published fossil mysticete he could find (and some modern mysticetes). Tsai, who is currently on a two month research trip to Taiwan, Japan, and Australia - said it would be OK for me to share it here with you all - so, enjoy!

Was Pelagiarctos a "killer" walrus? Part 4: No, probably not. The feeding ecology of Pelagiarctos reassessed

Sorry for the short break – I’ve been fairly busy the last two weeks doing a number of things, but most of all revising a manuscript that was accepted last week on barnacle-encrusted sea lion bones from Oregon. I’m also pleasantly satisfied with additional press attention our Pelagiarctos article got over the last couple of weeks, and the PLOS metrics indicates it’s already gotten 3,000 views, which is quite a few more than my 2011 fur seal article (under 100), although that’s probably because it’s 1) linked to in all the news articles and 2) not paywalled.

Comparison of the holotype (B, C) and referred mandible (A) of Pelagiarctos. From Boessenecker and Churchill (2013).

When Morgan and I started our research on the new specimen of Pelagiarctos, we realized that the most interesting application of the research would be reevaluating the interesting hypothesis by Barnes (1988) that Pelagiarctos thomasi was a specialized macrophagous predator. To recap, Barnes interpreted this novel hypothesis based on several lines of evidence: 1) Pelagiarctos thomasi is relatively rare in the Sharktooth Hill Bonebed relative to other pinnipeds (apex predators are rare because they require a large population of prey items to subsist upon), 2) sharp postcanine teeth roughly similar in morphology to those of bone-cracking hyaenids and borophagine canids, 3) a fused mandibular symphysis, suggesting an adaptation towards high bite force, and 4) large body size. In addition to these interpretations, Barnes (1988) further interpreted all the isolated teeth as being from males – as the holotype “chin” has large canines, and at least one of the postcanine teeth slipped right in to the empty alveolus of the holotype. The rest of the postcanines are of similar size, suggesting to Barnes that they were all from one gender. Most pinnipeds – and all fossil walruses for which we have sufficient sample sizes (even wee little Proneotherium) are sexually dimorphic, with larger males. Furthermore, one of the holotype canines was broken in life and then worn from continued use, which Barnes (1988) interpreted as the result of male-male combat. Barnes further speculated that the lack of females could be caused by geographic separation of sexes – certainly an intriguing possibility, but difficult to test with such a small sample size.

An example of a vagrant pinniped: a leopard seal that wound up on a New Zealand beach. Apparently this happens somewhat often, but regardless - New Zealand is not in the normal range of this animal. From keaphotography.org

We identified several other hypotheses which could just as parsimoniously explain the rarity of Pelagiarctos within the Sharktooth Hill Bonebed. Given the extremely large sample size of fossil vertebrates from the bonebed, and a century of intensive collecting, the rarity is probably a real phenomenon and thus probably not a result of preservational bias (least of which because Pelagiarctos is a large animal and has a higher preservation potential than the smaller but numerically more abundant Neotherium). Numerous studies of modern pinnipeds have demonstrated that they are prone to vagrancy – in the ocean, after all, it’s easy to get caught up in currents or forage further away than other members of your species. It’s also possible that Pelagiarctos was simply a pelagic, offshore pinniped, rarely straying into coastal waters off Orange County (“Topanga” Formation) or the Temblor Sea (Sharktooth Hill). Furthermore, the Sharktooth Hill Bonebed was deposited over a protracted period of time (~700,000 years) due to a depositional hiatus, and it’s possible that a short period of time could have seen introduction of Pelagiarctos (from further south, north, or further out in the Pacific) along with a brief change in climate, ecology, or circulation.

 

A sea lion and a fishy smorgasbord. Photo by David Doubilet.

The fused mandibular symphysis is a bit more ambiguous. The only modern pinniped with a fused symphysis is the extant walrus Odobenus rosmarus, and it’s not immediately clear why, or how it could be adaptive relative to feeding. My hunch is that it’s a secondary consequence of having a pachyosteosclerotic mandible – the lower jaw of Odobenus is thickened around the chin and dense, and development of this may have resulted in symphyseal fusion. The earlier walrus Alachtherium/Ontocetus has an unthickened chin and lacks fusion – but Valenictus chulavistensis (the strange sister taxon of Odobenus) has a fused symphysis, and also lacks a thickened “chin”. Extreme pachyosteosclerosis of the skull and mandible in Odobenus has been suggested as a possible adaptation for keeping the head negatively buoyant during benthic foraging. One other fossil odobenid has a fused symphysis – Dusignathus seftoni from the San Diego Formation. It’s not even clear what Dusignathus ate, so it’s not a very good analogue either. Barnes (1988) argued that a fused symphysis in Pelagiarctos thomasi suggested and adaptation for large bite forces – however, carnivorans with high bite forces such as borophagine canids and hyaenids (which Barnes compared the dentition of Pelagiarctos to), as well as sea otters – all have unfused symphyses, possibly to allow slight movement of the mandibles so as to avoid tooth damage (Scapino, 1981). So what was mandibular fusion in Pelagiarctos thomasi for? Who knows! That’s for someone else to figure out. Besides, the new specimen didn’t have a fused symphysis anyway.

 

Comparison of calculated trophic level and body mass in pinnipeds. Lower trophic level corresponds to feeding upon benthic invertebrates, and high trophic level corresponds to eating large fish and cephalopods. From Boessenecker and Churchill (2013).

 

A walrus after a successful seal kill. These events have only been witnessed a few times by humans. Apparently, walruses will use their powerful suction normally reserved for mollusks to literally suck the meat right from the bones. From moblog.net.

  

 Another gory shot of a walrus feeding on a poor seal. From Vlasman and Campbell, Diseases and Parasites of Mammals of the Eastern Arctic.

We reinvestigated the issue of body size as well. Morgan has been working on a method to estimate the body mass of fossil pinnipeds, and his preliminary results indicated that the length of the lower toothrow is the single best predictor of body mass in a dataset of modern pinnipeds (fortunate for us, since all we had was a mandible to work with). Morgan was able to estimate the body mass of Pelagiarctos sp. at approximately 350 kg (~770 lbs), which is similar to some modern sea lions (male South American sea lions, California sea lions). There are much larger sea lions, however – the Steller’s sea lion (Eumetopias jubatus), which weigh up to 1,150 kg (2,500 lbs). Steller’s sea lion is the fourth largest pinniped (after the two elephant seals and the modern walrus), but is not a macrophagous predator: it feeds predominantly on fish, although it will occasionally prey upon juvenile pinnipeds. The modern walrus also occasionally preys upon juvenile seals and marine birds – yet it is clearly adapted and specialized for mollusk predation. All large bodied otariids predominantly eat fish as well, and many adult male sea lions of other species as well will also occasionally consume warm blooded prey when given the opportunity. Morgan further investigated this by calculating the trophic level of modern pinnipeds and plotting it relative to body mass. The result is that there is no apparent trend between body mass and diet – with one exception: the largest pinnipeds fed both at the high and low trophic levels (e.g. fish, cephalopods, as well as benthic invertebrates – e.g. walruses and bearded seals). In other words, large body mass doesn’t necessarily indicate anything specific about feeding ecology or diet. A previous analysis by Peter Adam and Annalisa Berta (2002) only found a very poor correlation between morphology and diet. That evidence of adaptations for macrophagy is lacking within pinnipeds is highlighted by the leopard seal: it does not use its postcanine teeth to feed upon penguins and seals, and only uses them for filter feeding for krill the rest of the year (8 months or so out of the year). When it feeds upon large bodied prey, it nips with its incisors and canines – which are not really any different from those of fish-eating pinnipeds. In other words, the only modern pinniped which could be argued to be a macrophagous apex predator only has dental specializations for feeding upon krill.

Arguably the only macrophagous pinniped - the leopard seal spends most of the year eating krill, and doesn't use its delicate postcanine teeth for killing penguins and seals, and instead only for filter feeding. Figures from Hocking et al. (2013).

So, not a "killer" walrus after all, but still a pretty intimidating beast.

So what did Pelagiarctos feed on? Probably fish, cephalopods, the normal menu for large bodied pinnipeds. It very well probably did feed upon warm blooded prey – occasionally, anyway (again, like modern pinnipeds). We’re not arguing that Pelagiarctos did NOT eat warm blooded prey – rather, we’ve made the case that Pelagiarctos lacked any adaptations which would lend themselves to macrophagy. One last point of interest – within the Sharktooth Hill Bonebed, Pelagiarctos is not even the largest pinniped; it’s dwarfed by Allodesmus, which Morgan estimated at 1400 kg (~3,000 lbs)! That’s just enormous (and approximately ¾ the size of modern northern elephant seals). Allodesmus, on the other hand, had enormous orbits possibly indicating deep diving adaptations (to which its large body size may have helped with as well), and a long snout with simple teeth – a definite contrast to the short “bulldog” face and dentition of Pelagiarctos.

References:

Adam PJ, Berta A (2002) Evolution of prey capture strategies and diet in the Pinnipedimorpha (Mammalia, Carnivora). Oryctos 4: 83–107.

Barnes LG (1988) A new fossil pinniped (Mammalia: Otariidae) from the middle Miocene Sharktooth Hill Bonebed, California. Contributions in Science, Natural History Museum of Los Angeles County 396: 1–11.

Boessenecker RW, Churchill M (2013) A Reevaluation of the Morphology, Paleoecology, and Phylogenetic Relationships of the Enigmatic Walrus Pelagiarctos. PLoS ONE 8(1): e54311. doi:10.1371/journal.pone.0054311

Hocking, D.P., Evans, A. R., and E.M.G. Fitzgerald. 2013. Leopard seals (Hydrurga leptonyx) use suction and filter feeding when hunting prey underwater. Polar Biology 36:2:211-222.

Scapino R (1981) Morphological investigations into functions of the jaw symphysis in carnivorans. Journal of Morphology 167: 339–375

Was Pelagiarctos a "killer" walrus? Part 3: new specimen from Orange County

In early spring 2011, just as I was finishing up my master’s degree at Montana State, I received an email from Tom Deméré, the paleontology curator of the San Diego Natural History Museum, inviting Morgan and I to study a new fossil of Pelagiarctos from the “Topanga” Formation. Fortunately, I would get a chance to examine it closely in person soon afterward – in June, I would be attending and presenting my master’s taphonomy research at the 6^th triennial conference on secondary adaptations of tetrapods to life in the water (usually abbreviated SATLW or simply referred to as the aquatic tetrapods conference), which was being hosted by Tom Deméré and Annalisa Berta at San Diego State University and the museum.

Although I had successfully delivered my master’s defense presentation and graduated without a hitch a month and a half prior, I was still nervous to give my presentation because it was in front of a totally different audience – technically, the conference was about secondary adaptations, and I was giving a talk on taphonomy. However, I tooled it towards what we can reasonably infer from the marine vertebrate fossil record, including about exactly how aquatic organisms were based on their preservation – which, I concluded at the time was not much. The talk was also fairly long; although I had 18 minutes to speak, which is fairly long, I had not had the time to shorten it. 36 hours before driving down I-5, I was on the beach at Bolinas prospecting with Dick Hilton when I got a funny phone message from Tom ‘asking’ me if it would be okay to move my talk to the first day; so I said my goodbyes to Dick and raced home down Highway 1 so I could spend a day and a half polishing the presentation off. And then worried half the drive down I-5 that I didn’t shorten it enough.

 

  

A comparison of the new Topanga Formation specimen (A) and the holotype (B) of Pelagiarctos.

 

The talk went without a hitch, and later in the conference Morgan and I were able to sequester a few hours in the SDNHM type room to examine the new specimen of Pelagiarctos. It consisted of a fragmentary pair of mandibles, with the left mandible being nearly complete and having much of its dentition (missing only a premolar, the two molars, and an incisor). Unlike the type specimen from Sharktooth Hill (which Morgan and I got a chance to examine in person at LACM in January 2012), these mandibles were not fused together at the symphysis (intermandibular joint). Symphyseal fusion is not common in modern pinnipeds, where it is restricted to the modern walrus. I’ve also seen pathologic (diseased) mandibles of modern otariids where, due to some bone disease, the symphysis has fused along with a large mass of bone at the chin, accompanied by incisor and canine loss.

 

  

 The left mandible of the Topanga Formation specimen of Pelagiarctos. From Boessenecker and Churchill (2013).

The teeth present in the new specimen confirm that the large teeth referred to Pelagiarctos thomasi by Barnes (1988) were correctly referred. It’s not so surprising, since you could predict the mandible shape from the teeth: they are like giant versions of Neotherium teeth, and the mandible is like a giant Neotherium jaw. I never really doubted Barnes’ identifications – but it was nice to confirm them. The mandible of this new specimen is damned huge – it’s wide, deep, with a short toothrow. The canines are robust, again with grooves on the lateral and medial surfaces, giving the canine a figure-eight shape in cross section. The premolars are large, primitively retaining what’s called the metaconid cusp; most modern pinnipeds have teeth that are unicuspid (single cusp, usually conical), with small anterior and posterior accessory cusps in otariids. The main cusp (on lower postcanine teeth) is called the protoconid. The anterior and posterior cusps are the remnants of the paraconid (anterior) and hypoconid (posterior) cusps. The metaconid cusp is a fourth cusp which is present in many primitive pinnipeds, such as the early enaliarctines, as well as Proneotherium, Neotherium, and Pelagiarctos. The metaconid is located just behind the principal cusp (protoconid). Many modern phocids primitively retain all four cusps – the harbor seal is an excellent example. The crabeater seal additionally bears a number of extra “neomorphic” (=new or novel structure) cusps on the posterior tooth crowns, which are used for filter feeding.

 

 

The dentition of the Topanga Formation specimen. From Boessenecker and Churchill (2013).

More details of the dentition of the Topanga Formation Pelagiarctos, from Boessenecker and Churchill (2013); I had a fun time drawing the medial view of those teeth.

In addition to having these primitive features, a couple of new features not seen in earlier walruses are present: a lingual cingulum with small little “crenulations” forming a sawtooth type pattern, and the presence of a labial cingulum. A cingulum is a thickened ridge of enamel at the base of a tooth crown. Pelagiarctos is the only walrus with a labial (cheek side of the tooth) cingulum, and only one other walrus has a crenulated lingual (tongue side of the tooth) cingulum – the late Miocene walrus Imagotaria downsi. At this point the uninitiated reader might ask ‘what exactly makes this thing a walrus?’ The truth is, for the earliest known walruses, the only synapomorphies allowing identification as a member of the Odobenidae (walrus family) are skull features. Many of the features of the known specimens of Pelagiarctos appear in some sea lions – such as a mandible that is deepest near the canines. Although the fossils don’t have any specific features that are diagnostic at the family level – several features of the dentition are only found in early diverging “imagotariine” walruses. The Imagotariinae was a subfamily named by Ed Mitchell and used extensively in various papers by Barnes, but as pointed out by several studies over the past two decades it is a paraphyletic assemblage of primitive walruses. Nevertheless, it is a useful vernacular term; imagotariines are sea lion-like with primitive dentitions, and ranged in size from harbor seal size (Proneotherium) to elephant seal size (Pontolis magnus).

 

 

 

Comparison of walrus mandibles, including the Topanga Fm. Pelagiarctos specimen, Imagotaria downsi from the late Miocene Santa Margarita Sandstone of Santa Cruz County, Proneotherium repenningi from the early middle Miocene Astoria Formation of Lincoln County, Oregon, and Pontolis magnus from the late Miocene Empire Formation of Coos County, Oregon. From Boessenecker and Churchill (2013).

 

Because our new specimen was more complete than the holotype, we were able to include Pelagiarctos within a phylogenetic analysis for the first time. Previous analyses did not use many mandibular characters, so at first we constructed a matrix which focused on mandibular and dental characters, and only used pinniped species known by lower jaws (i.e. we didn’t include some species for which jaws were unknown). This meant we didn’t initially include the early walruses Prototaria and Pseudotaria from Japan. We originally did this because we felt we’d get more accurate results than if we included Pelagiarctos in an analysis where it couldn’t be coded for any cranial characters – it was a reasonable hunch at first. One of our reviewers suggested we use a more comprehensive dataset, so we merged our data set with that of Naoki Kohno’s (2006) analysis for his Pseudotaria muramotoi paper. We ended up with fantastic results, and better support for some of the relationships.

 

 

Cladograms from Deméré (1994), Kohno (2006), and our new study showing the varying position of Pontolis (underlined in red).

Most of the relationships in our analysis are consistent with previous studies like Deméré (1994), Deméré and Berta (2001), and Kohno (2006), with one exception. In Deméré (1994), Pontolis magnus grouped as a dusignathine walrus, and closely related to Dusignathus itself. In Kohno (2006), Pontolis instead formed a sister taxon relationship with Imagotaria. The Imagotaria-Pontolis clade is only one node below the dusignathines, so admittedly it is not a far distance. In our analysis, however, Pelagiarctos formed a sister taxon relationship with Imagotaria instead, based on two features: grooved canines, and a crenulated cingulum. Neither of these features are present in Pontolis. Instead, Pontolis plotted out as the last diverging “imagotariine” and the sister taxon to the dusignathine + odobenine clade – in other words, intermediate between the phylogenetic hypothesis of Deméré (1994) and Kohno (2006). It’s sort of a compromise between the two, in a way. Obviously, there are more cranial characters that need to be explored and new walruses to describe, so there is clearly further scope for a more comprehensive study of walrus phylogenetics, which is in the early planning stages.

Next up: the dramatic conclusion to this series on the new publication, focusing on the feeding ecology of Pelagiarctos, and the life restoration.

References:

Barnes LG (1988) A new fossil pinniped (Mammalia: Otariidae) from the middle
Miocene Sharktooth Hill Bonebed, California. Contributions in Science,
Natural History Museum of Los Angeles County 396: 1–11.

Boessenecker, R.W. and M. Churchill. 2013. A reevaluation of the morphology, paleoecology, and phylogenetic relationships of the enigmatic walrus Pelagiarctos. PLoS One 8(1) e54311. doi:10.1371/journal.pone.0054311.

Deméré TA (1994) The family Odobenidae: a phylogenetic analysis of living and
fossil forms. In: Berta A, Deméré TA, editors. Contributions in Marine Mammal
Paleontology honoring Frank C Whitmore, Jr: Proceedings of the San Diego
Society of Natural History. 99–123.

Kohno N (2006) A new Miocene odobenid (Mammalia: Carnivora) from
Hokkaido, Japan, and its implications for odobenid phylogeny. Journal of
Vertebrate Paleontology 26: 411–421.

Was Pelagiarctos a "killer" walrus? part 2: new publication in PLOS

Last thursday my new study which I collaborated with Morgan Churchill (University of Wyoming) on was published in PLOS One, regarding new fossil material of Pelagiarctos from the "Topanga" Formation of Orange County, California. There has been quite a bit of buzz about it, and it's gotten a surprising amount of media attention. To summarize it in one sentence - we describe the new material, reanalyze the paleoecological hypothesis of Barnes (1988), concluded it was not a specialized macrophagous predator, and conducted a phylogenetic analysis of the Odobenidae (walruses).

Brian Switek was kind enough to cover it on Laelaps, which you can see here. Also, there is an author spotlight on the PLOS EveryONE blog, viewable here.

Part of the new specimen of Pelagiarctos, which Tom Deméré (San Diego Natural History Museum) invited us to study.

Life restoration of Pelagiarctos, which I did last fall in my spare time. More on how I put this together at a later point.

This has been covered by a ton of news outlets, including the University of Otago news service, ScienceDaily, LiveScience, MSNBC, NBC, Huffington Post, Cosmos, Yahoo News, and even Fox News (perhaps a dubious honor...).

The full article can be viewed here at Plosone.org.

Boessenecker, R.W. and M. Churchill. 2013. A reevaluation of the morphology, paleoecology, and phylogenetic relationships of the enigmatic walrus Pelagiarctos. PLoS One 8(1) e54311. doi:10.1371/journal.pone.0054311.

And, the morphobank account is available here.

Was Pelagiarctos a “killer” walrus? Part 1: Sharktooth Hill Pinnipeds

As a teaser for a forthcoming paper by Morgan Churchill and myself, I thought I’d introduce a (short) new series of posts (fewer than the last series, I promise). As the publication is not out quite yet, I thought I could at least give an introduction to the extinct “killer” walrus from the Sharktooth Hill Bonebed.

The Sharktooth Hill bonebed in Kern County, California is a widespread horizon within the Round Mountain Silt member of the Temblor Formation. It’s exposed near Bakersfield, California, and is middle Miocene in age. It’s approximately 10-50 cm thick, generally lacks calcareous invertebrate fossils, but is extraordinarily rich in teeth and bones of sharks, bony fish, birds, sea turtles, pinnipeds, dolphins, sperm whales, baleen whales, and occasionally sea cows, desmostylians, and terrestrial mammals. I visited Sharktooth Hill several times as a high school student, trying to find “local” vertebrate fossils – digging well through the night in the trenches with tiki torches and a headlamp. At many localities frequented by amateur fossil collectors, the bonebed is exposed on a hillside and a large linear scar follows the position of the bonebed, dug out by collectors removing overburden to get to the fossil layer. Amateur fossil collectors have done so much digging that a trench reminiscent of World War 1 battlefields encircles many hills in the region where the bonebed is exposed. Although some collectors will spend days at a time digging through overburden - admittedly backbreaking work - some decide to risk it and tunnel into the trench to get at the bonebed. Some collectors have paid for this tactic with their lives: on my first visit in 2002, a cross was placed at one of the localities where a collector had tunneled in about ten feet and was killed when the hillside slumped down onto him; it took the authorities several days to dig out his body. The rest of the Round Mountain Silt is mostly barren with respect to vertebrate fossils, not only explaining the attention given by collectors to the bonebed itself – but also suggesting a “unique” environment temporarily persisted in order to concentrate vertebrate remains. A number of strange biologic explanations have been offered, including red tides, extensive shark predation, and even a marine mammal calving ground. Several authors have quite rightly scrutinized these biologic explanations, and have suggested sedimentologic processes as a cause (Mitchell, 1966; Prothero et al., 2008; Pyenson et al., 2009). These studies have specifically suggested that a depositional hiatus (slowdown in the accumulation rate of sediment) permitted marine vertebrate remains to be concentrated on the seafloor. I have some minor taphonomic reservations, but those are best discussed another day.

 

One of the Sharktooth Hill localities, wife for scale.

According to Barnes (1976), the Sharktooth Hill bonebed is the most extensively studied and richest marine mammal locality in the eastern North Pacific; a faunal list compiled by amateur collectors can be viewed here, and it includes roughly 140 vertebrate taxa. Some of the species on the list are not yet described or published (“Neotherium ernsti”, for example) and other taxa are based on old identifications and may not be borne out in the long run (aff. Herpetocetus). Regardless of issues pertaining to the taxonomic identity of some fossil vertebrates, the ballpark number is probably accurate. It’s also fairly spectacular: I recently tallied up fossil vertebrates from the Purisima Formation, and there are roughly 70 taxa present – still impressive as hell, but not quite as gargantuan as Sharktooth Hill. Depending upon whose publication you look at, there are anywhere from seven (Barnes, 1972; Barnes and Hirota, 1995) to four pinnipeds present (Deméré et al., 2003). Papers by L.G. Barnes and colleagues list several desmatophocids, including Allodesmus gracilis, Allodesmus kelloggi, Allodesmus kernensis, Desmatophocine B, and Desmatophocine C in addition to the imagotariine walruses Neotherium mirum and Pelagiarctos thomasi. According to Deméré et al. (2003), only four taxa are present – Allodesmus kernensis (with A. kelloggi and A. gracilis subsumed as junior synonyms), an indeterminate desmatophocid (Desmatophocine B), and the two walruses. While it’s nowhere near as diverse as the cetacean assemblage from the same locality, it’s fairly comparable with other fossil pinniped assemblages from the eastern North Pacific.

 

The skeleton of Allodesmus kelloggi as exposed in the field. From Mitchell (1966).

            In 1980, future chief preparator of the Los Angeles County Museum of Natural History (LACM) discovered a curious chunk of bone with teeth at Sharktooth Hill. Several years later, he brought it in to LACM and showed it to Dr. L. G. Barnes (colloquially known as ‘Larry’ within the field), and insisted that it was the piece of a snout of some extinct mammal – it even had two small holes which look like nostrils to the uninitiated. Barnes kindly pointed out that those were mental foramina on the “chin” end of a very large jawbone of a pinniped. Larry and Howell enthusiastically recalled this whole story for Morgan Churchill and I when we sat at the very same table last January, thirty or so years later (Larry Barnes has an incredible, near photographic and certainly encyclopedic memory of marine mammal fossil specimens). Howell Thomas donated the fossil for study, and within a few years was hired as the Chief Preparator, and Barnes began to study the specimen. At the time, the marine mammal assemblage was already enormous, and the pinniped assemblage well documented by hundreds of specimens. Most of the fossils could be assigned to the large seal-like Allodesmus, although a single jaw described by Barnes (1972) as “Desmatophocine B” didn’t appear to be referable. “Desmatophocine B” was probably similar to Allodesmus, which has a long narrow skull, enormous eye sockets, single-rooted teeth, and a relatively large body. Furthermore, we know Allodesmus retained the ability to rotate its hindflippers forward for sea-lion like terrestrial locomotion, and it was probably a sea-lion like underwater “flyer”. Numerous small pinniped elements appeared to be similar to a handful of elements described by Remington Kellogg (1931) as Neotherium mirum. 

 

 

Skulls of Allodesmus (left) and Neotherium (right) roughly to scale. From 

Barnes and Hirota (1995) and Kohno et al. (1995).

Neotherium was an enigma for over 60 years, and it wasn’t until more complete remains of the early walrus Imagotaria downsi were recovered from the Santa Margarita Sandstone near Santa Cruz, California, that Neotherium began to make sense. Imagotaria was a sea lion-like walrus that lived about 9-12 million years ago – a bit younger than the 15-16 Ma Sharktooth Hill Bonebed – and by the close of the 1970’s was known by a number of well preserved skulls and partial skeletons from Santa Cruz County. Fossils of Neotherium, although never as common as Allodesmus, continued to trickle in from the bonebed and were referred to Neotherium piecemeal, one or two bones at a time by Mitchell (1961), Mitchell and Tedford (1972) and Repenning and Tedford (1977). By the 1980’s, Barnes had amassed a collection of nearly every skeletal element of Neotherium, identifiable as miniature and slightly more primitive versions of that found in Imagotaria – including partial skulls and several mandibles (eventually a complete skull was published by Kohno et al. 1995). Barnes has been for many years working on a monograph on Neotherium – I’m looking forward to seeing it published. 

The holotype of Pelagiarctos thomasi. From Barnes (1988).

Howell Thomas’ mystery jawbone appeared more similar to Neotherium relative to Allodesmus, with the exception of its comparably gigantic size as well as having a fused intermandibular joint (mandibular symphysis) and deep grooves on the sides of the canines. Eventually, several isolated teeth that were similar to Neotherium, but several times larger in size – were discovered from the bonebed. Some of these teeth even fit right in to the tooth sockets in the mandible fragment. Barnes published the fossils in 1988 and described them as Pelagiarctos thomasi, the species name honoring Howell Thomas. The genus name Pelagiarctos refers to the primitive dental anatomy, as ‘arctos’ refers to bears, the traditional sister taxon of pinnipeds (the root arctos is frequently used in pinniped genus names – Arctocephalus, Phocarctos, Hydrarctos, Pteronarctos, etc.), as well as the inferred pelagic ecology of the animal.

 

The isolated teeth referred to Pelagiarctos by Barnes (1988).

Several aspects of the anatomy of Pelagiarctos, although based on scant material, suggested a different approach to feeding in this fossil walrus relative to other Sharktooth Hill Pinnipeds. The teeth of Pelagiarctos were huge – very robust canines, and postcanine teeth with multiple large cusps and sharp crests. He likened the premolars and molars to those of modern hyenas and extinct borophagine dogs, two groups which (by observation or inference) crack and ingest bones, suggesting that Pelagiarctos had dental adaptations for large bite forces related to feeding on large prey items. Furthermore, the robust mandible and fused symphysis further suggested high bite forces. Barnes (1988) additionally noted that Pelagiarctos is very large and numerically rare in the Sharktooth Hill Bonebed – only known by five teeth and a mandible fragment at the time of his study, as opposed to the hundreds of specimens known of other pinnipeds such as Allodesmus and Neotherium. This suggested to Barnes that Pelagiarctos was rare in California waters during the middle Miocene, further supporting his hypothesis that it was an apex predator (apex predators at the top of the food chain can never be very abundant because they rely on a constant stock of abundant prey items). Barnes further postulated that the type specimen was a male, as it had proportionally large canines; modern and fossil pinnipeds are sexually dimorphic, including early walruses like Neotherium, Imagotaria, and Proneotherium. One of the canines in the holotype is broken and polished down, suggesting the tooth had been broken and worn down after continued use in life – damage which Barnes attributed to male combat, which occasionally results in such damage in modern pinnipeds. Furthermore, Barnes identified some of the fossil teeth as males because they fit right into tooth sockets on the type specimen, and those that didn't were of similar size.

As a result of these hypotheses, numerous fanciful reconstructions of Pelagiarctos have been produced by paleoartists (fanciful depictions can be seen here, here, and here). and Pelagiarctos has achieved the nickname "killer" walrus by some enthusiasts. But what do we really know about Pelagiarctos? Stay tuned...

References –

Barnes LG (1972) Miocene Desmatophocinae (Mammalia: Carnivora) from California.

University of California Publications in Geological Sciences 89: 1-69.

Barnes L.G., 1976, Outline of eastern Northeast Pacific fossil cetacean assemblages:

Systematic Zoology, v. 25, p. 321–343,

Barnes LG (1988) A new fossil pinniped (Mammalia: Otariidae) from the middle Miocene Sharktooth Hill Bonebed, California. Contributions in Science, Natural History Museum of Los Angeles County 396: 1-11.

Barnes LG, Hirota K (1994) Miocene pinnipeds of the otariid subfamily Allodesminae in the North Pacific Ocean: Systematics and Relationships. The Island Arc 3: 329-360.

Deméré TA, Berta A, Adams P (2003) Pinnipedimorph evolutionary biogeography. Bulletin of the American Museum of Natural History 13: 32-76.

R. Kellogg. 1931. Pelagic mammals of the Temblor Formation of the Kern River region, California. Proceedings of the California Academy of Science 19(12):217-397

Kohno N, Barnes LG, Hirota K (1995) Miocene fossil pinnipeds of the genera Prototaria and Neotherium (Carnivora; Otariidae; Imagotarinae) in the North Pacific Ocean: evolution, relationships, and distribution. The Island Arc 3: 285-308.

E. D. Mitchell. 1961. A new walrus from the imperial Pliocene of Southern California: with notes on odobenid and otariid humeri. Los Angeles County Museum Contributions in Science 44:1-28

Mitchell ED (1966) The Miocene pinniped Allodesmus. University of California Publications in Geological Sciences 61: 1-46.      

Mitchell ED, Tedford RH (1972) The Enaliarctinae: a new group of extinct aquatic Carnivora and a consideration of the origin of the Otariidae. Bulletin of the American Museum of Natural History 151: 203-284.

Repenning CA, Tedford RH (1977) Otarioid seals of the Neogene. Geological Survey

Professional Paper 992: 1-87.

D. R. Prothero, M. R. Liter, L. G. Barnes, X. Wang, E. Mitchell, S. McLeod, D. P. Whistler, R. H. Tedford, and C. Ray. 2008. Land mammals from the middle Miocene Sharktooth Hill Bonebed, Kern County, California. New Mexico Museum of Natural History and Science Bulletin 44:299-314

Pyenson ND, Irmis RB, Lipps JH, Barnes LG, Mitchell ED, et al. (2009) The origin of a

widespread marine bonebed deposited during the Middle Miocene Climatic Optimum.

Geology 37: 519-522.

Sinus anatomy of modern porpoises revealed by CT imaging

Congratulations are in order to my colleague and friend Rachel Racicot, a Ph.D. student at Yale (working under Jacques Gauthier), for getting her master's research published in the Journal of Morphology. Rachel did her Master's at San Diego State with Annalisa Berta, and was just finishing up when I visited the San Diego NHM for the first time in 2007. Aside from functional morphology and the endocranial anatomy of odontocetes (particularly cranial sinuses, brain endocasts, and the inner ear), Rachel is also interested in fossil porpoises and is currently researching the strange "half-beaked" porpoise from the San Diego Formation. Rachel's master's research concerns the pterygoid sinus morphology of modern porpoises.

Ms. Racicot had no idea that the journal had selected her image to be put on the cover 

of the January 13 issue, and she was quite surprised when I congratulated her on it. Pleasantly 

surprised, I should say.

Before I continue, I should briefly introduce phocoenids. The Phocoenidae, or true porpoises, are a small bodied group of delphinoid cetaceans that are not terribly diverse (6 species, 3 genera) in comparison to oceanic dolphins (Delphinidae; ~40 species, ~12 genera). They differ from delphinids in having short rostra, having symmetrical skulls, large bumps on the premaxillae just before the bony nares, and have inflated braincases without large bony crests. Phocoenids are considered to be paedomorphic -that is, retaining juvenile features into adulthood, thus explaining A) their inflated, juvenile-like braincases, B) lack of strong bony crests, C) cranial symmetry, D) short rostra, and E) small body size. It should be noted that the delphinid Cephalorhynchus is thought to parallel phocoenid paedomorphosis. Modern phocoenids also have strange, spatulate teeth which almost resemble the teeth of nodosaurs and ankylosaurs. Many fossil phocoenids, on the other hand, have longer rostra, conical teeth, cranial asymmetry, and better developed cranial crests.

 

Schematic view of a neonatal harbor porpoise (Phocoena phocoena) skull showing in blue the various parts of the pterygoid sinus. From Racicot and Berta (2013).

The pterygoid sinus is present in all Neoceti, and even within basilosaurids. It originates as an outpocket of the eustachian tube (an air filled cavity present in the middle ear of all mammals - hold your nose with your fingers and blow, and you'll feel crackling in your eustachian tubes; they are also what "pop" when changing altitude as the pressure changes). Parts of the sinus system can be seen externally, such as the hamular lobe of the pterygoid sinus, which is not completely encased in bone and is visible in a prepared skull as large cavities surrounded by thin flanges of bone. The pterygoid sinus system is elaborated in odontocetes relative to mysticetes. Although known to exist, the anatomy of the pterygoid sinus system in odontocetes - and true porpoises (Phocoenidae) in particular - is difficult to assess. Since they are cavities within a solid object, it's difficult to study them by any conventional means as they remain hidden in the skull. Certain aspects of the pterygoid sinuses have, for example, been used in phylogenetics - in multiple phylogenetic analyses which have included phocoenids, a cladistic character has been used - presence or absence of a dorsal extension of the preorbital lobe of the pterygoid sinus between the maxilla and frontal bone (po on the above diagram). This is a phocoenid feature, and the bottlenose dolphin lacks this. The preorbital lobe is well developed in the neonatal specimen (neonate = newborn individual, rather than a juvenile or subadult), although the dorsal extension is not as well developed as in the adults (an example of ontogeny recapitulating phylogeny).

Digital 'endocast' of the right pterygoid sinus (in medial view) from six skulls of Phocoena phocoena; anterior is to the left. Neonate specimen shown in F. The sinus shape looks pretty weird (but then again, so does the rest of a cetacean skull). From Racicot and Berta (2013).

 So, what's it for? Previous hypotheses for the function of the sinus includes A) an acoustic barrier to reflect sounds produced during echolocation forward through the melon, B) an acoustic barrier between sound producing and sound receiving structures (e.g. nasal region and petrotympanic complex, respectively), and C) to acoustically isolate the petrotympanic complex from sounds produced during echolocation (which, admittedly, sounds similar to B). A fourth hypothesis posits that the pterygoid sinus serves as a means to regulate pressure around the middle ear during diving.

 The paired sinuses (right and left) of Phocoena phocoena specimens in anterior view. Note that there is right-left asymmetry in each specimen. From Racicot and Berta (2013).

To test the sound reflecting ability of the sinus, Racicot and Berta calculated the minimum thickness necessary to reflect sound at the typical highest frequency sounds produced by Phocoena phocoena (~150 kHz). Many aspects of the cetacean skull make sense in the light of acoustic impedence - sound waves tend to bounce off of objects or features where there is a stark change in density. For example, echoes in air are sound waves bouncing off a solid surface. In water where the medium is much denser, sound not only travels faster, but flesh and bone are so similar in density that sounds travel through the vertebrate body rather than bouncing off of it - making things like external ears (which take advantage of sound waves bouncing due to acoustic impedence, and funnels sound in) useless. So, within a skull, a wall of air within a sinus is different enough in density to reflect sound, analogous to a solid object in air.

They calculated that the preorbital lobe would need to be 2.5mm thick at a minimum, which is less than what they observed in phocoenid sinuses - indicating they would function well at reflecting sounds. As for the asymmetry of the sinuses, they remarked that this could be explained by the fact that experiments have determined that porpoises produce sounds in an asymmetric fashion, preferring to use one nasal passage over the other, potentially explaining why the sinuses are asymmetrical.

Wild speculation time: it's also possible that aysmmetrical sinuses may be a vestige of cranial asymmetry. Fossils show that the earliest phocoenids had asymmetrical skulls in a similar fashion to delphinids; perhaps this is an example of phylogenetic inertia - the external skull changed at a faster pace than the sinuses, reaching symmetry first. However, paedomorphosis typically progresses by delaying adult morphology later and later during ontogeny, and retaining juvenile features longer and longer instead. In other words, paedomorphosis would suggest that asymmetry was once an adult feature which at some point was lost because juvenile symmetry prevailed - which doesn't totally jive with asymmetrical sinuses being retained, unless the two are decoupled somehow, progressing along different ontogenetic trajectories. Or, is asymmetry so ingrained within odontocetes that it's a juvenile feature in phocoenids, with symmetry really being secondarily gained via hypermorphosis, with asymmetry being pushed earlier on in ontogeny? Interesting questions, but they remain unanswered. We need more fossils and further studies of modern phocoenid cranial anatomy.

Another last thought - it's interesting to note that phocoenids are considered paedomorphic, but have relatively extensive pterygoid sinuses. The primitive condition among Neoceti, of course, is possessing less well developed sinuses (pterygoid sinuses in Neoceti and Basilosauridae are acquired stepwise in a piecemeal fashion). In other words - sinus development is not showing a paedomorphic trend - in fact, it's showing the opposite trend - it's a peramorphic feature, probably undergoing something like hypermorphosis (development is postponed and extended later into ontogeny) or acceleration (faster development of a feature during ontogeny). Perhaps hypermorphosis is not likely, given the short period it takes for phocoenids to mature.

References:

Racicot, R.A., A. Berta. 2013. Comparative Morphology of Porpoise (Cetacea: Phocoenidae) Pterygoid Sinuses: Phylogenetic and Functional Implications. Journal of Morphology 274:49-62.