Tuesday, February 12, 2013

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).


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.

No comments: