Dissecting the “Killer” Walrus

Compared to its seal and sea lion cousins, the walrus looks like an oddball. There’s a good reason for that. The wrinkly, tusked marine mammal is the last surviving member of a pinniped group that proliferated during prehistory but is down to just a single species, Odobenus rosmarus. Over the past 20 million years, walruses have included tuskless fish hunters, quad-tusked suction feeders, and molluskivores quite similar to the sole surviving species. Their blubbery ranks were also thought to contain a particularly vicious walrus that preyed upon other marine mammals, but a new study by University of Otago’s Robert Boessenecker and  the University of Wyoming’s Morgan Churchill have undermined the bloodthirsty reputation of Pelagiarctos.

Paleontologist Lawrence Barnes named the fossil pinniped in 1988. Discovered by a fossil preparator among the remnants of a 16 to 14.5 million year old seabed in California’s Kern County, Pelagiarctos thomasi was initially known from little more than a broken portion of the lower jaws and a few teeth. Based upon the anatomy of those deep, fused jawbones and the details of the mammal’s dentition, though, Barnes was able to identify Pelagiarctos as a strange, archaic walrus.

Barnes saw Pelagiarctos as a bulky predator that fed on seabirds and other marine mammals found in the same deposit, such as the smaller walrus Neotherium mirum and perhaps young individuals of the especially common pinniped Allodesmus. The anatomy and rarity of Pelagiarctos inspired Barnes’ hypercarnivirous hypothesis. The fact that the deep jaw bones of Pelagiarctos were fused at the front, along what’s called the mandibular symphysis, hinted that the mammal had reinforced jaws capable of delivering a strong bite with its large canine teeth. Furthermore, Barnes interpreted the rear teeth of Pelagiarctos as remarkably similar to those of hyenas and bone-crushing fossil dogs called borophargines, and pointed out that the rarity of Pelagiarctos was consistent with a predatory lifestyle. The fact that California’s Miocene rock turned up plenty of Allodesmus and Neotherium but only one partial Pelagiarctos specimen was consistent with the idea that the primeval walrus was a specialized predator that Miocene marine ecosystems could only support in small numbers.

For years following Barnes’ description, researchers were only able to locate a few additional teeth of Pelagiarctos. The stout-toothed walrus certainly was rare. But a slightly more complete set of lower jaws was accidentally discovered in 1997 during construction in Mission Viejo, California. This specimen forms the basis of the new PLoS One study by Boessenecker and Churchill. Contrary to the rapacious reputation of Pelagiarctos, the walrus fossil doesn’t show any sign of a pinniped-crunching lifestyle.

While the lower jaws of the Pelagiarctos thomasi specimen Barnes described were fused, those of the Mission Viejo animal were not. This could be a sign that the two animals belonged to different species, or that jaw fusion varied within the species, much as it does in the modern walrus. Boessenecker and Churchill refer the Mission Viejo animal to Pelagiarctos sp. to be on the safe side. Identity issues aside, though, the teeth and jaws of this Pelagiarctos indicate that the walrus was more of a generalized predator rather than a big game specialist.

The teeth of Pelagiarctos show wear patterns created by biting. (If the animal was a suction feeder, like the modern walrus, then its teeth would have been polished by the intake of grit and sediment along with their small prey.) That might seem to fit the profile Barnes suggested, but other details undermine that interpretation.

As Boessenecker and Churchill point out, a fused mandibular symphysis isn’t necessarily a good indicator of diet. In marine mammals, such as the modern walrus, the fusion strengthens the jaw for suction feeding, while the same characteristic might help big cats resist the strains of struggling prey. Then again, carnivorans that often consume bone and other hard foods – such as hyenas, borophagines, and sea otters – often have relatively loose mandibular symphyses, which might give them the flexibility to move their jaws into positions that minimize the risk of breaking their teeth. Among carnivoran mammals, at least, lower jaw fusion can’t be directly tied to a particular diet. And the fact that the two existing Pelagiarctos jaw sets differ in terms of lower jaw fusion hints that the trait might have been variable and, by itself, is not a good indicator of behavior or diet for the walrus.

The postcanine teeth of Pelagiarctos also lack the cusps and other specializations of hyenas and other macropredatory carnivores. As might be expected given the fact that Pelagiarctos was an early walrus that evolved after the lineage split from other pinnipeds, but before the origin of the modern walrus lineage, the marine mammal had teeth and jaws that most closely resembled those of other early walrus species. Pelagiarctos doesn’t show any sign of being a macropredatory specialist, and was not actually a giant apex predator. Boessenecker and Churchill calculate that Pelagiarctos would have weighed about 350 kg, comparable to a South American sea lion, intermediate between the 280 kg Neotherium and the much bigger Allodesmus, which weighed as much as 1,400 kg.

Of course, teeth are tools that can be employed in various ways. Leopard seals are known for their pinniped and penguin-killing prowess, yet they also suck up krill. The mollusk-slurping modern walrus, on the other hand, has been known to sometimes kill and eat seals. With its robust canines, Pelagiarctos could have killed and consumed other pinnipeds, but the important point is that there’s no sign that this walrus was adapted to a dedicated mammal-hunting lifestyle. Like other early walrus species, Boessenecker and Churchill conclude, Pelagiarctos was probably “a generalist predator, feeding on fish, invertebrates, and the occasional warm-blooded prey item.”

Why Pelagiarctos was so rare, however, remains a mystery. After ruling out the apex predator hypothesis, Boessenecker and Churchill suggest that the walrus might have only occasionally inhabited the regions now preserved at Mission Viejo and Sharktooth Hill. After all, they point out, the Sharktooth Hill bonebed where Pelagiarctos was originally found probably accumulated over the course of 700,000 years or so, hinting that rare creatures only inhabited the area during brief periods when conditions were just right. It’s entirely possible that the usual habitat of Pelagiarctos was elsewhere, and the California finds represent occasional excursions into the area. If we’re going to understand this fossil walrus and its natural history, we’ll first have to find the vestiges of those hideouts where more complete Pelagiarctos may still lie undiscovered.

For more, see Boessenecker’s series on the new research, starting here.

References:

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