The Whale that Looked Like a Walrus

Odobenocetops (shown above in an illustration from de Muizon & Domning, 2002) was definitely one of the odder forms of whale. It was found off the coast of Peru in the early Pliocene, about 3 or 4 million years ago, where it seems to have converged in a number of details on the modern walrus (Odobenus rosmarus). Or perhaps it is more accurate to say that the modern walrus converged on it - while Odobenus also appeared in the early Pliocene (Berta et al., 2005), Odobenocetops may have had a slight head start on it - the records in the Paleobiology Database suggest that the earliest records of Odobenus (from Belgium and New Jersey) are of somewhat uncertain age.

The walrus feeds on molluscs, particularly soft-shelled bivalves. The thick upper lip bears a strong array of sensitive bristles for finding molluscs buried in the bottom sediment. However, unlike most molluscivores the walrus doesn't directly crush the shells of its prey. Instead, the shell of the mollusc is held in the thick lips while the tongue is used like a piston to remove the animal from its shell by suction. Odobenocetops had a broadened palate and evidence of a thickened upper lip (probably with similar bristles) like a modern walruses, and probably fed in the same manner. The interesting thing is that Odobenocetops also evolved enlarged tusks like the walrus. Before the discovery of Odobenocetops, most authors had believed that the walrus' tusks were unrelated to its unusual feeding method, having probably evolved instead through sexual selection. That Odobenocetops also combined suction feeding and tusks suggests that their combination might not be as accidental as previously thought. What exactly the role of the tusks may be in feeding is more uncertain. De Muizon & Domning (2002) suggested that they might function as guides for the sensory bristles, or they may have been used to guide prey animals towards the mouth.

One of the most unusual features of toothed whales (in a group well-provided with unusual features) is a tendency towards a loss of bilateral symmetry, with one side of the skull being more developed than the other. The reasons for this asymmetry are uncertain, but the most popular theory is that it is related to the development of the sonar system. Odobenocetops possessed one of the most dramatic examples of skull asymmetry in the cetaceans. The left tusk of the type specimen of O. peruvianus is estimated to have been about 20 cm in length (because the tusks were quite fragile, fossilised specimens are invariably broken). However, the right tusk was over twice as long, at least 50 cm. The type of the other known species, O. leptodon, is even more dramatic - the left tusk is about the same size as known for O. peruvianus at 25 cm, while the right tusk was over one metre! Oddly enough, the remainder of the skull was rather less asymmetrical than in other odontocetes, as Odobenocetops had lost the melon and therefore the sonar of other toothed whales. Sonar was probably unnecessary for a diet of less mobile animals than fed on by other whales.



Relationship-wise, Odobenocetops is included in its own family, but was closely related to the Monodontidae, the family including the narwhal and white whale (also known as beluga - image above of white whale from Whale Trust). Monodontids also have rather mobile lips compared to other whales. Even more significantly, Odobenocetops and monodontids both have a completely mobile neck with unfused vertebrae, in contrast to other cetaceans which have the first two to five vertebrae fused into a single mass. This extra head motility would have doubtless been critical in allowing Odobenocetops to become an efficient sediment feeder.

REFERENCES

Muizon, C. de, & D. P. Domning. 2002. The anatomy of Odobenocetops (Delphinoidea, Mammalia), the walrus-like dolphin from the Pliocene of Peru and its palaeobiological implications. Zoological Journal of the Linnean Society 134: 423-452.