Muizon and others (2002) is the most thorough in a series of publications concerning the fossil, walrus-like cetacean, Odobenocetops. These authors discuss discuss at great length the functional morphology of this bizarre animal. Odobenocetops is thought to be closely related to the Monodontidae (a family that includes the beluga and narwhal), but unlike all other monodontids, Odobenocetops possesses numerous features that are evolutionary reversals back to a primitive odontocete anatomy, and others that are remarkably convergent with that of walruses.
Similar to the walrus, Odobenocetops has a pair of downward pointing tusks. In addition, Odobenocetops also has a highly vaulted palate (the roof of the mouth). A vaulted palate is associated with suction feeding in walruses. Modern walruses never chew their food; instead they hold a mollusc (such as a clam) in their lips and suck out the flesh from the shell by creating a vacuum using their tongue and the vaulted palate. The authors suggest that the presence of such a vaulted palate in Odobenocetops is evidence that it was a molluscivorous suction feeder, analogous to the modern walrus.
With the exception of tusks, Odobenocetops possessed no other teeth. In addition, there are many features that indicate that Odobenocetops had a very poor echolocating system, if any at all. Echolocation is a system that modern odontocetes use for navigation and prey capture; they interpret the echoes of their vocalizations to create an audio “picture.” The facial region of the skull is not concave and the nose was far forward in Odobenocetops. Together, these characteristics suggest that its melon was absent or very reduced. The melon is a fatty sac situated in front of the nose of dolphins and porpoises that helps focus and direct the vocalizations used in echolocation.
Additionally, Odobenocetops had reversed much of what is called cranial telescoping. Telescoping of the cetacean skull refers to the manner in which the bones of the front of the skull (e.g. premaxilla, maxilla, and frontal) and the bony nares (nose opening) have shifted back during evolution to lie on top of the bones that compose the orbits for the eyes and the braincase. This condition, however, is reversed in Odobenocetops so that the bony nares and surrounding bones are restricted to the front part of the skull.
The presence of elongate, backward pointing tusks in two different molluscivorous (eats clams and similar shelled organisms), suction-feeding marine mammals sheds some interesting light on the evolution of tusks. Previously, Deméré (1994) attributed long tusks in fossil and modern walruses to sexual selection; features that function in attracting mates or in competition for mates. However, de Muizon and others (2002) argue that tusks evolved as a sort of sled runner, which would keep the head at the optimal angle to the sea floor. Both of these hypotheses are interesting, but require further testing utilizing information from both walruses and odontocetes.
Deméré, T. A. 1994. Two new species of fossil walruses (Pinnipedia:Odobenidae) from the Upper Pliocene San Diego Formation, California; pp. 77–98 in Berta, A., and T. A. Deméré (eds.), Contributions in Marine Mammal Paleontology Honoring Frank C. Whitmore, Jr. Proceedings of the San Diego Society of Natural History, 29.
Summary written by Robert Boessenecker.
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