Palaeontologists who examined a new fossil found in southern California have thrown doubt on earlier claims that a "killer walrus" once existed.
Geology PhD student Robert Boessenecker from New Zealand's University of Otago and co-author Morgan Churchill from the University of Wyoming have today published their paper about the fossil in the online scientific journal PLOS ONE.
The paper reports that the new fossil-find, of the extinct walrus Pelagiarctos from southern California, prompts a different hypothesis to an earlier one that a "killer walrus" existed, preying on other marine mammals and/or birds.
Fossils of the walrus were originally found in the 1980s. The large, robust size of the jaw bone, along with the sharp pointed cusps of the teeth similar to modern bone-cracking carnivores like hyenas, suggested that Pelagiarctos fed upon other marine mammals rather than the typical diet of fish as in modern walruses.
However the new fossil, a lower jaw with teeth, and more complete than the original fossil, suggests to the Otago and Wyoming palaeontologists that the Pelagiarctos was more of a fish eater as it lacked adaptations for being a "killer walrus".
The evidence pointed to the tooth shape being unlikely to have been adapted for feeding upon large prey; instead it was an example of primitively retained tooth shape.
"This new find indicates that this enigmatic walrus would have appeared similar in life to modern sea lions, with a deep snout and large canines," says Mr Boessenecker.
The researchers estimated Pelagiarctos to be similar in size to some modern male sea lions (about 350 kg or 770 lbs).
"However, modern pinnipeds (seals, sea lions, and walruses) of small and large body sizes are dietary generalists, and tend to have diets rich in fish - including sea lions similar in body size to Pelagiarctos, which means that its large body size alone doesn't make Pelagiarctos an apex predator."
The new study also analysed the evolutionary relationships of Pelagiarctos for the first time, and found it to be an early sea lion-like walrus that was most closely related to another sea lion-like walrus, Imagotaria downsi, also from California.
The study was supported by a University of Otago Doctoral Scholarship, and grants from the Geological Society of America, The Palaeontological Society, and a National Science Foundation EAPSI Fellowship.