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Extinct otter-like 'marine bear' might have had a bite like a saber-toothed cat

March 01, 2016

New research suggests that the feeding strategy of Kolponomos, an enigmatic shell-crushing marine predator that lived about 20 million years ago, was strangely similar to a very different kind of carnivore: the saber-toothed cat Smilodon. Scientists at the American Museum of Natural History used high-resolution x-ray imaging and computerized biting simulations to show that even though the two extinct predators likely contrasted greatly in food preference and environment, they shared similar engineering in jaw structure, suitable for anchoring against prey with the lower jaw and forcefully throwing the skull forward to pry loose its food. The study is published today in the journal Proceedings of the Royal Society B.

The only known specimens of Kolponomos--primarily skulls and teeth of two species--were recovered from ancient marine deposits along the Pacific coast of Oregon, Washington, and possibly Alaska. Because of its peculiar morphology and the small number of fossils, the animal's place in the evolutionary tree remains a mystery.

"When Kolponomos was first described in the 1960s, it was thought to be a raccoon relative," said Camille Grohé, a National Science Foundation and Frick Postdoctoral Fellow in the American Museum of Natural History's Division of Paleontology and a co-author on the new paper. "But later research on the skull base led some to think it might be a seal or a bear relative instead, and studies of its teeth show that they are very similar in both shape and wear to the teeth in sea otters."

Sea otters pry their prey--hard-shelled marine invertebrates like clams and mussels--off of surfaces using their hands and rock tools, then crush the shells with their teeth or against their chests, again using tools. By studying Kolponomos fossil material from the National Museum of Natural History in Washington, D.C., and comparative specimens from the American Museum of Natural History, the research team originally set out to test if the extinct predator used otter-like shell-crushing to eat. But the scope of the research expanded after Grohé's collaborator Z. Jack Tseng noticed something curious in parallel to work he was conducting on the saber-toothed cat Smilodon.

"I started seeing a great deal of similarity between the jaws of Kolponomos and Smilodon," said Tseng, a National Science Foundation and Frick Postdoctoral Fellow in the American Museum of Natural History's Division of Paleontology and the lead author on the new paper. "Both of them have a distinctive profile with a deep jaw bone that tapers off toward the back, and both have an expansion of the mastoid processes and the skull's back surface, suggesting large attachment sites for muscles that let the animal move its head powerfully but with control. We definitely didn't expect to bring Smilodon into this study of feeding in a clam-eating marine carnivore, but that's what we ended up doing."

At the Museum's Microscopy and Imaging Facility, the researchers used computed tomography (CT) to scan the skulls of Kolponomos and six other carnivores: Smilodon, grey wolf, sea otter, river otter, brown bear, and leopard. They then used computerized methods to build sophisticated biomechanical models to look at how efficiently the animals could perform various bites, including the jaw-anchored killing shear-bite that is characteristic of saber-tooth cats.

They found that the jaw mechanics of Kolponomos and Smilodon are more similar to each other than to any of the other animals in the study, pointing to a unique feeding strategy in addition to the previous idea that Kolponomos might have crushed its prey like sea otters do today. Taken together, the researchers suggest that Kolponomos might have pried prey off of rocks with its lower jaw, swung its skull forward to dislodge it, and then crunched it with its chewing teeth.

"Our biomechanical data show that the chewing bites of sea otters and Kolponomos are not very similar," Tseng said. "They probably still have an overlapping diet based on tooth wear, but their evolutionary solutions for getting to those hard-shelled animals are dramatically different."

The researchers stress that this finding does not imply shared ancestry between Kolponomos and Smilodon, but rather an intriguing case of convergence--the independent evolution of similar traits.

"This innovative study, showing unexpected feeding similarities between such wildly distinct carnivores, could only happen by applying new technologies to understand specimens from some of the world's greatest archives of ancient life," said John J. Flynn, a curator in the Museum's Division of Paleontology and Dean of the Richard Gilder Graduate School, also an author on this paper.

This work was funded by the U.S. National Science Foundation grant # DEB-1257572 and the American Museum of Natural History's Frick Postdoctoral Fellowships.

The authors have dedicated this study to the memory of Museum artist Chester Tarka, who illustrated Kolponomos newportensis. Tarka, a decorated Army veteran of the Normandy Invasion and Battle of the Bulge in WWII, passed away recently at the age of 96 and was buried with military honors in Saratoga, New York.
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