Published December 4, 2017
Research led by Jack Tseng, PhD, assistant professor of pathology and anatomical sciences, found that a wolf-sized otter that lived about 6 million years ago may have been a dominant predator in its time.
The study, published Nov. 9 in Scientific Reports, analyzed the jaws of Siamogale melilutra, an otter that weighed about 100 pounds — bigger than any living otter.
“We started our study with the idea that this otter was just a larger version of a sea otter or an African clawless otter in terms of chewing ability, that it would just be able to eat much larger things. That’s not what we found,” says Tseng, also a research associate with the American Museum of Natural History and the Natural History Museum of Los Angeles County.
When scientists used computers to simulate how biting would strain S. melilutra’s jaws, they concluded that the animal had much firmer jaw bones than expected. This stiffness would have given the otter a surprisingly strong bite — even for its size.
“We don’t know for sure, but we think that this otter was more of a top predator than living species of otters are,” Tseng says. “Our findings suggest that Siamogale could crush much harder and larger prey than any living otter can.”
Modern otters have a varied diet, with different species dining on foods that range from plants and rodents to fish, crabs and clams.
Based on the new study’s findings, S. melilutra’s jaws would have been strong enough to crush the shells of big mollusks or the bones of birds and small mammals like rodents, though what exactly it ate is unknown.
To better understand S. melilutra, Tseng and colleagues compared the prehistoric creature to its living counterparts.
The team used computed tomography (CT) scans of skulls to create 3-D, computerized models showing how the jaw bones of living otter species bend under biting forces.
It also made a model for S. melilutra, using CT scans of fossils as a guide. The work included a painstaking, digital reconstruction of the cranium based on a crushed fossil.
A comparison of all the otter jaw simulations revealed a linear relationship between jaw stiffness and animal size: Smaller otters had sturdier jaws.
But S. melilutra was an outlier. The massive mammal’s modeled jaws were six times sturdier than expected. This strength, paired with the creature’s size, would have made it a formidable hunter.
Besides providing insight into S. melilutra, the new study also raises general questions about the relationship between jaw power and diet in animals.
Typically, scientists expect to find more powerful jaws in creatures that eat harder foods. But according to the new study, these two traits don’t match up in living otters — jaw strength correlated with size, regardless of meal choice.
Tool use may help explain this discrepancy, allowing some otters with a relatively weak bite to tackle tough foods.
“Sea otters, for example, swim on their backs and use their chests as a platform for crushing their food with stones,” Tseng notes.
But tool use can’t completely account for the pattern that the scientists saw, and more research needs to be done to understand the unexpected trend.
For now, Tseng believes it’s still possible to draw some conclusions about S. melilutra based on its unusual mandibular strength.
“We think the anatomy means something because it doesn’t fall within the usual pattern that we see in other otters,” he says.
“The strong jaws suggest that the primitive otter probably did not have the tool-using capability, and combined with the giant size, it was likely a top predator.”
The research team included scientists from the following institutions:
The study was funded by the U.S. National Science Foundation, Yunnan Natural Science Foundation, Institute of Vertebrate Paleontology and Paleoanthropology, the National Natural Science Foundation of China and the governments of Zhaotong and Zhaoyang.