About 6 million years ago, a giant, wolf-sized otter may have been a dominant predator, according to a new study that analyzed the animal’s jaws.
The research provides insight into the ecological niche that the oversize otter may have filled in the wetlands of southwest China where it lived. The otter, named Siamogale melilutra, weighed about 110 pounds, much larger 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 Z. Jack Tseng, Ph.D., who led the project.
Tseng is an assistant professor of pathology and anatomical sciences in the Jacobs School of Medicine and Biomedical Sciences at the University at Buffalo, and a research associate with the American Museum of Natural History and the Natural History Museum of Los Angeles County.
The study was published in the November 9th issue of Scientific Reports. The research team included Denise F. Su of the Cleveland Museum of Natural History; Xiaoming Wang of the Natural History Museum of Los Angeles County, American Museum of Natural History, and Chinese Academy of Sciences; Stuart C. White of UCLA; and Xueping Ji of the Yunnan Institute of Cultural Relics and Archaeology in China.
When scientists used computers to simulate how biting would strain S. melilutra’s jaws, they found that the animal had much firmer jawbones than expected. The 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 imply that Siamogale could crush much harder and larger prey than any living otter can."
Modern otters have a varied diet. Different species eat different foods, ranging from plants and rodents to fish, crabs and clams. Based on the computer analyzed data and 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 animals like rodents. But they don’t know exactly what the giant otter would have eaten.
To better understand the S. melilutra, Tseng and colleagues compared the prehistoric critter to its living family members.
The team used computed tomography (CT) scans of skulls to create 3D, computerized models showing how the jaw bones of 10 of the 13 known living otter species bend under biting forces. One rare otter was left out because researchers couldn’t find bones to scan, and the other two were left out of the study because they were so similar to other species.
The team also made a model for S. melilutra, using CT scans of fossils as a guide. The work included a 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 the animal's size. The smaller otters had sturdier jaws. But the S. melilutra was an outlier: the mammal’s modeled jaws were six times sturdier than expected. The strength and the huge size would have made it a great hunter.
"At the time that the otter lived, the area where its remains were found included a swamp or a shallow lake surrounded by evergreen forest or dense woodland," said Su, a paleoecologist at the Cleveland Museum of Natural History and one of the leaders of the Shuitangba Project that discovered the fossil otter. "There was a diverse aquatic fauna at Shuitangba, including fish, crab, mollusks, turtles and frogs, as well as many different species of water birds, all of which could have been potential prey for S. melilutra."
In the wet and forested environment, the otter’s jaw power could have given it an edge over predators that couldn’t hunt or smash the shells of aquatic prey.
"Carnivores are known to evolve powerful jaws, often for the purpose of cracking the bones of their prey," said Wang, a curator in the Vertebrate Paleontology Department of the Natural History Museum of Los Angeles County. "In the shallow swamp of South China, it's possible that an abundance of big clams drove these giant otters to acquire their rare traits, including their crushing teeth and robust jaws." Wang, along with Su, White and Ji, was a member of the research team that first reported the discovery of the giant otter's fossils in January.
Besides providing inside into S. melilutra, the new study raises general questions about the relationship between an animal’s jaw power and their diet.
Scientists expect to find more powerful jaws in creatures that eat harder foods. According to the new study, these two traits don’t match up in living otters: jaw strength correlated with size, regardless of meal choice.
Modern otters may use tools to help them crack open harder to eat foods, despite having a weaker bite.
"Sea otters, for example, swim on their backs and use their chests as a platform for crushing their food with stones," Tseng says.
But tool use can’t completely account for the pattern that the scientists saw and more research is needed to understand the unexpected trend.
Tseng believes it is 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."