Six million years ago, a hundred-pound otter was on the prowl somewhere in the swampy wetlands of what’s now southwestern China.
Unlike today’s playful sea otters, which use stones to smash open urchins off the Pacific coasts of North America and Asia, this ancient creature could have chomped right through mollusk shells with its powerful jaws.
Meet Siamogale melilutra, a prehistoric otter ancestor that was unearthed in China’s Yunnan Province and first described earlier this year.
In a new study, researchers examined the animal’s mandible, or jawbone, and determined that it was probably an apex predator of the late Miocene, with strong jaws and teeth made for crushing that would have allowed it to consume a wide range of prey.
"We still think it probably consumed mollusks, but at a level of capability that's way beyond what we see in living otters," says study leader Z. Jack Tseng of the SUNY University at Buffalo.
The discovery not only provides insight into the life of this ancient giant, but also offers clues to the behaviors of modern otters—including some species’ remarkable use of tools.
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Today, otters generally fall into two groups: Molluscivores dine on hard-shelled invertebrates like crabs, clams, and urchins, while piscivores feast primarily on fish.
To better understand the feeding habits of Siamogale, Tseng and his team rounded up mandibles and skulls from ten of the 13 living otter species and created computerized 3-D models of their jaws, along with those of the fossil animal.
When muscles rotate the jaw, energy passes through the bone and into the teeth. Some of that energy naturally gets lost through friction and heat and, in the case of mandibles, to slight deformations in the shape of the bone itself.
More efficient chewing would therefore involve stiffer mandibles, which bend less as they bite, and biting efficiency can shed light on dietary preferences. (Also find out how sabertoothed cats killed prey despite their weak bites.)
Molluscivores, they reasoned, would have stiffer jaws than piscivores, because they must break through the hard exoskeletons of their prey. Instead, they found a simple linear relationship for all living otters, even after accounting for body size: The smaller the otter, the stiffer the jaw.
"That came as a surprise to us," says Tseng, whose team reports their findings this week in the journal Scientific Reports.
The next surprise was that Siamogale's jaw wound up being six times stiffer than its body size would have predicted. Because otters are so well adapted to hunting in water, this oversized animal was probably still chowing down on shellfish, but it would have been able to bite through much tougher and larger prey than its modern counterparts.
Though none are quite as remarkable as Siamogale, some modern otters do fall outside the linear pattern that Tseng and his team discovered.
For example, African clawless otters have sturdier jaws than their size would suggest, but only slightly. That no doubt reflects their taste for crabs, which have relatively sturdy shells. Meanwhile, sea otters, which regularly tuck into meals of prickly sea urchins, actually have bendier jaws than their body size would indicate.
That might be because sea otters have outsourced the work of breaking through the urchins' tough exoskeletons to their hands. One of very few known tool users among marine mammals, sea otters use stone hammers to smash open their prey.
It's only speculation for now, but the researchers think that the development of tool use may have reduced the need for sea otters' jaws and skulls to produce more powerful bites.
"We don't have hard evidence," admits Tseng, "but it's suggestive."
"I think they're right on the money," says College of Charleston paleontologist Bobby Boessenecker, who wasn't involved in the study. He points out, though, that the fossil record for sea otters remains too sparse to say for sure.
Still, it's not a crazy idea. Some anthropologists have argued that as ancient humans began to use tools, they began to show alterations in skull anatomy. By outsourcing part of the work of digestion to our hands, our skulls transitioned from being primarily food processing machines to working as brain protection devices.
"Somehow," says Tseng, "diet and brain evolution are associated."
Editor's Note: This story has been updated to correct the relationship between otter body size and jaw stiffness.