Rare Otter Fossil Found in the Mexican Desert

The newfound teeth may help solve a mystery about how the water-loving mammals made it across the continent six million years ago.

In a surprise to paleontologists, ancient teeth discovered in the hot scrublands of central Mexico belong to a water-loving otter that roamed North America six million years ago.

Found 120 miles from the nearest coastline, the fossil opens up an entirely new discussion about the movements of mammals across the continent millions of years ago.

The teeth belong to an extinct otter species called Enhydritherium terraenovae, says Jack Tseng at the University of Buffalo, whose team describes the find this week in the journal Biology Letters. This species had previously been found only in coastal regions in Florida and California, suggesting it was dependent on coastal environments like its modern relatives.

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This fossil jawbone found in Mexico belonged to the extinct otter species Enhydritherium terraenovae.

In March, Tseng and his colleagues were in the land-locked Juchipila Basin in central Mexico, looking for fossils that will help explain a time period beginning around six million years ago when mammals were frequently migrating between North and South America.

While in the field, graduate student Adolfo Pacheco-Castro, from the National Autonomous University of Mexico, came to him with an unusual specimen.

“I knew it was some kind of weasel family member, but otter never crossed my mind,” Tseng says. (Also read about a giant catfish fossil recently found in Egyptian desert.)

Luckily, field expedition member Xiaoming Wang at the Natural History Museum of Los Angeles County had recently been involved with another otter-related research project. “He’s got an eye for otters and knew what it was,” Tseng quips.

Finding otter fossils anywhere is already a feat, says Robert Boessenecker, a paleontologist at the College of Charleston who was not involved with the study.

“Any otter specimens in general are as rare as hen’s teeth,” Boessenecker says.

But finding evidence of Enhydritherium in Mexico is even more remarkable. Previous fossils suggest the Americas were seeing massive north-to-south migrations during this time—now, this singular fossil indicates there was possibly some east-to-west movement going on, too.

“Based on what we know about living otters, they need to be close to the water,” Tseng says.

Finding Enhydritherium on both coasts and now in between means this otter may not have been living only in coastal regions, but could have used smaller bodies of water to make its way from coast to coast. Tseng points out that the shape of limb bones from Florida specimens suggest the animal was not specially adapted to marine life and had the ability to move easily over land.

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A view of the fossil site in the Juchipila Basin in Mexico, where the ancient otter teeth were discovered.

The discovery also works against a previous hypothesis that the ancient otters took a more circuitous ocean migration route between coasts, Boessenecker adds.

Many more surprises are probably waiting to be uncovered in the rocks of central Mexico, Tseng notes, because not much paleontological field exploration has been done there.

“It just goes to shows you how one find can completely change how you interpret the ecology of an extinct species.”

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