Site of the dig in Zacatecas where the jawbone (inset) was found. Site of the dig in Zacatecas where the jawbone (inset) was found. jack tseng

A sea otter’s jawbone surprises scientists

Discovered in Zacatecas, it suggests an east-west migration route millions of years ago

A sea otter’s jawbone was discovered last March in the state of Zacatecas, some distance away from the nearest sea.

But it was also remarkable for another reason: it is estimated to be six million years old.

The discovery led to a startling realization for paleontologists researching the intercontinental migration of ancient mammals.

Adolfo Pacheco Castro, a professor at the Geosciences Center at the National Autonomous University of Mexico, or UNAM, found the jawbone at an archaeological site in Zacatecas’ Juchipila Basin.

He showed the bone to Jack Tseng, an assistant professor at the University at Buffalo who was doing research at the same dig.

“I thought it was a badger,” Tseng said, “but a colleague on the site had just finished a study of otters, and he said it was sea otter-like. But what would a sea otter be doing in central Mexico?”

The bone, which still held several teeth, was then taken to Mexico City, cleaned off and studied.

“One tooth was a lower first molar, the most diagnostic tooth in a carnivore,” Tseng said, and one that can provide a lot of useful information.

Further studies found that the tooth was almost identical to one from an ancient sea otter found in Florida.

Similar finds had only been made along the California coast, but paleontologists did not know how the small mammals got across the continent.

Tseng and his team are now theorizing that the animal found in Zacatecas may have been part of an east-west migration.

Based on the discovery Tseng, Pacheco and a team of colleagues published a paper this week in the journal Biology Letters.

They suggest there was an east-west passage for the otter, and potentially other mammals, along the northern edge of the Trans-Mexican Volcanic Belt, which runs across the country at the latitude of Mexico City.

“This is an entirely new idea that no one else has proposed,” Tseng said. “We think it’s very likely other animals utilized this route.”

Specialists had hypothesized that otters had migrated north and around through northern Canada, or south to Panama, then crossing over to the west, travelling eventually to California.

The possibility of an east-west migratory route in Mexico in the Miocene geologic period (roughly 23 million to 5.3 million years ago) has implications for a much larger event, called by specialists the Great American Biotic Interchange, when land bridges were formed and animals dispersed to and from North and South America.

The jawbone discovered in Zacatecas could be the breakthrough paleontologists were waiting for, as discoveries of this kind in Mexican territory have so far been scarce.

“Compared to the U.S., Mexico is a blank slate in terms of paleontology,” Tseng said. Regions can be difficult to work in because of the topography, and not many long-term field projects exist here.

“This is the beginning of the study,” Tseng said. “Now that we have evidence of these animals moving through Mexico, we can look for evidence of other animals doing the same.”

Source: Science Daily (en)

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