Published July 5, 2017
The passage is along the northern edge of the Trans-Mexican Volcanic Belt, which runs across Mexico at the latitude of Mexico City.
The discovery was made this past March during a dig in the central Mexican state of Zacatecas. During the dig, a paleontologist uncovered a jawbone and called over to Tseng, who was researching intercontinental immigration of fossil mammals.
"I thought it was a badger, but a colleague on the site had just finished a study of otters, and he said it was otter-like," Tseng says. "But what would a sea otter be doing in central Mexico?"
It turns out that the otter may have been part of an immigration event from Florida to California some 6 million years ago.
Based on the discovery, Tseng and his colleagues wrote a paper that proposed the passage through Mexico. Titled "Discovery of the Fossil Otter Enhydritherium terraenovae (Carnivora Mammalia) in Mexico Reconciles a Paleozoogeographic Mystery," it was published June 14 in the journal Biology Letters.
"This is an entirely new idea that no one else has proposed," Tseng says. "We think it's very likely other animals utilized this route."
This breakthrough came from a fortunate detail — the jawbone held several teeth.
"One tooth was a lower molar, the most diagnostic tooth in a carnivore," Tseng says. "If we are lucky enough to find a fossil molar tooth that is complete, there is a lot of useful information."
The tooth is almost identical to one found from another Enhydritherium terraenovae (an ancient sea otter) fossil found in Florida. Similar finds have been made along the coasts, in Florida and California, but paleontologists did not know how the animals got across the continent.
One hypothesis was that they moved up and around and through northern Canada, a 5,000-mile trip. Another hypothesis was that they made it down to Panama and crossed over to the west.
The possibility of an east-west migratory route in Mexico in the Miocene geologic epoch (roughly 23 million to 5.3 million years ago) has implications for a much larger biologic event — the Great American Biotic Interchange, when land bridges were formed and animals dispersed to and from North America and South America. It shows that the region's fossil sites could have recorded details of this biological interchange of historic proportions.
The region of the dig site is the Juchipila Basin, which is 535 miles southeast of Laredo, Texas.
The region is difficult to work in because of the topography and flora, like cactus. Therefore, not many long-term field projects exist there.
"Compared to the U.S., Mexico is a blank slate in terms of paleontology," Tseng says. "This is the beginning of the study. Now that we have this evidence of these animals moving through Mexico, we can now look for evidence for other animals doing the same."
Tseng says he expects some people will not agree with the new interpretation of an east-west corridor through Mexico for other mammals. But more research may confirm it.
"We are aware it is a single discovery," Tseng says. "It essentially opens up a can of worms. We are throwing a different factor in. We now have a connection between Florida and California, and it's not in a straight line."