Fossils from Middle America are rare, and these extinct mammals help shed light on the impacts of climate change and environmental fluctuations, as well as the evolution and migration of organisms.
Starting around three million years ago, South America connected to North America after tens of millions of years of isolation. This led to the Great American Biotic Interchange, when some northern animals moved south and some southern animals moved north.
These dispersals occurred in pulses associated with waxing and waning glacial periods and rising and falling sea levels, gradually laying the foundation for the ecosystems we see in the Americas today. But fossil sites in Middle America, where the exchange actually took place, are very rare.
Hoyo Negro, a deep pit within the enormous Sac Actun cave system, is one of these rare sites. Between around 40,000 and 13,000 years ago, during the Late Pleistocene Epoch, the pit acted as a trap, collecting the remains of animals that fell in while wandering through the caves, including ground sloths, elephant-like gomphotheres, tapirs, saber-tooth cats and even the famous skeleton of Naia, the most complete early human in the Americas.
At the end of the last Ice Age, massive glaciers melted in northern latitudes, and sea level rose dramatically. By 10,000 years ago, the entire cave system was flooded by rising water levels, leaving many of the fossils submerged more than 130 feet under water.
These conditions have made Hoyo Negro a difficult-to-reach fossil site that is worth exploring for its bounty of exquisitely well-preserved fossils.
East Tennessee State University’s Dr. Blaine Schubert is the lead author of a study recently published in the prestigious journal Biology Letters, in which researchers describe two unexpected species from Hoyo Negro.
These species are Protocyon troglodytes, a large relative of coyotes and wolves, and Arctotherium wingei, a relatively small short-faced bear and currently the only bear known from the Yucatán Peninsula. Both of these extinct species have only been found in South America, and their discovery in the Yucatán extends their known range by more than 1,200 miles.
Some ursids (bears) and canids (wolves and their relatives) moved into South America from North America during the Interchange, but the fact that these two South American species were living in the Yucatán at the end of the Ice Age suggests their history of movement was more complicated.
There are many unanswered questions about the timing and patterns of migration during the various phases of the Interchange, and the researchers are hopeful that further expeditions to Hoyo Negro and other similar sites will lead to long-awaited answers. This ongoing project has its next expedition in November.
Recovering and researching these fossils was an international effort. Schubert, a professor of Geosciences and director of the ETSU Museum of Natural History at the Gray Fossil Site, was brought onto the project for his experience with cave paleontology and fossils in the Americas.
But once he recognized South American species among the fossils, he explains, “the significance of the site became clear and it was important to get researchers involved from South America who were more experienced with these animals.”
The full team of researchers included other scientists from ETSU Geosciences and the Don Sundquist Center of Excellence in Paleontology, Drs. Joshua Samuels and Chris Widga, as well as others from California, Washington, Argentina and Mexico. In addition, the Instituto Nacional de Antropologia e Historia (INAH) in Mexico is integral in managing the fossil site and the fossils themselves.
Without the support of INAH, Schubert says, this project would not be possible.
“We’re talking about the American Interchange here, so it is important that we have an interchange of scientists from the Americas,” he said.
It was also essential to utilize the expertise of specific cave divers.
“I’m a caver. I’m a diver. But I’m not a cave diver … not even close,” Schubert notes. The diving team was composed of highly trained technical divers who travel through the horizontal passages and down into Hoyo Negro itself to document fossil remains in the pit.
“From images, researchers make digital models of the site, pick which fossils they want collected and send the divers back in to gather them. Diving into a pitch-black underwater cave surrounded by tropical jungle is not at all the classic image of paleontology.
For ETSU paleontologists, Hoyo Negro is also an intriguing comparison with the fossil site in their own backyard, the 5-million-year-old Gray Fossil Site in East Tennessee. The Gray Site was also a cenote, or sinkhole, that opened up in a warm, forested region, and it even shares some animals with Hoyo Negro, including tapirs, ground sloths and a much earlier species of short-faced bear.
“There may have been a time when you could have put on scuba gear and gone into the Gray Fossil Site cave system looking for fossils,” Schubert says.