Some 13,000 years ago in what’s now the Yucatán Peninsula, a deep pit inside a cave became the final resting place for a menagerie of exotic animals.

Now, their exquisitely preserved bones, trapped for centuries under water, are offering some of the first solid clues to how large Ice Age beasts were mixing and migrating between North and South America after the Isthmus of Panama connected the two continents.

“We’re going to go from a place with no records to having the best records for a lot of megafauna from Mexico, Central America, and northern South America,” says East Tennessee State University’s Blaine Schubert, who presented the findings this week at the Society of Vertebrate Paleontology’s annual meeting in Calgary.

The animal bones are also painting a more detailed picture of the strange world inhabited by Naia, an Ice Age girl found in the cave who is the oldest, most complete human skeleton yet discovered in the Americas. (See how humans first entered the Americas.)

Giant Underwater Cave Was Hiding Oldest Human Skeleton in the Americas In a deep underwater cave, three divers make a stunning discovery: the oldest complete human skeleton ever found in the Americas. See the ancient remains, venture through the remarkable deep-water chamber, and see how a skeleton belonging to a teenage girl from 13,000 years ago led scientists to a major revelation about the earliest Americans.

Like the saber-toothed cats, giant sloths, and other wild creatures trapped in the cave, Naia most likely wandered in looking for fresh water and took a fatal fall into the 90-foot-deep pit. Later, rising seas brought on by melting glaciers raised the Yucatán Peninsula’s water table by hundreds of feet, flooding the caves and entombing the skeletons.

Cave divers first came across Hoyo Negro—Spanish for “black hole”—in 2007, and were stunned to find a massive water-filled chamber rife with articulated animal remains and the skeleton of Naia. Expeditions through the years have mapped at least 28 animal skeletons within the pit, only a handful of which have been fully excavated.

Carnivore Complexity

Now some of those bones have been recovered, and the latest fossil examinations are giving paleontologists crucial new insight into the Great American Interchange, a dizzying migration of ancient animals between North and South America.

After the Isthmus of Panama tectonically rose from the sea some 3 to 5 million years ago, the ecosystems that existed on the two continents—left to stew in their own evolutionary juices for tens of millions of years—were at last able to mix and mingle.

This complex exchange of life settled into the Americas’ modern ecosystems: South America gave North America armadillos, and North America gave South America llamas. But paleontologists still know very little about this massive interchange, since fossils are notoriously hard to come by in the tropical forests that cover the region.

By contrast, Hoyo Negro is a fossil bonanza. The underwater cave preserved entire animals, because the carcasses had nowhere to go, and the low-oxygen waters ensured that the remains laid undisturbed for more than ten thousand years.

Paleontologists and cave divers have found the remains of saber-toothed cats, peccaries, mountain lions, tapirs, and elephant-like animals called gomphotheres within Hoyo Negro’s depths. Some parts of the underwater cave even preserved the footprints of ancient bears, crusted over in a film of calcite.

Earlier this year, the joint U.S.-Mexico team identified a new species of ground sloth in the cave, which they named Nohochichak xibalbahkah, Mayan for “great-clawed dweller of the underworld.” (Read about another ancient sloth found elsewhere in the Yucatán.)

At the paleontology meeting, Schubert revealed that bears also entered these caves—and, in the case of Hoyo Negro, sometimes never exited. Divers have found three exquisitely preserved skulls of the extinct bear species Arctotherium wingei. A cousin of the Andean spectacled bear, the ancient species was slightly smaller than today’s grizzly bear. (Read more about its sister species—the biggest bear that ever lived.)

The skulls are so well-preserved, Schubert says, that visitors to his lab often mistake them for high-quality reconstructions. Divers also found the skull of a stocky, coyote-like canid previously only known from South America.

Together, the remains represent the first hard evidence of carnivores leaving North America for South America, diversifying into new South American species, and then returning northward—adding greater complexity to the Great American Interchange.

It’s likely that Hoyo Negro will turn up yet more surprises: Schubert recently received a National Geographic grant for further fieldwork at the site. The team hopes to collect more bones—and venture deeper into the darkness.