Diver Alex Alvarez examines the remains of a mastodon, found about 20 feet (6 meters) from the human skull during the 2007 Hoyo Negro cave expedition. Other animal bones were found nearby, including the remains of a likely prehistoric bear, according to Alvarez's teammate Alberto Nava.
(Related: "Mastodons Driven to Extinction by Tuberculosis, Fossils Suggest.")
There are no openings in the cave big enough for a grown mastodon—a smaller, less hairy cousin of the woolly mammoth—to have entered. One possibility, the team says, is that the animal had been hunted and butchered by Paleo-Indian hunters, then carried into the cave in chunks.
Evidence from other sites do seem to confirm that humans once hunted mastodons in North America, according to anthropologist David Meltzer, who wasn't involved in the study.
Overall, the discoveries in Hoyo Negro are "exciting," said Meltzer, a Paleo-Indian expert at Southern Methodist University in Texas.
But, he added, the team's ideas about the bones being 10,000 years old are still "hypotheses to test, rather than conclusions to accept, at this point."
(Also see "Portal to Maya Underworld Found in Mexico?")