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Two divers explore deep inside a blue hole in the Bahamas. (Credit: Brian Kakuk/U. Florida)

fossils

Divers pull amazing tortoise skeleton from sinkhole

An extinct tortoise species accidentally tumbled into a water-filled limestone sinkhole in the Bahamas about 1,000 years ago. It’s finally made its way out—with much of its DNA intact.

As the first sample of ancient DNA retrieved from an extinct tropical species, scientists say the genetic material could provide insight into the history of the Caribbean tropics and the reptiles that dominated them.

It could also offer clues to the region’s future, as the tropics undergo significant transformation due to climate change.

tortoise skull
The fossil skull of the Bahamian tortoise. (Credit: U. Florida)

“This is the first time anyone has been able to put a tropical species into an evolutionary context with molecular data,” says David Steadman, an ornithology curator at the Florida Museum of Natural History at the University of Florida.

“And being able to fit together the tortoise’s evolutionary history together will help us better understand today’s tropical species, many of which are endangered.”

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Scientists call the finding “boundary-pushing” and say that even after DNA was extracted from the tortoise bones, they weren’t optimistic that much information could be gleamed from it.

“Not only did we have DNA, we were surprised to find we could amplify it and sequence DNA beyond what we had available,” Steadman says.

“The tortoise went through a pretty ugly existence.”

Most ancient DNA has come from mammals that lived in temperate regions. “The two things that are really good for the long-term preservation of DNA are coldness and dryness,” Steadman says. “And the tropics typically provide neither one.”

tortoise carapace
(Credit: U. Florida)

A plastic 3D model created from the ancient tortoise’s shell rests easily in two hands, about the size of a football. Bite marks from crocodiles and other predators are visible on the surface. “The tortoise went through a pretty ugly existence,” Steadman says.

After retrieving the tortoise from Sawmill Sink, a deep blue hole in the Bahamas with steep vertical walls, scuba divers found not only the shell intact, but the entire skeleton. “That’s really unheard of in the fossil record, especially in the West Indies,” Steadman says.

Access to the tortoise’s skeleton and DNA allowed Florida Museum herpetologist emeritus and study coauthor Richard Franz to describe its anatomy and structure in as much detail as modern species. Divers found other giant tortoises preserved in the water, but performed DNA analysis on only one for the published study.

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“In the fossil record, so many species are described just from a few fragments that exist, and while it’s a lot better than nothing, you don’t get to characterize the entire critter,” Steadman says. “Whereas, with this tortoise, well, here it is.”

As reported in the Proceedings of the Royal Society B, the tortoise skeleton contained bone collagen, a protein, which allowed scientists to radiocarbon date the animal and find out when it died. Several other tortoises that were also found in the Bahamas—though not as well preserved—helped researchers determine the species went extinct about 780 years ago, soon after the arrival of human settlers in the area.

“There’s a correlation that the arrival of humans spelled the demise of the tortoises,” Steadman says. “It’s probably a blend of direct hunting and habitat loss as the humans started burning the forests in the dry season.”

The chemical composition of the water in Sawmill Sink prevented the decay of animals that fell into the water and sunk to the bottom 80 feet down. The secret: water with no oxygen. The water in Sawmill Sink is stratified, or has several layers. The decay of plants and animals removes the oxygen from the water deeper than 70 feet, helping to preserve the fossils.

Although the conditions in Sawmill Sink are an exception rather than a rule, the findings give scientists more hope of finding material from other extinct tropical species.

“We now know so much about the tortoise’s anatomy, how it lived, and its evolutionary context,” he says. “To be able to do that with other species is a goal.”

Source: University of Florida

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