Ancient Tropical Tortoise Found in a Deep Blue Sinkhole in the Bahamas

The Sawmill Sink yields a well-preserved skeleton and shell that allow researchers to obtain DNA samples

The Sawmill Sink hole, on Great Abaco Island in the Bahamas, had something special waiting at the bottom for divers from the University of Florida: the 1,000-year-old skeleton of an extinct tortoise species, one so well preserved it yielded its ancient DNA intact.

The find, described by University of Florida researchers in the journal Proceedings of the Royal Society B, is being called the first DNA sample extracted from an extinct tropical species.

"This is the first time anyone has been able to put a tropical species into an evolutionary context with molecular data," said study co-author David Steadman, in a statement.

"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," he added.

The tortoise, Chelonoidis alburyorum, with a shell described as about the size of a football, seems to have lived a harsdscrabble life. Evidence of bite marks from predators such a crocodiles adorned the shell's surface.

The fact that anything from the ancient animal, much less DNA, was intact was a happy accident of chemistry. For DNA to remain intact for a long time, it needs to exist in cold, dry conditions - quite the opposite of life in the West Indies. But Sawmill Sink is a deep blue hole – deep enough that, at its bottom, 80 feet down, say researchers, water devoid of oxygen is created by the decay of plants and animals. That lack of oxygen kept the tortoise preserved, allowing researchers 1,000 years later to find it and sample its DNA.

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Bone collagen preserved in the skeleton helped the researchers date the find, while other, more poorly preserved tortoises allowed them to determine that Chelonoidis alburyorum went extinct about 780 years ago. Not coincidentally, the scientists say, soon after humans settled in the area.

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

Will the tropics yield more such well-preserved treasures of nature? Only time will tell.

"We now know so much about the tortoise's anatomy, how it lived and its evolutionary context," Steadman said. "To be able to do that with other species is a goal."

Top Photo: The fossil skull of the Bahamian tortoise, which yielded the first ancient tropical DNA.

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