“Today, Aldabra Atoll, an island in the Indian Ocean near Madagascar and Tanzania, is a predator-free paradise for more than 100,000 giant tortoises. Gone are the seafarers who over-hunted them to near extinction in the 18th and 19th centuries. Gone too are the large crocodiles that may have preyed upon them in prehistoric times, as a new study suggests.
Dennis Hansen, an ecologist from the University of Zurich, was exploring the coral atoll and its azure lagoon, part of the Seychelles, when he came across fossils that intrigued him. They included parts of a giant tortoise shell with circular bite marks and the jaw of an ancient crocodylian. From the finds, he and a colleague concluded that some 90,000 to 125,000 years ago, the ancient crocodiles may have feasted upon the giant tortoises of Aldabra.
The finding, which was published Tuesday in the journal Royal Society Open Science, may offer new insights into the ancient past of the world’s most numerous giant tortoise and the threats it once faced from predators.
Measuring more than 3 feet long and 550 pounds on average, the Aldabra giant tortoise is behind only the Galapagos tortoise in size. It has a thick, domed shell which protects its soft body. It also has a long neck and stout legs covered in scales.
Dr. Hansen found about 180 fossils near a pond on the atoll. One was a jawbone of an ancient crocodile that he first thought was a tortoise longbone. But when he turned it over he saw it had holes where teeth could fit. He sent some of the fossils to his colleague Torsten Scheyer, a paleontologist also at the University of Zurich, for further examination.
“It was pretty clear from the start that these were crocodylian remains and something distinctly larger than what was found previously on the atoll,” Dr. Scheyer said.
Although smaller crocodile fossils had been found on the island, the remains that Dr. Hansen found suggested they belonged to beasts that were about 11 and a half feet long, larger than today’s West African crocodiles but not as big as Nile crocodiles or saltwater crocodiles.
Dr. Hansen also found a fragment of a giant tortoise fossil belonging to the part of the shell that would have protected the reptile’s neck. The bite mark on the plate, he said, offered two possible scenarios for what might have occurred.
The first, is that the tortoise was ambushed as it drank.
“Maybe the crocodile, in classic crocodile fashion, jumped out of the water and grabbed it in the only place where it could grab it,” said Dr. Hansen.
The second, is that the tortoise died near the pond from starvation or heat stroke in the arid environment and a nearby crocodile scavenged its rotting carapace.
Stephanie K. Drumheller-Horton, a paleontologist at the University of Tennessee, Knoxville, said the findings were supported by other examples of crocodile bite marks in the fossil record.
“Aldabra giant tortoises would have been tough nuts to crack, even for crocodiles, who can generate incredibly high bite forces,” Dr. Drumheller-Horton said. “It makes sense that you would see bite marks around the edges of the turtle shells, where the crocodiles could more easily get at exposed limbs.”
Alexander Hastings, the assistant curator of paleontology at the Virginia Museum of Natural History, said the authors showed that it was possible that large crocodiles feasted on giant tortoises. But, he added, they would need to find evidence of tortoises with healed wounds in order to show explicitly that the crocodiles were actively hunting and killing the giant tortoises.
“It was thought something that big and that well-armored would be pretty safe from predation, especially on such an isolated island,” Dr. Hastings said. “This study raises at least the possibility that they may have not been so safe.”