Like modern freshwater turtles, Eileanchelys probably didn't have flippers, which in turtles are first seen close to a hundred million years ago in the first sea turtles.
The study team found other clues to Eileanchelys' aquatic lifestyle during fieldwork funded by the National Geographic Society's Committee for Research and Exploration. (National Geographic Society owns National Geographic News.)
Prehistoric sediments the fossils were embedded in were strewn with the remains of small fish, sharks, salamanders, and other aquatic animals, said study co-author Susan Evans, a professor of paleontology at University College London.
"The fact the turtles were found with an aquatic fauna means we can be pretty sure they were aquatic animals," Evans said.
The species is thought to have inhabited ponds and coastal lagoons at a time when Skye was tropical.
"Salt water might have got into those lagoons, bringing in things like sharks," Evans said. "But the fact we've got things like salamanders there, which simply won't tolerate salt water, tells us it was mostly a freshwater environment."
Intact fossils of primitive turtles are extremely rare, according to Evans.
"To get a whole rock full of the things, with skulls as well, was really remarkable," the paleontologist said.
Taken with several other recent finds, the Skye discovery suggests turtles diversified earlier than had been thought, the study team writes in the most recent issue of the journal Proceedings of the Royal Society, Series B.
For example, another turtle, Condorchelys, was described in 2008 from Middle Jurassic-aged rocks in Argentina.
"A few years ago we didn't know any turtle from the Middle Jurassic," said the Natural History Museum's Anquetin.
Walter Joyce of the Peabody Museum of Natural History at Yale University says the new study makes a compelling case that "by this stage in evolution, turtles had started moving into aquatic habitats."
The paleontologist said the find is also very exciting, given that "a 65-million-year gap used to exist in the fossil record between the oldest known turtles from the Late Triassic and basically modern turtles, in the Late Jurassic."
"Together with recent finds from the U.S., Argentina, and Russia, the new fossil from Scotland is finally giving us a glimpse of how early turtles evolved," Joyce said.
"The new turtle is really quite spectacular in preservation, considering that several complete skeletons are preserved, instead of the usual scrap that has to be pieced together," he added.
Torsten Scheyer is a researcher at the Palaeontological Institute and Museum at the University of Zurich, Switzerland.
He said the very oldest turtles probably had a similar lifestyle to that of modern tortoises, and that it wasn't until the Late Jurassic that aquatic turtles began to flourish.
"We lack quite a bit of data in between, so this is why these turtles from the Middle Jurassic are very important for us," Scheyer said.
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