By James Morgan
Science reporter, BBC News
The new turtle species is a missing link between land and water-based turtles
The earliest turtles known to live in water have been discovered on a Scottish island.
The 164 million-year-old reptile fossils were found on a beach in southern Skye, off the UK's west coast.
The new species forms a missing link between ancient terrestrial turtles and their modern, aquatic descendants.
The discovery of Eileanchelys waldmani, which translates as "the turtle from the island", is reported in the Royal Society journals.
The turtles were found embedded in a block of rock at the bay of Cladach a'Ghlinne, on the Strathaird peninsula.
It contained four well-preserved turtle skeletons, and the remnants of at least two others.
The team uncovered four remarkably well preserved turtle skeletons
Together, these are the most complete Middle Jurassic turtles described to date.
The historic specimens are now being housed in the National Museums Scotland, Edinburgh.
They were uncovered by a team from London's Natural History Museum and University College London (UCL).
"Why did turtles enter the water? We have no idea. It's a mystery - like asking why cetaceans went back into the sea," said Jérémy Anquetin, of the department of palaeontology at the Natural History Museum.
"Little by little, we are filling the gaps.
"Now, we know for sure that there were aquatic turtles around 164 million years ago.
"Eileanchelys may represent the earliest known aquatic turtle.
"It is part of a new revision we are having about turtle evolution."
The Isle of Skye looked very different 164 million years ago
The new species helps bridge a 65 million-year gap in the story - between the terrestrial "basal" turtles, from the late Triassic, and the aquatic "crown-group" turtles of the late Jurassic.
The former were "heavy-built" land-dwellers, with skulls which were "more reptilian", says Mr Anquetin.
The latter were lighter, and closer in appearance to the aquatic, freshwater turtles we know today.
I am blown away that the team was able to recover such extraordinary material
What happened in between was a mystery, until very recently.
In the last two years, fossils of three new turtle species, all dating to to the Middle Jurassic, have been discovered in Russia, Argentina, and now Scotland.
"The Scottish fossils are the most complete of them all", says Anquetin.
"They tell us a lot about how the primitive 'stem turtles' diversified into the varied forms we see today."
So what would these "missing links" have looked like? Certainly nothing like the marine turtles which are occasionally seen on the Skye coastline today.
The rocks at Cladach a'Ghlinne hold many ancient aquatic fossil specimens
On the outside, E. waldmani would resemble a modern freshwater turtle - "like the ones you can buy in the pet shop", says Mr Anquetin.
"The differences are on the inside - in the cranial anatomy. They are small differences but very important. There is no other turtle like this one."
The fossils have now been recognised as a new species, named Eileanchelys to incorporate "Eilean", the Gaelic word for "island".
"I liked the idea of giving it a name in Gaelic," explains Anquetin.
"So I tried to find words that sound good in Gaelic and Latin.
"I chose 'Eilean', so the whole name means 'the turtle from the island'."
However, the turtles would have lived in a land unrecognisable from the rugged, wind-battered coastlines of modern day Skye.
In the Middle Jurassic, the land mass was much further south, basking in a warm, sub-tropical climate.
The fossils were discovered on the rugged shores of Skye's Strathaird peninsula
The turtles probably lived in a landscape of shallow lagoons and freshwater lakes, according to the authors.
Their claims are founded on the geology of the rocks in which the turtle fossils were found - alternating mudstones, shales and occasional limestone horizons - sediments which were laid down in closed water systems.
Other aquatic species, such as sharks and salamanders, were found alongside the turtle fossils.
But remains of terrestrial vertebrates, such as lizards and dinosaurs, were "exceptionally rare".
"If [we accept all this evidence], E. waldmani plausibly represents the first aquatic turtles."
The remains are "spectacular in preservation".
Their findings are reported in the journal Proceedings of the Royal Society B: Biological Sciences.
"This new turtle is very exciting", said Dr Walter Joyce, an expert in turtle evolution, formerly of the Peabody Museum of Natural History, Yale University.
"Keep in mind 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.
"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.
"The find confirms that basal turtles were a global phenomenon. It also confirms my research that the split into the primary groups that we see today did not occur until later than originally thought.
"Finally, although it is really difficult to assess the ecological habitat preferences of turtles, the authors make a compelling case that by this stage in evolution turtles had started moving into aquatic habitats."
He added: "I am blown away that the team was able to recover such extraordinary material from the icy cold shores of Scotland, an area generally not known for its turtle fossils!"
The fossilised turtle shell reveals the transition from land to sea