More here from the Mount Desert Island Biological Laboratory in Bar Harbor, Maine.
From Nature News:
According to the fossil record, turtles first appeared in the Triassic period around 230 million years ago, and have changed little since. Crucially, turtles lack temporal fenestrae, suggesting that they are the living relatives of a more primitive reptilian form that pre-dates the diapsids. Consequently, they were set aside in their own clade, somewhere between amphibians and the rest of the reptiles.
However, there are two other possibilities that would bring turtles into the diapsid fold. One is that turtles are more closely related to birds and crocodiles than to lizards. Most of the genetic studies undertaken so far support this.
The other possibility is that turtles are much more closely related to lizards and snakes, with this group sharing a more recent common ancestor than the rest of the diapsid clade.
Disappearing act
Genetic analysis of a turtle, a lizard and an alligator1 now supports the latter hypothesis. This implies that the ancestors of turtles did once have temporal fenestrae in their skulls, but that these have disappeared as turtles evolved.
“What we’ve been able to provide is unambiguous evidence for one hypothesis over the others,” says molecular palaeobiologist Kevin Peterson of Dartmouth College in Hanover, New Hampshire, a coauthor on the new study.