diapsid

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All reptiles living today are diapsid, with the exception of turtles, previously thought to be anapsid, making them the only living primitive reptiles.
When all of the data -- much of it provided by deBraga, whose research focuses on Paleozoic reptiles -- was crunched through the computer, "we found that turtles flipped from anapsid to diapsid," Rieppel explains.
Turtles fell squarely within the modern diapsids rather than in their expected position on a branch outside the group, the scientists report in the Nov.
Their results lend support to a 1996 analysis of reptile fossils, which also suggested that turtles evolved from diapsids, says Olivier C.
Both cases suggest that turtles evolved from diapsids and then lost the characteristic skull holes.