Turtles misplaced on reptile family tree.

Turtles long have been thought to be the most primitive living reptiles. making them an important model for scientists who study reptiles ranging from the extinct dinosaurs to modern-day snakes, lizards, and crocodiles. However, paleontologist Olivier Rieppel of Chicago's Field Museum and Michael deBraga, a zoology student at Erindale College, Ontario, Canada, have discovered that turtles may not be so primitive. They found that the turtle clade (group that shares common ancestry), testudines, has been assigned to the wrong place in the reptile family tree, a discovery that "may not only change the way we look at turtles," suggests Rieppel. "but may change our view of reptiles in general."

Rieppel and deBraga conducted a detailed computer analysis of reptiles which revealed that turtles share more characteristics with diapsids -- modern-day reptiles -- than with anapsids, the more primitive reptile clade to which they long have been assigned. One major difference between anapsid and diapsid reptiles is in the cheek or temporal area of the skull. Anapsid reptiles have no holes there; diapsid reptiles have two -- upper and lower -- between their cheek bones.

All reptiles living today are diapsid, with the exception of turtles, previously thought to be anapsid, making them the only living primitive reptiles. Because of this assumption, previous cladistic or computer analyses of the relationships between turtles and other reptiles based on bone characteristics only have included those reptiles living in the Paleozoic time period, which ended 225,000,000 years ago.

For the analysis, Rieppel and deBraga compiled an extensive data matrix for amniota (reptiles, birds, and mammals) that included 33 amniote groups and 168 characteristics of their bones -- an important physical characteristic in defining clades. Their study included reptiles living today and those that existed during the Mesozoic time period, which ended 65,000,000 years ago. Including these more "modern" reptiles in the computer analysis yielded surprising results. 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.

An important finding supporting Rieppel and deBraga's conclusion about turtles has to do with the pattern of ankle bone formation in turtles, lizards, and an endangered lizard-like species known as tuatara, which survives on islands off the coast of New Zealand. "Turtles, tuatara, and lizards share a pattern of bone formation in the ankle which is unique in reptiles, birds, and mammals," Rieppel points out. "This is a very strong signal for a close relationship."