<P> Testudines (turtles) </P> <Tr> <Td> (crown group) </Td> </Tr> <Tr> <Td> (total group) </Td> </Tr> <P> The placement of turtles has historically been highly variable . Classically, turtles were considered to be related to the primitive anapsid reptiles . Molecular work has usually placed turtles within the diapsids . So far three turtle genomes have been sequenced . The results place turtles as a sister clade to the archosaurs, the group that includes crocodiles, dinosaurs, and birds . </P>

Explain why a classification system containing class reptilia and class aves is not valid