The therocephalians and cynodonts are thought to have diverged in the
Middle Permian, and each group independently evolved mammal-like features, including a
secondary palate and the loss of a
postorbital bar (these features were retained in mammals, which are considered a
derived group of cynodonts). Mammalian features that both groups inherited from a common ancestor include the loss of teeth on the
palate, the expansion of the epipterygoid bone at the base of the skull (an area called the
alisphenoid in mammals), and the narrowing of the
skull roof to a narrow
sagittal crest running between large
temporal openings. ==References==