Wikipedia
Diarthrognathus ("Two joint jaw") was a genus of synapsid, known from fossil evidence found in South Africa. and first described in 1958 by A.W. Crompton. The creature lived during the Late Triassic to Early Jurassic periods, about 200 million years ago. It was carnivorous and small, slightly smaller than Thrinaxodon which was under long.
Diarthrognathus possesses a jaw structure that is similar to both mammals and more basal synapsids. Its primitive jaw joint is located between the quadrate and articular bones, and its derived, mammalian jaw joint is located between the squamosal and dentary bones.
The articular and quadrate bones evolved to become two of the middle-ear bones in mammals. The transition exemplified by Diarthrognathus suggests that natural selection favored animals with a more powerful bite.
At one time, Diarthrognathus was thought to be synonymous with Pachygenelus. However, in 1980, newly discovered fossils revealed sufficient differences to warrant separate genera.
The double jaw joint of Diarthrognathus neatly bridges the reptiles and mammals, and thus rebuts a claim by creationists such as Duane Gish who thought such a transition was impossible. This "twin-jointed jaw" can also be seen in other late cynodonts, as well as in early mammaliforms.