Temporal range: 201–189 Ma Lower Jurassic
Diarthrognathus ("Two joint jaw") is an extinct genus of synapsids, 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 50 centimetres (20 in) 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.
The double jaw joint of Diarthrognathus neatly bridges early synapsids 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.
- Diarthrognathus - Paleobiology Database
- Diarthrognathus - Encyclopædia Britannica.
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