Diarthrognathus ("Two joint jaw") is an extinct genus of tritheledontid cynodonts, known from fossil evidence found in South AfricaDiarthrognathus - Encyclopædia Britannica. and first described in 1958 by A.W. Crompton.Rieppel, Olivier. Evolutionary Theory and the Creation Controversy, p. 190 (Springer, 2010). The creature lived during the Early Jurassic period, about 200 million years ago. It was carnivorous and small, slightly smaller than Thrinaxodon, which was under long.Crompton, A.W. "Masticatory Function in Non-Mammalian Cynodonts and Early Mammals" in Functional Morphology in Vertebrate Paleontology, p. 64 (J. Thomason, ed., Cambridge University Press 1997).
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.Prothero, Donald. Evolution: What the Fossils Say and Why It Matters, p. 278 (Columbia University Press, 2013).
The articular and quadrate bones evolved to become two of the middle-ear bones in mammals.The Mesozoic Era: Age of Dinosaurs, p. 183 (Britannica Educational Publishing, Rosen Publishing Group, 2010). The transition exemplified by Diarthrognathus suggests that natural selection favoured animals with a more powerful bite."How Animals Got Their Bite", New Scientist, p. 146 (July 18, 1963).
At one time, Diarthrognathus was thought to be synonymous with Pachygenelus. However, in 1980, newly discovered fossils revealed sufficient differences to warrant separate genera.Martinelli, Agustín and Bonaparte, José."A new tritheledontid (Therapsida, Eucynodontia) from the Late Triassic of Rio Grande do Sul (Brazil) and its phylogenetic relationships among carnivorous non-mammalian eucynodonts", Ameghiniana, Vol. 42, p. 191 (2005).
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.Kitcher, Philip. Abusing Science: The Case Against Creationism, p. 111 (MIT Press 1982). This "twin-jointed jaw" can also be seen in other derived cynodonts, such as early mammaliaforms.Colbert, Edward and Morales, Michael. Evolution of the Vertebrates: A History of the Backboned Animals Through Time, p. 228 (Wiley-Liss, 4th edition, 199)
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