''Diarthrognathus'' ("Two joint jaw") is an extinct

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of tritheledontid

cynodont Cynodontia () is a clade of eutheriodont therapsids that first appeared in the Late Permian (approximately 260 Megaannum, mya), and extensively diversified after the Permian–Triassic extinction event. Mammals are cynodonts, as are their extin ...

s, known from fossil evidence found in

South Africa South Africa, officially the Republic of South Africa (RSA), is the Southern Africa, southernmost country in Africa. Its Provinces of South Africa, nine provinces are bounded to the south by of coastline that stretches along the Atlantic O ...

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 The Early Jurassic Epoch (geology), Epoch (in chronostratigraphy corresponding to the Lower Jurassic series (stratigraphy), Series) is the earliest of three epochs of the Jurassic Period. The Early Jurassic starts immediately after the Triassic� ...

period, about 200 million years ago. It was

carnivorous A carnivore , or meat-eater (Latin, ''caro'', genitive ''carnis'', meaning meat or "flesh" and ''vorare'' meaning "to devour"), is an animal or plant whose nutrition and energy requirements are met by consumption of animal tissues (mainly mu ...

and small, slightly smaller than ''

Thrinaxodon ''Thrinaxodon'' is an extinct genus of cynodonts which lived in what are now South Africa and Antarctica during the Late Permian - Early Triassic. ''Thrinaxodon'' lived just before, during, and right after the Permian–Triassic mass extinction ...

'', 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 The articular bone is part of the lower jaw of most vertebrates, including most jawed fish, amphibians, birds and various kinds of reptiles, as well as ancestral mammals. Anatomy In most vertebrates, the articular bone is connected to two o ...

bones, and its derived, mammalian jaw joint is located between the

squamosal The squamosal is a skull bone found in most reptiles, amphibians, and birds. In fishes, it is also called the pterotic bone. In most tetrapods, the squamosal and quadratojugal bones form the cheek series of the skull. The bone forms an ancestra ...

and

dentary In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone ...

bones. 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 Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the Heredity, heritable traits characteristic of a population over generation ...

favoured animals with a more powerful bite. At one time, ''Diarthrognathus'' was thought to be synonymous with ''

Pachygenelus ''Pachygenelus'' is a genus of extinct cynodont. Fossils have been found from the Karoo basin in South Africa and date back to the Early Jurassic. The genus was named in 1913 on the basis of a partial lower jaw found from South Africa, with the ...

''. However, in 1980, newly discovered fossils revealed sufficient differences to warrant separate genera. The double jaw joint of ''Diarthrognathus'' neatly bridges early synapsids and mammals, and thus rebuts a claim by

creationists Creationism is the religious belief that nature, and aspects such as the universe, Earth, life, and humans, originated with supernatural acts of divine creation, and is often pseudoscientific. Gunn 2004, p. 9, "The ''Concise Oxford Dictionary ...

, such as Duane Gish, who thought such a transition was impossible. This "twin-jointed jaw" can also be seen in other derived cynodonts, such as early

mammaliaform Mammaliaformes ("mammalian forms") is a clade of synapsid tetrapods that includes the crown group mammals and their closest extinct relatives; the group radiated from earlier probainognathian cynodonts during the Late Triassic. It is defined a ...

s.Colbert, Edward and Morales, Michael. ''Evolution of the Vertebrates: A History of the Backboned Animals Through Time'', p. 228 (Wiley-Liss, 4th edition, 199)

References

{{Taxonbar, from=Q3765186 Tritheledontidae Early Jurassic synapsids of Africa Fossil taxa described in 1958