''Ordosiodon'' is an extinct

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial n ...

therocephalia Therocephalia is an extinct suborder of eutheriodont therapsids (mammals and their close relatives) from the Permian and Triassic. The therocephalians ("beast-heads") are named after their large skulls, which, along with the structure of their ...

therapsid Therapsida is a major group of eupelycosaurian synapsids that includes mammals, their ancestors and relatives. Many of the traits today seen as unique to mammals had their origin within early therapsids, including limbs that were oriented mor ...

s from the

Early Triassic The Early Triassic is the first of three epochs of the Triassic Period of the geologic timescale. It spans the time between Ma and Ma (million years ago). Rocks from this epoch are collectively known as the Lower Triassic Series, which is a ...

of China. It includes two species, ''O. lincheyuensis'' and ''O. youngi''. ''O. lincheyuensis'', the

type species In zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the species that contains the biological type specimen( ...

, was named by Chinese paleontologist C. C. Young (Yang Zhongjian) in 1961 on the basis of a partial lower jaw. The jaw was discovered by a petroleum survey team in October 1958 from the Lower

Ermaying Formation The Ermaying Formation is a sedimentary succession of Anisian (Middle Triassic) age. It is found in the Shaanxi Province of China. It is composed of an up to 600 m thick sequence of mudstone and sandstone. It is famous for its fossils of tetrapod ...

Shanxi Province Shanxi (; ; formerly romanised as Shansi) is a landlocked province of the People's Republic of China and is part of the North China region. The capital and largest city of the province is Taiyuan, while its next most populated prefecture-level ...

, which dates back to the

Olenekian In the geologic timescale, the Olenekian is an age in the Early Triassic epoch; in chronostratigraphy, it is a stage in the Lower Triassic series. It spans the time between Ma and Ma (million years ago). The Olenekian is sometimes divid ...

stage. Young named the genus after the

Ordos Desert The Ordos Desert () is a desert/steppe region in Northwest China, administrated under the prefecture of Ordos City in the Inner Mongolian Autonomous Region (centered ca. ). It extends over an area of approximately , and comprises two sub-de ...

, the region where the jaw was found. ''Ordosiodon'' was originally identified as a type of

diademodontid Diademodontidae is an extinct family of Triassic gomphodonts. The best-known genus is ''Diademodon'' from South Africa. '' Titanogomphodon'' from Namibia may also be a member of Diademodontidae. The Chinese genera ''Hazhenia'' and ''Ordosiodon'' ...

by Young (diademodontids are herbivorous

cynodont The cynodonts () ( clade Cynodontia) are a clade of eutheriodont therapsids that first appeared in the Late Permian (approximately 260 mya), and extensively diversified after the Permian–Triassic extinction event. Cynodonts had a wide variet ...

s that are much more closely related to mammals than are therocephalians). Young noted several unusual features of ''Ordosiodon'' that set it apart from diademodontids, including the lack of a

diastema A diastema (plural diastemata, from Greek διάστημα, space) is a space or gap between two teeth. Many species of mammals have diastemata as a normal feature, most commonly between the incisors and molars. More colloquially, the condition ...

or gap in the teeth, the lack of molar-like sectorial teeth at the back of the jaw, conical

caniniform In mammalian oral anatomy, the canine teeth, also called cuspids, dog teeth, or (in the context of the upper jaw) fangs, eye teeth, vampire teeth, or vampire fangs, are the relatively long, pointed teeth. They can appear more flattened however ...

teeth, and relatively small postcaniniform teeth. ''Ordosiodon'' was later identified as a juvenile diademodontid to account for these differences. In 1979 a partial skull and skeleton was found from the same area as ''O. lincheyuensis'' and named ''Ordosia youngi'' in honor of Young. ''O. youngi'' was identified as a therocephalian and placed in the family Ordosiidae. Distinguishing characteristics of ''O. youngi'' include an elongated

secondary palate The secondary palate is an anatomical structure that divides the nasal cavity from the oral cavity in many vertebrates. In human embryology, it refers to that portion of the hard palate that is formed by the growth of the two palatine shelves med ...

on the underside of the skull, a short snout, and a temporal (cheek) region that is wider than the back of the skull. The postcaniniform teeth increase in size farther back in the jaws. In 1981 the jaw of ''Ordosiodon'' was recognized as that of a therocephalian similar to ''Ordosia youngi''. ''Ordosia'' became a

synonym A synonym is a word, morpheme, or phrase that means exactly or nearly the same as another word, morpheme, or phrase in a given language. For example, in the English language, the words ''begin'', ''start'', ''commence'', and ''initiate'' are al ...

of ''Ordosiodon'' and ''Ordosia youngi'' was reclassified as ''Ordosiodon youngi''. The names ''

Ordosia ''Ordosia'' is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived from 501 to 490 million years ago during the Dresbachian faunal stage of the late Cambrian Period The Cambrian Period ( ; som ...

'' and Ordosiidae are now occupied by a genus and family of

Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ag ...

trilobite Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the ...

References

Bauriids Triassic synapsids of Asia Early Triassic synapsids Taxa named by Yang Zhongjian Fossil taxa described in 1961 Therocephalia genera {{Paleo-Therapsid-stub