Dicynodontia

Article

August 14, 2022

The dicynodonts (Dicynodontia) are an extinct infraorder of herbivorous therapsids of the also extinct suborder Anomodontia, having lived from the mid-Permian to the late Triassic. They are herbivores with a pair of tusks, hence their name, literally meaning "two dog's teeth". The latter also have a horny beak, typically toothless, unique among synapsids. Dicynodonts are among the few surviving therapsids of the Permian-Triassic extinction, dominating the following period until its end. With over 70 known genera, they are the most diverse group of non-mammalian therapsids, ranging in size from a rat to that of an elephant.

Discoveries and naming

Dicynodonts have been known since the mid-1800s. South African geologist Andrew Geddes Bain gave the first description of dicynodonts in 1845. At the time, Bain was overseer of military road construction under the Corps of Royal Engineers and found many so-called "reptilian" fossils during his investigations in South Africa. Bain describes these fossils in an 1845 letter published in Transactions of the Geological Society of London, calling them "bidental" because of their two prominent tusks. In the same year, the English paleontologist Richard Owen named two species of dicynodonts from South Africa: Dicynodon lacerticeps and Dicynodon bainii. Since Bain is concerned about the Corps of Royal Engineers, he wants Owen to describe his fossils in more detail. Owen would not publish a description until 1876 in his Descriptive and Illustrated Catalog of the Reptile Fossils of South Africa in the Collection of the British Museum. At this time, many other dicynodonts are already described. In 1859, another important species called Ptychognathus declivis was named from fossils from South Africa. The same year, Owen named the group Dicynodontia. In his Descriptive and Illustrated Catalog, Owen honors Bain by erecting the scientific name Bidentalia as a replacement name for Dicynodontia. The name Bidentalia quickly fell into disuse over the following years, replaced by the popularity of the first term proposed by Owen. It will be necessary to wait until 2009 so that the taxon is reused to describe one of the subgroups of dicynodonts.

Features

The dicynodont skull is highly specialized, light but strong, with the temporal synapsid openings at the back of the skull greatly enlarged to accommodate larger jaw muscles. The front of the skull and the lower jaw are usually narrow and in almost all primitive forms toothless. Instead, the front of the mouth is fitted with a horny beak, as in turtles and ceratopsians. Food is processed by the retraction of the lower jaw as the mouth closes, producing a powerful shearing action, which would have enabled dicynodonts to cope with hard plant material. Many genera also have a pair of tusks, which could be an example of sexual dimorphism. Several genera, such as Placerias, that lack true tusks instead bear tusk-like extensions on the side of the beak. Their bodies are short and stout, barrel-shaped and with strong limbs. In large genera, such as Dinodontosaurus, the hind limbs are held straight, but with the forelimbs bent at the elbow. Both the pectoral girdle and the ilium are large and strong.

Endothermy and fur

Dicynodonts have long been suspected of having been warm-blooded animals. Their bones are highly vascularized and possess Haversian canals, and their proportions