Dicynodontia

The Dicynodontia are a taxon of Therapsids or mammal-like reptiles. Dicynodonts were small to large herbivorous animals with two tusks, hence their name, which means 'two dog tooth'. They are also the most successful and diverse of the non-mammalian therapsids, with over 70 genera known, varying from rat- to ox-sized.

Characteristics
The Dicynodont skull is highly specialised; light but strong, with the synapsid temporal openings at the rear of the skull are greatly enlarged, to accommodate larger jaw muscles.

The front of the skull and the lower jaw are generally narrow and, in all but a number of primitive forms, toothless. Instead, the front of the mouth is equipped with a horny beak, as in turtles and ceratopsian dinosaurs. Food was processed through retraction of the lower jaw when the mouth closed, producing a powerful shearing action (Crompton and Hotton 1967), which would have enabled dicynodonts to cope with tough plant material.

Many genera also have a pair of tusks, which it is thought may have been an example of sexual dimorphism (Colbert 1969 p.137)

The body is short, strong and barrel-shaped, with strong limbs. In large genera (such as Dinodontosaurus) the hindlimbs were held erect, but the forelimbs bent at the elbow. Both the pectoral girdle and the ilium are large and strong. The tail is short.

Evolutionary history
Dicynodonts first appear during Middle Permian, and underwent a rapid evolutionary radiation, becoming the most successful and abundant land vertebrates of the Late Permian. During this time they including a large variety of ecotypes, including large, medium-sized, and small herbivores and short-limbed mole-like burrowers.

Only two families survived the end Permian extinction, one of which, the Lystrosauridae, were the most common and widespread herbivores of the Induan (earliest Triassic). These medium-sized animals evolved into and were replaced by the Kannemeyeridae, stocky, pig- to ox-sized animals that were the most abundant herbivores worldwide from the Olenekian to the Ladinian age. By the Carnian they had been supplanted by Traversodont cynodonts and rhynchosaur reptiles. During the Norian (middle of the Late Triassic), when - perhaps due to increasing aridity - they drastically declined, and the role of large herbivore was taken over by sauropodomorph dinosaurs.

With the decline and extinction of the Kannemeyerids, there were to be no more dominant large synapsid herbivores until the middle Paleocene epoch (60 Ma) when mammals, descendants of cynodonts, began to diversify after the extinction of the dinosaurs.

It used to be thought that Dicynodonts died out completely before the end of the Triassic. Recently however, evidence has come to light showing the dicynodonts survived into the Cretaceous in southern Gondwana (now Queensland) (Thulborn and Turner, 2003).

Taxonomy

 * Infraorder Dicynodontia
 * Genus Colobodectes
 * Genus Angonisaurus
 * Superfamily Eodicynodontoidea
 * Family Eodicynodontidae
 * Superfamily Kingorioidea
 * Family Kingoriidae
 * Clade Diictodontia
 * Superfamily Emydopoidea
 * Family Cistecephalidae
 * Family Emydopidae
 * Superfamily Robertoidea
 * Family Diictodontidae
 * Genus Diictodon
 * SUBORDER Cynodontia
 * Family Robertiidae
 * Genus Robertia
 * Clade Pristerodontia
 * Genus Dinanomodon
 * Genus Odontocyclops
 * Genus Propelanomodon
 * Family Aulacocephalodontidae
 * Family Dicynodontidae
 * Genus Dicynodon
 * Family Kannemeyeriidae
 * Family Lystrosauridae
 * Family Oudenodontidae
 * Family Pristerodontidae
 * Family Shanisiodontidae
 * Family Stahleckeriidae

Phylogeny
The cladogram presented here follows a synthesis from Mikko's Phylogeny Archive.