Dicynodont
Dicynodontia is an extinct clade of anomodonts, an extinct type of non-mammalian therapsid. Dicynodonts were herbivorous animals with a pair of tusks, hence their name, which means 'two dog tooth'. Members of the group possessed a horny, typically toothless beak, unique amongst all synapsids. Dicynodonts first appeared in Southern Pangaea during the mid-Permian, ca. 270–260 million years ago, and became globally distributed and the dominant herbivorous animals in the Late Permian, ca. 260–252 Mya. They were devastated by the end-Permian Extinction that wiped out most other therapsids ca. 252 Mya. They rebounded during the Triassic but died out towards the end of that period. They were the most successful and diverse of the non-mammalian therapsids, with over 70 genera known, varying from rat-sized burrowers to elephant-sized browsers.
The dicynodont skull is highly specialised, light but strong, with the synapsid temporal openings at the rear of the skull 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 by the retraction of the lower jaw when the mouth closed, producing a powerful shearing action,[2] 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.[3]: 137 Several genera, such as Stahleckeria, that lacked true tusks instead bore tusk-like extensions on the side of the beak.[4][3]: 139
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.[citation needed]
Dicynodonts have long been suspected of being warm-blooded animals. Their bones are highly vascularised and possess Haversian canals, and their bodily proportions are conducive to heat preservation.[5] In young specimens, the bones are so highly vascularised that they exhibit higher channel densities than most other therapsids.[6] Yet, studies on Late Triassic dicynodont coprolites paradoxically showcase digestive patterns more typical of animals with slow metabolisms.[7]
More recently, the discovery of hair remnants in Permian coprolites possibly vindicates the status of dicynodonts as endothermic animals. As these coprolites come from carnivorous species and digested dicynodont bones are abundant, it has been suggested that at least some of these hair remnants come from dicynodont prey.[8] A new study using chemical analysis seemed to suggest that cynodonts and dicynodonts both developed warm blood independently before the Permian extinction.[9]
Dicynodonts are the only vertebrates beside mammals to have true tusks. Their development prior to the evolution of the mammalian tooth replacement offers an insight into the acquisition of tusks.[11]
