Dicynodontia

Lore & Background

Dicynodonts were herbivorous members of the anomodont group of non-mammalian therapsids. Their name, meaning "two dog tooth," refers to the pair of tusks they typically bore. They possessed a horny, toothless beak unique among synapsids, and their skull was highly specialized: light but strong, with greatly enlarged temporal openings at the rear to accommodate larger jaw muscles. The front of the skull and lower jaw were generally narrow and, except in primitive forms, toothless, with food processed by a powerful shearing action as the lower jaw retracted. The tusks were enlarged maxillary caniniform teeth; in later forms, these independently evolved into ever-growing structures like mammal tusks multiple times. Some dicynodonts lacked true tusks and instead had tusk-like extensions on the side of the beak. A novel skull bone, the preparietal, formed the anterior margin of the pineal foramen. Ancestrally, they had three sacral vertebrae, but this number rose to over six in some species through duplication and incorporation of caudal vertebrae. Size varied enormously: the smallest were rat-sized, common in the Permian, while Triassic members were typically much larger, with the youngest known, *Lisowicia*, being elephant-sized. Many Triassic dicynodonts may have held their hindlimbs in a relatively erect posture while having more splayed forelimbs, though *Lisowicia* likely had erect forelimbs as well. Possible hair remnants in Permian carnivore coprolites have been suggested to have come from dicynodonts, but mummified skin of *Lystrosaurus* shows a surface covered in overlapping tubercules. Tracks suggest they had fleshy foot pads. Their bones are highly vascularised with Haversian canals, and a 2017 chemical analysis suggested they developed warm blood independently before the Permian extinction. A preserved embryonic *Lystrosaurus* individual suggests they laid eggs, likely soft and leathery like monotreme eggs, and the relatively large egg size indicates young were precocial and that dicynodonts did not produce milk. Fossil communal latrines, trackways showing adults and juveniles walking together, and large aggregations of both adults and juveniles suggest some dicynodonts engaged in post-hatching parental care and were gregarious.

Reader's Guide

Dicynodontia represents the most successful and diverse clade of non-mammalian therapsids, with over 80–90 genera known. Their highly specialized skull, featuring a horny beak and often tusks, allowed them to process tough plant material through a powerful shearing action. They varied enormously in size, from rat-sized burrowers to the elephant-sized Lisowicia. Their evolutionary history includes a near-total extinction at the end-Permian, followed by a rebound in the Triassic, and eventual decline. Evidence suggests they laid soft-shelled eggs, may have been warm-blooded, and some engaged in parental care and lived in groups. Their fossils have been known to science since the mid-1800s, with early descriptions by Andrew Geddes Bain and Richard Owen.

Did You Know?

• The name Dicynodontia means 'two dog tooth', referring to their typical pair of tusks.
• Dicynodonts had a horny, toothless beak unique among all synapsids.