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Dicynodontia

Extinct clade of therapsids

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Fossilized skeletons of two adult male Diictodon that crawled into an underground burrow and curled-up together, probably to conserve heat during winter hibernation. It has been proven that in this sp
Nkansahrexford · CC BY 3.0

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Characteristics

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 was equipped with a keratinous 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, which would have enabled dicynodonts to cope with tough plant material. Dicynodonts typically had a pair of enlarged maxillary caniniform teeth, analogous to the tusks present in some living mammals. In the earliest genera, they were merely enlarged teeth, but in later forms they independently evolved into ever-growing teeth like mammal tusks multiple times. In some dicynodonts, the presence of tusks has been suggested to be sexually dimorphic. Some dicynodonts such as Stahleckeria lacked true tusks and instead bore tusk-like extensions on the side of the beak. Like some other anomodonts, the skull roof shows the development of a novel skull bone, the "preparietal" that is not found in earlier synapsids nor in the cynodont ancestors of mammals. It is unpaired and forms the anterior (forward) margin of the pineal foramen. Similar bones, also called "preparietals" are found in gorgonopsid and biarmosuchian therapsids, though these are thought to have evolved convergently in each group. Dicynodonts ancestrally had three vertebrae in the sacrum, though this rose to over 6 vertebrae in some dicynodonts as a result of both vertebrae duplication and incorporation of caudal vertebrae into the sacrum.

Dicynodonts vary enormously in size. The smallest were rat-sized, a size common amongst Permian members of the group, while Triassic members of the group were typically much larger, with the youngest known member Lisowicia also being the largest known, around the size of an elephant, with a body mass estimated at 5,880 kilograms (12,960 lb). Many at least Triassic dicynodonts may have held their hindlimbs in a relatively erect posture while having more splayed forelimbs, though Lisowicia likely to have had erect forelimbs as well.

Soft tissue anatomy

Possible hair remnants in Permian carnivore coprolites have been suggested to have possibly been from dicynodonts, as digested dicynodont bones are abundant in the coprolites. However specimens of Lystrosaurus fossilized with mummified skin have found that the skin surface was covered in numerous overlapping tubercules. Pentasauropus dicynodont tracks suggest that dicynodonts had fleshy pads on their feet.

Possible endothermy

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. In young specimens, the bones are so highly vascularised that they exhibit higher channel densities than most other therapsids. Yet, studies on Late Triassic dicynodont coprolites paradoxically showcase digestive patterns more typical of animals with slow metabolisms. A 2017 study using chemical analysis suggested that cynodonts and dicynodonts both developed warm blood independently before the Permian extinction.

Reproduction and social behavour

A preserved tightly curled embryonic individual of Lystrosaurus from the Early Triassic of South Africa suggests that dicynodonts laid eggs, as is thought to be ancestral for synapsids and amniotes, though the lack of preserved eggshell suggests that the eggs were probably soft and leathery like in living monotreme mammals, and as is suggested to be ancestral for amniotes. The relatively large size of the eggs suggests individuals of Lystrosaurus were precocial (relatively independent from birth), and the egg had large yolk reserves for development, indicating that dicynodonts did not produce milk like mammals (the eggs of milk-producing monotremes are relatively small compared to body size) and perhaps at least some non-mammalian cynodonts.

Fossil communal latrines, preserved trackways showing adults and juvenile dicynodonts walking alongside each other, as well as large fossil aggregations of both adult and juvenile dicynodonts, suggest that at least some dicynodonts engaged in post-hatching parental care and were gregarious.

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Monotypic taxonTaxonomic group with only one subordinate taxonSpecies descriptionFormal description of a discovered speciesParaphylyType of taxonomic groupIUCN Red ListInventory of the global conservation status of biological speciesSubtropicsGeographic and climate zoneCommon nameName generally used for a taxon, group of taxa or organism(s)