Angonisaurus

Angonisaurus was discovered in the African Karoo Basin in 1983. Because of the lack of good lithostratigraphic marker beds, biostratigraphy has proven to be the most reliable aid for stratigraphic subdivision of this Lower–Middle Triassic succession.[3] Angonisaurus was found in the present day locations of the towns of Sterkstroom and the Molteno.[3] Within the area, there are three subzones, Angonisaurus are biostratigraphically constrained to the uppermost Cynognathus Assemblage Zone (subzone C).[4]

Gondwana and Laurasia during the Triassic

Angonisaurus was named by its discoverers Cox and Li in 1983. In the name Angonisaurus, "an" means without, and "gon" means angle(d) or knee, which refers to the erect hind limbs with forelimbs that bend at the elbow.[3]

Identification was based on a number of characters shared by the South African specimens and the holotype of Angonisaurus cruickshanki, including the broad occipital bone; robust squamosal; interparietal contribution to the skull roof; postorbitals that do not contact the squamosals on the skull roof, such that the parietals form a significant portion of the temporal bar; pineal foramen located within a deep conical depression; wide interorbital bar; and the triangular, tuskless caniniform process. However, there are marked differences between the South African specimens and the Tanzanian holotype, including the absence of a midline groove that extends along the entire length of the temporal bar, the absence of a dorsal margin of the occiput that overhangs the remainder of the occipital plate, and the more gracile caniniform processes in the South African specimens. Because the type of A. crucikshanki was the only known specimen of Angonisaurus at the time, Hancox and Rubidge[6] expressed uncertainty as to whether these differences reflected a species-level distinction between South African and Tanzanian Angonisaurus, or if they stemmed from intraspecific variation, and they did not offer a species-level identification for BP/1/5530 and BP/1/5531.[3][7]

Not much is known about Angonisaurus that distinguishes it functionally different from other herbivorous dicynodont relatives. 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, which would have enabled dicynodonts to cope with tough plant material.[8]

• List of therapsids