Aulacephalodon
Aulacephalodon is an extinct genus of medium-sized dicynodonts, or non-mammalian synapsids, that lived during Permian period, about 299-252 million years ago. Individuals of Aulacephalodon are commonly found in the Lower Beaufort Group of the Karoo Supergroup of South Africa and Zambia. Rising to dominance during the Late Permian, Aulacephalodon were the dominant terrestrial vertebrate herbivores until they became extinct during the Triassic.[1] No living relatives of Aulacephalodon exist today. Two species have been named, the type species, A. bainii, and a second species, A. peavoti. However, debate exists among paleontologists if A. peavoti is a true member of the genus Aulacephalodon.[1] Therefore, a majority of the information known about Aulacephalodon is in reference to discoveries about A. bainii.
The name Aulacephalodon combines the Greek words aulak- (aulax), meaning "a furrow", kephale, meaning "head," and odon, meaning tooth. Together, Aulacephalodon means "furrow-head tooth." The species Aulacephalodon bainii was named in honor of Andrew Geddes Bain (1797-1864), a Scottish geologist and road engineer who is credited with discovering the first dicynodont skull in South Africa.
Aulacephalodon belong to the family Geikiidae, a family of dicynodonts generally characterized by their short, broad skulls and large nasal bosses.[2] Two autamorphies used to define Geikiidae include (1) the posterior portion of the zygomatic arch thickened and/or downturned; (2) the labial fossa present.[2] Both features are found in Aulacephalodon and allow for suggestions to be made about possible sexual dimorphism evident in A. bainii.
Among certain anomodont therapsids, Aulacephalodon and Pelanomodon are distinguished from their related genera, Oudenodon and Rhachiocephalus by the specialization of their biting mechanism and the relative size of their nasal and prefrontal bosses.[3] Aulacephalodon and Pelanomodon bit using the transverse anterior tips of the jaws, while Oudenodon and Rhachiocephalus bit using the sides of their horn covered jaws.[3] The difference in jaw morphology between the two groups of genera is a notable dichotomy among these related therapsids. Aulacephalodon was unique among dicynodonts and its related therapsids because members of this genus were one of the only dicynodonts to possess canine tusks.[3]
Aulacephalodon is considered to be medium-sized relative to other dicynodont species, unique to other dicynodont species due to the canine tusks they possessed.[1] Fossilization tends to have preserved only skulls and complete or fragmented bones of Aulacephalodon bainii, requiring paleontologists to use the unique features of the cranium when identifying specimens believed to belong to the genus. Aulacephalodon had short, broad skulls with a recorded range of 135 mm to 410 mm.[4] Comparison of juvenile and mature individuals suggest that Aulacephalodon demonstrated a positive allometric growth pattern for their cranial features and a negative allometric growth pattern for their tusk and orbit size as ontogenetic age increases.[4] Disproportionately large eyes in juveniles of a species is considered a characteristic of higher vertebrates.[4] Diagnostic features of Aulacephalodon include (1) the size of their nasal bosses, (2) the shape and articulation of cranial bones, and (3) the length-breadth ration of the skull.[4] The most complete restoration of Aulacephalodon is a skeleton of A. peavoti from the Field Museum of Natural History in Chicago.
