A juvenile specimen of Anteosaurus magnificus Watson, 1921 (Therapsida: Dinocephalia) from the South African Karoo, and its implications for understanding dinocephalian ontogeny

Abstract

Anteosaurid dinocephalians were the apex terrestrial predators of the latter part of the Guadalupian (middle Permian) and became extinct at the end of that epoch. The group was relatively diverse in Russia, but represented by only two genera, Australosyodon and Anteosaurus, in the Karoo rocks of South Africa. A newly discovered skull of Anteosaurus magnificus from the Abrahamskraal Formation is unique among specimens of this taxon in having most of the individual cranial bones disarticulated, permitting accurate delimitation of cranial sutures for the first time. The relatively large orbits and unfused nature of the cranial sutures suggest juvenile status for the specimen. A computer-aided 3D reconstruction of the skull, and comparison with 11 additional individuals, enabled an allometric study of cranial growth in the species. Positive allometry for four of the measurements suggests rapid growth in the temporal region, and a significant difference in the development of the postorbital bar and suborbital bar between juveniles and adults. Pachyostosis was an important process in the cranial ontogeny of Anteosaurus, significantly modifying the skull roof of adults. This condition is more obvious in large individuals of the species, but it is recognized that variation may also be independent of growth and could be related to sexual dimorphism. Growth of the skull in Anteosaurus shows similar morphological trends to that of the Russian Titanophoneus and the Chinese Sinophoneus. The overall morphology of the juvenile Anteosaurus is clearly reminiscent of the adult skull of the Russian medium-sized Syodon, a condition that is more likely a result of similar skull sizes and the lack of strong pachyostosis in adult Syodon.