An early geikiid dicynodont from theTropidostomaAssemblage Zone (late Permian) of South Africa

Abstract

Based on specimens previously identified asTropidostoma, a new taxon of dicynodont (Bulbasaurus phylloxyrongen. et sp. nov.) from the Karoo Basin of South Africa is described.Bulbasaurusis a medium-sized dicynodont (maximum dorsal skull length 16.0 cm) restricted to theTropidostomaAssemblage Zone (early Lopingian) of the Beaufort Group.Bulbasauruscan be distinguished fromTropidostomaby an array of characters including the presence of a tall, sharp premaxillary ridge, large, rugose, nearly-confluent nasal bosses, a nasofrontal ridge, massive tusks, robust pterygoids, prominently twisted subtemporal bar, and absence of a distinct postfrontal. Inclusion ofBulbasaurusin a phylogenetic analysis of anomodont therapsids recovers it as a member of Geikiidae, a clade of otherwise later Permian dicynodonts such asAulacephalodonandPelanomodon.Bulbasaurusexhibits many of the characters typical of adultAulacephalodon, but at substantially smaller skull size (these characters are absent in comparably-sizedAulacephalodonjuveniles), suggesting that the evolution of typical geikiid morphology preceded gigantism in the clade.Bulbasaurusis the earliest known geikiid and the only member of the group known from theTropidostomaAssemblage Zone; discovery of this taxon shortens a perplexing ghost lineage and indicates that abundant clades from the later Permian of South Africa (e.g., Geikiidae, Dicynodontoidea) may have originated as rare components of earlier Karoo assemblage zones.