A new traversodontid cynodont with a peculiar postcanine dentition from the Middle/Late Triassic of Namibia and dental evolution in basal gomphodonts

摘要

We here present a new gomphodont cynodont from the Ladinian/Carnian Upper Omingonde Formation of central Namibia.Etjoia dentitransitusgen. et sp. nov. is a medium-sized (skull length of 88.5 mm) omnivorous gomphodont represented by an almost complete skull and a few cervical vertebrae. This new taxon is diagnosed by divergent first left and right lower incisors that bear large apically inclined denticles, a third lower gomphodont postcanine with a well-separated mesial cingulum, four distal upper and lower sectorial teeth, and a lingually offset last upper sectorial postcanine.Etjoiais particular in having a transitional dentition between basal gomphodonts and traversodontids. It shares with some non-traversodontid gomphodonts a high number of sectorial postcanines and ovoid mesial-most upper gomphodont postcanines with a centrally positioned transverse crest.Etjoiawas, however, recovered as a basally branching Traversodontidae in phylogenetic analyses performed on different data matrices. It shares with traversodontids upper gomphodont postcanines with a high transverse crest, sub-rectangular and mesiodistally elongated lower gomphodont postcanines, and a mesially displaced transverse crest in the lower gomphodont postcanines. We explored dental evolution in basal cynognathians based on a dentition-oriented data matrix. The acquisition of gomphodont postcanines, which allowed tooth-to-tooth occlusion, in basal-most gomphodonts was followed by a loss of shearing planes between the upper and lower gomphodont postcanines in non-diademodontid gomphodonts, and the development of deep basins and tall transverse crests in gomphodont teeth and shearing planes between the upper and lower gomphodont postcanines in basal traversodontids. These dental transformations likely resulted from three shifts in feeding ecologies, from a carnivorous to an omnivorous diet in basal-most gomphodonts, followed by an adaptation to a specialized feeding ecology possibly made of insects and hard plant material in basal-most neogomphodonts (traditional 'trirachodontids'), then an increase in the consumption of plant-material in basal traversodontids.