Evolution of postcanine complexity in Gomphodontia (Therapsida: Cynodontia)

Abstract

Gomphodonts form a Triassic radiation of small to medium-bodied(<0.5–2.5 m in length) quadrupedal cynodonts characterized by labiolinguallyexpanded gomphodont postcanines. They were the dominant cynodont group inMiddle and Late Triassic ecosystems from the Southern Hemisphere and the firstpredominantly herbivorous cynodonts to evolve. Gomphodonts were also the firsttherapsids to develop hypsodonty and a dentition with complex occlusal patterns,and their highly diagnostic upper and lower postcanines show many differentmorphologies. Here, we explored dental complexity in gomphodont cynodontsthrough time using geographic information system analysis and orientation patchcount applied on 3D crown surfaces of upper and lower gomphodont postcaninesbelonging to 32 gomphodont taxa. This study reveals that the peak in postcaninecomplexity was reached early in the evolution of gomphodonts with the emergencein the Early Triassic of omnivorous or insectivorous forms with postcaninesmade of well-separated cusps and cingular cuspules. Traversodontids evolved simplerpostcanines via coalescence of cusps into crests and the development of largeocclusal basins, and the Middle Triassic radiation of traversodontids led to a sharpdecrease in mean postcanine complexity. Simplification of the postcanines in traversodontidsis interpreted as being related to a gradual increase in the consumptionof plant material. Interestingly, the trend of insectivory/omnivory highpostcanine complexity and herbivory low dental complexity in gomphodonts isopposite to the trend of dental complexity reported in some extant mammals, withomnivorous having low dental complexity and herbivorous higher. Postcaninecomplexity remained relatively stable throughout the evolution of traversodontidsand only slightly diminished in the Late Triassic due to the presence of minuteforms with particularly simple postcanines in the Rhaetian. The major phylogeneticdiversity and taxonomic richness of Gomphodontia are represented in twoperiods of time: at the end of the Anisian, an age in which the postcanine complexityis simplifying, and at the early Carnian when the postcanine complexity intraversodontids, the only Gomphodontia represented, is stable.