Abstract

Shuotheriids are Jurassic mammaliaforms that possess pseudotribosphenic teeth in which a pseudotalonid is anterior to the trigonid in the lower molar, contrasting to the tribosphenic pattern of therian mammals (placentals, marsupials, and kin) where the talonid is posterior to the trigonid. The origin of the pseudotribosphenic teeth remains unclear, obscuring our perception of shuotheriid affinities and the early evolution of mammaliaforms. Here we report a new Jurassic shuotheriid represented by two skeletal specimens. Complete pseudotribosphenic dentitions allow re-identification of dental structures using serial homology and tooth occlusal relationship. Contrary to the conventional view we show that dental structures of shuotheriids can be homologized to those of docodontans and partly support homologous statements for some dental structures between docodontans and other mammaliaforms. The phylogenetic analysis based on new evidence removes shuotheriids from the tribosphenic ausktribosphenids (including monotremes) and clusters them with docodontans to form a new clade, Docodontiformes, that is characterized by pseudotribosphenic features. Within the phylogeny, docodontiforms and “holotherians” (Kuehneotherium, monotremes, and therians) evolve independently from a Morganucodon-like ancestor with triconodont molars by labio-lingual widening their posterior teeth for more efficient food processing. The pseudotribosphenic pattern passed a cusp semitriangulation stage, whilst the tribosphenic pattern and its precursor went through a stage of cusp triangulation. The two different processes resulted in complex tooth structures and occlusal patterns that elucidate the earliest diversification of mammaliaforms.