Abstract

Haramiyavia clemmenseni from the Rhaetic (Late Triassic) of East Greenland, as one of the earliest-known mammaliaforms, has held an important place in understanding the timing of the earliest radiation of the group. Reanalysis of the type specimen, using high resolution computed tomography (μCT), has revealed new details, such as the presence of the dentary condyle of the mammalian jaw hinge, and the postdentary trough for mandibular attachment of the middle ear - a transitional condition of the predecessors to crown Mammalia. Our tests of competing phylogenetic hypotheses with these new data show that Late Triassic haramiyids are a separate clade from multituberculate mammals, and are excluded from the Mammalia. Consequently, hypotheses of a Late Triassic diversification of the Mammalia, which are dependent on multituberculate affinities of haramiyidans, are rejected. Scanning Electron Microscopy (SEM) study of tooth wear facets and kinematic functional simulation of occlusion with virtual 3D models from CT scans confirm that Haramiyavia, had a major orthal occlusion with the tallest lingual cusp of the lower occluding into the lingual embrasure of the upper molars, followed by a short palinal movement along the cusp rows alternating between upper and lower molars. This differs from the minimal orthal, but extensive palinal, occlusal movement of multituberculate mammals, previously regarded as relatives of haramiyidans. The disparity of tooth morphology and the diversity of dental functions of haramiyids and their contemporary mammaliaforms suggest dietary diversification is a major factor in the earliest mammaliaform evolution.