New Specimens of Reigitherium bunodontum from the Late Cretaceous La Colonia Formation, Patagonia, Argentina and Meridiolestidan Diversity in South America

We describe the first maxillae and additional new specimens of Reigitherium bunodontum, a small meridiolestidan from the Late Cretaceous La Colonia Formation, Patagonia, Argentina. The new material supports a dental formula of I?, C1, P4, M3, resolves postcanine positional uncertainty and corrects previous interpretations. Our phylogeny recovers Reigitherium as a meridiolestidan allied to other bunodont Mesungulatoidea, as the sister group of the Paleocene Peligrotherium. Posterior premolars/molars of Reigitherium, and to a smaller degree Peligrotherium, are dominated by an incomplete transverse ridge running between the protoconid-metaconid in the lowers and the paracone-stylocone in the uppers, semi-symmetrical basins developing mesially and distally from these central ridges. The trigonid-derived single transverse crest results from a mesial shift of the robust metaconid, an enhancement of the basin crest stretching from the protoconid/metaconid, and a shallower trigonid basin. The mesungulatoid condition, with its complete absence of talonid, contrasts sharply with that of therians with lophs, or transverse ridges, which involved at least one talonid-derived loph resulting in two transverse crests per tooth. Mesungulatoid meridiolestidans achieved complex tooth-on-tooth occlusion with a predicted increase in herbivory/omnivory, departing from the traditional sharp-cusp insectivores plesiomorphic for meridiolestidans and Mesozoic mammals in general. Reigitherium’s dramatic remodeling of the primitive meridiolestidan molar morphology, extensive continuous occlusal surface, accessory cuspules, and highly textured crenulated enamel illustrates one of most distinctive adaptations to herbivory among Mesozoic mammals.