A subantarctic reigitheriid and the evolution of crushing teeth in these enigmatic Mesozoic mammals

Mesozoic mammals from Gondwana remain poorly understood, with most species
known only from isolated teeth. Nevertheless, these tantalizing fossils
point to a diversity of families, that were distinct from Laurasian forms.
Among these, the enigmatic South American family Reigitheriidae, was
proposed based on the unique crushing teeth of its sole representative,
Reigitherium from Northern Patagonia. Here, we report a new mammal,
Yeutherium pressor gen. et sp. nov., which is only the second known taxon
of Reigitheriidae. This is based on a partial maxilla bearing an upper
molar from the Late Cretaceous Dorotea Formation in the Río de Las Chinas
Valley, Sub-Antarctic Chile. Parsimony and Bayesian phylogenetic analyses
indicate a close relationship between Reigitherium and Yeutherium. Shared
features between both taxa—such as expanded pre- and postparacristae, a
lingual cingulum at the paracone base, interradicular crests, and enamel
crenulations—help define Reigitheriidae. Cingula development and molar
morphology of Yeutherium differ from that of the related Mesungulatidae,
suggesting independent evolution of the crushing adaptations in
Reigitheriidae. Additionally, we suggest a reinterpretation of the dental
formula of Reigitheriidae. The discovery of Yeutherium highlights the
specialised dentition of Reigitheriidae as a significant functional
innovation in a radiation of Gondwanan mammals that is starting to be
understood.