New cladotherian mammal from southern Chile and the evolution of mesungulatid meridiolestidans at the dusk of the Mesozoic Era

MARTINELLI AG; SOTO-ACUÑA S; GOIN FJ; KALUZA J; BOSTELMANN JE; FONSECA PH; REGUERO MA; LEPPE M; VARGAS AO

Nature Scientific Reports

NATURE PUBLISHING GROUP

Lugar: Londres; Año: 2021 vol. 11

In the last decades, several discoveries have uncovered the actual complexity of mammalian evolution during the Mesozoic Era, including important Gondwanan lineages: the australosphenidans, gondwanatherians, and meridiolestidans (Dryolestoidea). Most often, their presence and diversity is documented by isolated teeth and partial jaws. Here, we describe a new meridiolestidan mammal, Orretherium tzen gen. et sp. nov., from the Late Cretaceous of southern Chile, based on a partial jaw with five cheek teeth in locis and an isolated upper premolar tooth. Phylogenetic analysis places Orretherium as the earliest divergence within Mesungulatidae, before other forms such as the Late Cretaceous Mesungulatum and Coloniatherium, and the early Paleocene Peligrotherium. The in locus tooth sequence (last two premolars and three molars) is the first recovered for a Cretaceous taxon in this family, and suggests that reconstructed tooth sequences for other Mesozoic mesungulatids should be reviewed and may represent more than one species. Evidence on tooth eruption and replacement shows that mesungulatids evolved a heterochronic delay of molar eruption with regard to basal dryolestoids, with therian-like simultaneous eruption of the last premolar and last molar. Unlike widespread Gondwanan gondwanatherians and Laurasian dryolestidans, meridiolestidans seem endemic of southern South America by the Cretaceous; however, given increasing records of their abundance and diversity in Patagonia, a broader Gondwanan distribution of this clade could be expected.