Tiny pouched colonizers growing larger: an evolutionary history of Eurasian herpetotheriid metatherians in the Eocene
Résumé
Herpetotheriidae are small extinct stem-marsupials with a superficially “opossum-like” dentition. Here, we shed light on three crucial phases of their previously obscure European evolutionary history. (i) Each metatherian family known in Europe likely dispersed once from North America (Gernelle et al. 2024). We report the earliest unequivocal herpetotheriid from a French locality that slightly predates the Paleocene-Eocene Thermal Maximum (PETM). This new genus and species is the smallest European herpetotheriid known so far and probably represents a case of mammalian dwarfism prior to the PETM. (ii) All early Eocene metatherian species from Western Europe then rapidly became ubiquitous in this part of the continent, contrary to most eutherian ones. In parallel, relatively large species such as Peratherium musivum and Peratherium maximum appear. We illustrate the importance of studying changes in molar cusp – basin proportions and correlated characters that occurred during the evolution of these larger herpetotheriids. This leads us to hypothesize different diets for Eocene European metatherians than those initially inferred using Messel individuals, such as Peratherium aff. maximum (Gernelle et al. 2025). (iii) New phylogenetic analyses suggest that the enigmatic Anatoliadelphyidae from the middle Eocene of the insular Pontide terrane (Anatolia) fauna are derived from a European herpetotheriid stock, rather than South American emigrants. Because anatoliadelphyids were larger and probably durophagous compared with herpetotheriids, a promising approach to clarify the herpetotheriid origin of anatoliadelphyids would be to restudy the older Peratherium aff. maximum from Messel and Geiseltal. This taxon displays unusually large carnassial teeth for a herpetotheriid, which is a functionally relevant diagnostic character in anatoliadelphyids. Altogether, the typically dentally conservative herpetotheriids underwent morphological diversification while increasing in size, likely following niche invasions during early to middle Eocene.
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