Abstract
The early Eocene is marked by a succession of major global warming events that culminated in a period of climatic optimum (EECO) corresponding to the warmest period of the Cenozoic. Due to their short duration and high intensity, these hyperthermal events are often considered as the best analogues of current global warming, and are correlated with important diversification of mammals in the North American fossil record. In Europe, the early Eocene localities are less well known and it is difficult to trace the impact of these climatic events. Lophiodontidae are a family of perissodactyl mammals found in abundance in European Eocene localities, particularly in southern France, making them a model of choice for studying the impacts of hyperthermal events on the evolutionary history of European mammals. The first part of this work consists of an exhaustive study of the intraspecific variability of this family. More than thirty polymorphic dental characters have been identified, as well as the presence of marked sexual dimorphism. The genus Paralophiodon has been revised in light of this high variability and is considered invalid. In all, several hundred specimens from 22 localities in southern Europe were studied and 2 new species belonging to the genus Eolophiodon have been identified. The revision of the lophiodontids of the Occitanie region, placed in a biostratigraphic framework, allow us to rediscuss of the controversial age of the “grès d’Aigne”, and to propose a late Ypresian-early Lutetian age (MP10b-MP11) for this formation, thus filling the previous known gap in the European fossil record. The study of basal lophiodontids, coupled with the results of a new phylogenetic analysis, places the lophiodontids close to chalicothera within the suborder Ancylopoda, suggesting an Asian origin of the family and an arrival of lophiodontids in southern Europe soon after PETM. The study of unpublished material and phylogenetic analyses constrained by the new biostratigraphic data make it possible to distinguish three major phases in the evolutionary history of lophiodontidae that seem to be correlated with the major climate changes of the Eocene. Basal radiation of lophiodontidae is contemporary to the hyperthermal events of the Ypresian and is marked by the multiplication of small size forms and is restricted to the Mesogean Province. The second radiation of lophiodontidae appears contemporaneous with the EECO and its marked by the appearance of the genus Lophiodon and the replacement of small sub-cursorial forms by more massive taxa with sligthly more molarized premolars. This radiation marks the end of the endemism of lophidontidae that dispersed throughout all Western Europe. Lophiodontidae radiate one last time at Lutetien during the post-EECO cooling, during the "Intra Eocene Mammal Turnover I". This radiation sees the introduction of giant forms (one to two tons), graviportal and with molarized teeth, interpreted as a response to the relative opening of the environment during the post-EECO cooling leading to drastic changes in diets.