18 19 Cainotheriids are a family of small artiodactyls, known in the fossil record from the late 20 Eocene to the middle Miocene in Western Europe. Contrary to several European endemic 21 ungulate groups that became extinct at the end of the Eocene or close to the Eocene-Oligocene 22 transition (Grande Coupure), cainotheriids crossed this boundary and diversified during the 23 Oligocene. The evolutionary history of cainotheriids around the Grande Coupure remains 24 poorly documented and only a few works deal with the modalities of their evolution, notably 25 because of the scarcity of available Eocene remains. A new fossiliferous karstic network named 26 Dams was discovered during field prospections in the Quercy area (Tarn-et-Garonne, France).

The Eocene–Oligocene transition (EOT) represents a period of global environmental changes particularly marked in Europe and coincides with a dramatic biotic turnover. Here, using an exceptional fossil preservation, we document and analyse the diversity dynamics of a mammal clade, Cainotherioidea (Artiodactyla), that survived the EOT and radiated rapidly immediately after. We infer their diversification history from Quercy Konzentrat–Lagerstätte (south-west France) at the species level using Bayesian birth–death models. We show that cainotherioid diversity fluctuated through time, with extinction events at the EOT and in the late Oligocene, and a major speciation burst in the early Oligocene. The latter is in line with our finding that cainotherioids had a high morphological adaptability following environmental changes throughout the EOT, which probably played a key role in the survival and evolutionary success of this clade in the aftermath. Speciation is positively associated with temperature and continental fragmentation in a time-continuous way, while extinction seems to synchronize with environmental change in a punctuated way. Within-clade interactions negatively affected the cainotherioid diversification, while inter-clade competition might explain their final decline during the late Oligocene. Our results provide a detailed dynamic picture of the evolutionary history of a mammal clade in a context of global change.