Diaceratherium lemanense (Rhinocerotidae) from Eschenbach (eastern Switzerland): systematics, palaeoecology, palaeobiogeography

This paper describes a juvenile rhinocerotid skull from Eschenbach (eastern Switzerland). Computed tomography revealed the presence of the unerupted adult teeth P4 and M3 and enabled their three dimensional virtual reconstruction. The morphological features of this skull (postglenoid and posttympanic apophyses in contact, deep nasal notch, very slight occipital elevation, molar lingual cingulum absent, strong molar crochet) ascribe it to an evolved form of Diaceratherium lemanense (POMEL 1853) when compared with other European specimens. The diacerathere lineage is restricted to the Western European basins, except for the occurrences of the primitive D. cf. lamilloquense in the late Oligocene deposits of Nong Ya Plong (Thailand, Asia) and Diaceratherium askazansorense in the early Miocene of the Askazansor Formation (south-western Betpakdala, southern Kazakhstan, Asia). The Eschenbach locality represents a new biostratigraphical pinpoint at the Aquitanian (early Miocene), probably at the base of the MN2a Mammal Zone (Granitischer Sandstein Formation, upper part of the Lower Freshwater Molasse, USM, eastern Swiss Molasse Basin). A review of the European diacerathere record is proposed from the First Appearance Data of D. lamilloquense (MP29, La Milloque) to the Last Appearance Data of D. cf. aurelianense (MN4b, Eggingen-Mittelhart). During this time interval, the diacerathere lineage points to climatic stress and environmental changes. An ecological and spatio-temporal evolution in three phases between the Terminal Oligocene Crisis (disappearance of the ronzothere lineage) and the Proboscidean Datum Event at the late early Miocene (first European occurrence of the overtaking Brachypotherium) is suggested: 1) the Diaceratherium genus arrive in Europe during the latest Oligocene (the faunal renewal phase); 2) high diversity of the diacerathere group during the Aquitanian (the diversification phase); 3) wide geographical expansion of the last representatives (D. aurelianense) of the lineage during the Burdigalian (the stabilisation & geographical expansion phase).