Discovery of the oldest European ruminant in the late Eocene of Bulgaria: Did tectonics influence the diachronic development of the Grande Coupure?

We describe the oldest known European ruminant, Bachitherium thraciensis sp. nov., from late Eocene (latest Bartonian or early Priabonian) strata in Bulgaria. The new specimen, which possesses the most primitive dental morphology known in the Bachitheriidae family, predates its western European relatives by at least 4.5 Myr. The discovery suggests that ruminants dispersed into Europe in a series of east to west waves, preceding and following the Eocene–Oligocene boundary. Probably originating from Asia, the oldest ruminant (reported here) migrated into southeastern Europe. During a second wave, comprising the Grand Coupure dispersal event, ruminants moved into western Europe at the Eocene–Oligocene boundary. A third wave composed by Bachitheriidae and Tragulidae arrived in western Europe, 2 Myr later, in the earliest Oligocene. The origin of this east to west diachronism was probably facilitated by orogenesis that, episodically, allowed dispersal of ruminants from Asia, through southeastern Europe, and into western Europe, in what we herein term the “Bachitherium dispersal event”. However, none of the abundant perissodactyl fauna from southeastern Europe migrated to western Europe during this time, because, like their relatives in central Asia, they may not survive to terminal Eocene cooling and aridity. •The oldest European ruminant is described (Bachitherium thraciensis nov sp.).•The specimen predates its western European relatives by at least 4.5 Myr.•A new paleobiogeographical scenario is proposed for Eocene–Oligocene dispersals.•Balkan Europe was a different biogeographical province from West Europe until MP23.•The Paratethys closing at MP23 allowed the western dispersal of Balkan vertebrates.