Asian mammals would have arrived in Europe by the islands of Balkanatolia

More than 40 million years ago, the distribution of mammals on the Eurasian continent was mainly divided into two large faunal groups, associated with two distinct provinces: Western Europe and Asia. This zonation was largely linked to the presence of inlets surrounding the continental zones, which would have acted as natural barriers, preventing the mixing of the two groups. This separate evolution between the west and the east of the Eurasian continent, however, came to an abrupt end at the beginning of the Oligocene, around 33 million years ago. From this date, we observe a decline in animals endemic from Western Europe as well as the rapid appearance of species from Asia. This sudden change in European fauna is known as the Grande Coupure.

The identification of a third group of mammals sows confusion

However, several paleontological studies report the presence of mammals of Asian affinity in Europe 5 to 10 million years before the Great Cut, calling into question the idea of ​​the two large faunal groups. Some researchers therefore suggest the presence of a third group of mammals, established before the Great Cut in the region covering present-day Southeastern Europe up to Anatolia. This group is marked by a strong endemismwith characteristics that isolated it from the European and Asian groups, until the time of the Great Cut.

Researchers have therefore set about reconstructing the chronology of the dispersal of mammals before the Great Coup, based in particular on new paleontological discoveries and on the revision of the ages of certain paleontological sites in the south-eastern part of Europe ( from the Balkans to the Caucasus). Alexis Licht, from the Center for Research and Education in Environmental Geosciences (Cerege), and his colleagues present the discovery of the oldest fossils of ungulates in Anatolia, attesting to the presence of mammals of clear Asian affinity in this region 38 to 35 million years ago.

Balkanatolia: islands isolated from Europe and Asia

The results of the study, published in Earth Science Review, suggest the presence of an isolated faunal group, so far not clearly defined. The distribution of the fossils indeed shows that these mammals of Asian origin were present at the level of a province which remained isolated for a long time from the rest of Europe and Asia. This biogeographical province, now entangled with the Eurasian continent through tectonic collision, was characterized 38 million years ago by several continental blocks grouped together under the term Balkanatolia. These blocks derive from the fragmentation of supercontinents Gondwana and Laurasia and assembled with the Eurasian continent at the end of the Cretaceous around 65 million years ago. They occupied the eastern and central part of the Neotethys Ocean then being closed.

Several studies suggest that the continental blocks of Balkanatolia formed during the Eocene a set of land weakly emerged extending from the Alpine region to the Lesser Caucasus. Balkanatolia seems to have known several episodes of immersion during this period and could thus have passed from the configuration of a very discontinuous archipelago to that of a large island. However, it would have remained isolated from the rest of the Eurasian continent during most of the Eocene, allowing the development of an endemic fauna, unrelated to Asia.

A first colonization allowed by a drop in the water level

Alexis Licht and his colleagues suggest, however, that Balkanatolia, then populated by a very specific fauna, would have been colonized by Asian mammals at the end of the Bartonian, around 38 million years ago, thanks to the establishment of a passage emerged between Asia and Balkanatolia. This event was followed by a change in the fauna in Western Europe, long before the Great Coup, thereby indicating the probable opening of another passage, this time between Balkanatolia and Europe.

The colonization of Balkanatolia by Asian mammals would have been associated with a marine regression, that is to say a drop in sea level. For the authors, this eustatic change is attributed to the combination of two factors: a glacial episode causing a drop in the water level and an uplift related to the start of the tectonic collision between the African plate and the Eurasian plate. This collision will give in particular born in the Alps and will subsequently lead to the incorporation of land from Balkanatolia along the compression front.

These two factors, climatic and tectonic, would have allowed the establishment of a corridor continuum of emerged land allowing the passage of Asian mammals towards Balkanatolia, then towards Western Europe. The researchers believe that this passage would have been reused during the Great Cut, when the climate was experiencing a new glaciation33.5 million years ago.