Around 34 million years ago, a biological upheaval took place in Europe. A good half of the native mammals disappeared and were replaced by newcomers from Asia, including the forerunners of today's pigs and ruminant species. However, the main reason for this was not competition with the newly immigrated species, but rather climatic factors, as researchers have found using ungulate fossils. According to this, rapid climate change in particular led to the extinction of the endemic animals.
During the “Great Break” (Grande Coupure) 33.9 million years ago, at the geological transition from the Eocene to the Oligocene, around every second endemic mammal species in Europe died out. For a time there were hardly any mammals living on what was then an island continent before new species from Asia settled and spread here. Scientists have long suspected that the cause of this upheaval was that the original native species and the immigrated species competed with each other for food and living space and that the newcomers displaced the local species.
A research group led by Romain Weppe from the University of Montpellier has now tested this assumption using ungulates. To do this, they analyzed more than 2,000 fossils from 90 different species and 39 genera of even-toed ungulates found in the southwest of France. The fossilized remains are between 25 and 42 million years old and therefore date from the time of upheaval. Weppe and his colleagues determined the species' relationships and compared their appearance in the region over time and their feeding habits. The animals' teeth provide information about this, among other things.
Sudden climate change triggers species extinction
As the analyzes showed, around 77 percent of the ungulate species originally endemic to Western Europe died out within just one million years during the “Great Breakdown”. Instead, immigrated ungulate species from Asia settled and quickly dominated the region. Ruminants, pigs and hippos in particular came to Western Europe via newly formed land bridges in the Alps and the Balkans, the researchers report. However, these animals initially did not represent direct competition for the native species in their way of life.
According to the study, the main cause of the species upheaval was different: While the climate in Eocene Europe was tropical all year round and beneficial for the native ungulates, in the subsequent Oligocene different seasons suddenly developed with a changing and overall colder and drier climate. The trigger for this was the migration of the continental plates, which formed new seas and changed the ocean currents. Due to this rapid climate change, other plants began to thrive in Europe and the food supply changed. As a result, many of the endemic even-toed ungulates became extinct, report Weppe and his colleagues. Only when they disappeared did previously alien species, which were better adapted to the harsh climate and food supply that prevailed, have the opportunity to settle in Europe.
Passive replacement by previously alien species
Accordingly, climatic and environmental factors were probably more responsible for the species upheaval than competition with the introduced species. “It was a largely passive replacement rather than an active displacement of local species,” explains Weppe. However, the few surviving endemic ungulate species actually competed with the newcomers if they had similar eating habits or hunted the native animals, as the analyzes showed.
Source: Romain Weppe (University of Montpellier) et al., Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2309945120