How Asian wildlife replaced European animals 34 million years ago
Where do the animals around us today come from? A major event took place around 34 million years ago, known as the "Great Divide." At that time, much of Europe's fauna became extinct, replaced by animals of Asian origin. We have just published our work in the journal PNAS, which provides a better understanding of this pivotal stage.
Romain Weppe, University of Montpellier
We focused on fossils of artiodactyl mammals (represented today by pigs, ruminants, camels, hippopotamuses, and cetaceans). These are particularly abundant in deposits in southwestern France, such as in the Quercy region of Occitanie.
Our research indicates that 77% of European endemic species have become extinct and have been replaced by immigrant Asian species consisting of pigs and ruminants.
Until now, the scientific community believed that this mass extinction was mainly due to active competition with Asian species, which would have limited access to food resources for endemic species and thus led to their extinction. Our study refutes this hypothesis.
It seems that the climatic upheavals of this period, known as the Eocene-Oligocene transition, were the main factor in the extinction of endemic artiodactyl species in Western Europe.
A massive and historic extinction
It is now estimated that 99% of species that have ever existed on Earth are now extinct. This is why understanding the reasons for species extinction has become a central topic in evolutionary biology and paleontology. After the mass extinction of non-avian dinosaurs, the Eocene-Oligocene transition 34 million years ago coincides with one of the major extinction events of the last 66 million years.
This transition is associated with global cooling of the planet, estimated at around 5°C, a significant drop in sea levels, and the formation of the Antarctic ice cap. This drop, along with the tectonic movements initiated by the rise of Africa, allowed land to emerge that could serve as migration routes for Asian mammals to Western Europe. It is important to remember that between 50 and 34 million years ago, the two continents were separated by a sea.
Two hypotheses are regularly proposed to explain the extinction of European island species: rapid global cooling during the Eocene-Oligocene transition and competitive interactions with immigrant fauna from Asia. In order to determine the factors underlying this extinction event, we compiled and analyzed a collection of artiodactyl mammal fossils (more than 2,100 fossils analyzed) from the Quercy phosphorite region. The Quercy phosphorites cover more than 1,800 km² of numerous fissures and cavities filled with clayey and phosphatic sediments. During the second half ofthe 19thcentury, many of these were completely or partially emptied due to an intense phase of phosphate mining, yielding numerous fossil remains of vertebrates, insects, and plants.
This fossil-rich area is now world-renowned for the exceptional preservation and abundance of fossils it contains. In fact, there are more than 180 fossil deposits in this region that have recorded local faunal assemblages, sometimes separated in time by less than 1 million years.
Together, these deposits provide a unique window in time for studying the processes and mechanisms of mammalian diversification, as they cover a continuous period from the Middle Eocene to the early Upper Oligocene (-42 to 24 Ma), thus perfectly framing the Eocene-Oligocene transition.
In our study, we therefore estimated the diversity dynamics of artiodactyl mammal species in Western Europe, using statistical methods that take into account the quality of preservation of the fossil record.
During the Eocene epoch, artiodactyls were one of the most diverse groups of endemic mammals in Western Europe. The tropical and favorable conditions of this period allowed them to develop various ecological adaptations (arborealism, occasional bipedalism, amphibiosis, insectivory). Towards the end of the Eocene, the diversity of the group even peaked at levels equivalent to those found in the African plains today.
However, this thriving group suffered a dramatic extinction at the Eocene-Oligocene transition. Our results show that 77% of endemic artiodactyl species became extinct during this event, and that the ecological niches that were freed up allowed immigrant and modern artiodactyl fauna to settle, mainly consisting of pigs and ruminants. Our results also refute the hypothesis of competition between endemic and immigrant species, suggesting instead that climate change was responsible for the decline of European endemic species. Indeed, the sudden change and reduction in the type and abundance of available food resources left endemic species no time to adapt, giving immigrant species the opportunity to replace them.
Future studies examining and compiling fossil occurrences of other mammal groups such as rodents, carnivores, and perissodactyls (now including horses, rhinos, and tapirs) will undoubtedly improve our understanding of the major European fauna renewal that took place 34 million years ago.
The DEADENDER project is supported by the French National Research Agency (ANR), which funds project-based research in France. Its mission is to support and promote the development of fundamental and applied research in all disciplines, and to strengthen the dialogue between science and society. For more information, visit theANR website.
Romain Weppe, Paleontologist at the Institute of Evolutionary Sciences in Montpellier, University of Montpellier
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