How Asian fauna replaced European animals 34 million years ago

Where did the animals that surround 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 sheds light on this pivotal event.

Romain Weppe, University of Montpellier

We were interested in fossils of artiodactyl mammals (represented today by pigs, ruminants, camels, hippopotamuses and cetaceans). The latter are particularly abundant in deposits in south-west France, such as in the Quercy region of Occitanie.

Our research shows that 77% of endemic European species have become extinct, replaced by Asian immigrant species 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 the Eocene-Oligocene transition were the main factor in the extinction of artiodactyl species endemic to Western Europe.

A massive and historic extinction

Today, it is estimated that 99% of the species that once 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, estimated at around 5°C, a significant drop in sea levels and the formation of the Antarctic ice cap. This drop, together with tectonic movements initiated by the rise of Africa, led to the emergence of lands that may have served as migration routes for Asian mammals towards Western Europe. You have to imagine that between 50 and 34 million years ago, the two continents were separated by a sea.

Two hypotheses are regularly put forward to explain the extinction of European island species: rapid global cooling during the Eocene-Oligocene transition, and competitive interactions with immigrant fauna from Asia. With the aim of determining the factors underlying this extinction event, we have compiled and analyzed a collection of artiodactyl mammal fossils (over 2,100 fossils analyzed) from the Quercy phosphorite region. The Quercy phosphorites cover over 1800km2 of numerous fissures and cavities filled with clay and phosphate sediments. During the second half of the ^19th century, many of these cavities were completely or partially emptied due to an intense phase of phosphate mining, and have yielded numerous fossil remains of vertebrates, insects and plants.

Today, this fossiliferous area is world-renowned for the preservation and exceptional abundance of fossils it contains. In fact, there are more than 180 fossil deposits in this region, recording local faunal assemblages, some separated in time by less than 1 million years.

Together, these deposits provide a unique time window for studying the processes and mechanisms of mammalian diversification, since they cover a continuous period from the Middle Eocene to the beginning of the Upper Oligocene (-42 to 24 Ma), perfectly framing the Eocene-Oligocene transition.

In our study, we have therefore estimated the diversity dynamics of artiodactyl mammal species in Western Europe, using statistical methods that take into account the preservation quality of the fossil record.

In the Eocene, artiodactyls are one of the most diverse groups of endemic mammals in Western Europe. The favorable tropical conditions of this period enabled them to develop a variety of ecological adaptations (arboricolia, occasional bipedalism, amphibiosis, insectivory). Towards the end of the Eocene, the group's diversity even reached levels equivalent to those found on 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 freed up allowed immigrant and modern artiodactyl fauna to establish themselves, mainly consisting of pigs and ruminants. Our results also refute the hypothesis of competition between endemic and immigrant species, but instead suggest that climate change is responsible for the decline in European endemic species. In fact, climate change and the abrupt reduction in the type and abundance of food resources available have not given endemic species time to adapt, giving immigrant species the opportunity to replace them.

Future studies examining and compiling the fossil occurrences of other mammal groups, such as rodents, carnivores and perissodactyls (now including horses, rhinoceroses and tapirs), will undoubtedly improve our understanding of the major European faunal renewal of 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. To find out more, visit theANR website.

Romain Weppe, Paleontologist at the Institut des Sciences de l'Évolution, Montpellier, University of Montpellier

This article is republished from The Conversation under a Creative Commons license. Read theoriginal article.