How Asian wildlife replaced European animals 34 million years ago
Where do the animals around us today come from? A major event occurred about 34 million years ago, known as the “Great Cut-off.” At that time, a large portion of Europe’s fauna went extinct, replaced by animals of Asian origin. We have just published our research in the journal PNAS, which provides a better understanding of this pivotal moment.
Romain Weppe, University of Montpellier
We focused on fossils of artiodactyl mammals (which today include pigs, ruminants, camels, hippopotamuses, and cetaceans). These fossils are particularly abundant in deposits in southwestern France, such as in the Quercy region of Occitanie.
Our research indicates that 77% of Europe’s endemic species have gone extinct and been replaced by immigrant species from Asia, primarily pigs and ruminants.
Until now, the scientific community believed that this mass extinction was primarily due to intense competition with Asian species, which limited access to food resources for endemic species and thus led to their extinction. Our study refutes this hypothesis.
It appears that the climatic upheavals of this period, known as the Eocene–Oligocene transition, were the primary factor behind the extinction of endemic artiodactyl species in Western Europe.
A massive and historic extinction
It is now estimated that 99% of the species that have ever existed on Earth are now extinct. This is why understanding the causes of species extinction has become a central focus in evolutionary biology and paleontology. Following 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 past 66 million years.
This transition is associated with a global cooling of the planet, estimated at around 5 °C, a significant drop in sea levels, and the formation of the Antarctic ice sheet. This drop, along with the tectonic movements triggered by the uplift of Africa, allowed landmasses to emerge that served 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 frequently 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. To determine the factors underlying this extinction event, we compiled and analyzed a collection of artiodactyl mammal fossils (over 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 phosphate-rich sediments. During the second half ofthe 19thcentury, many of these were completely or partially emptied due to an intense phase of phosphate mining and yielded numerous fossil remains of vertebrates, insects, and plants.
This fossil-bearing area is now world-renowned for the exceptional preservation and abundance of the fossils it contains. In fact, there are more than 180 fossil deposits in this region that have preserved local faunal assemblages, some of which are separated in time by less than 1 million years.
Together, these sites provide a unique chronological window for studying the processes and mechanisms of mammalian diversification, as they span a continuous period from the Middle Eocene to the early Upper Oligocene (42 to 24 Ma), thus perfectly encompassing the Eocene-Oligocene transition.
In our study, we therefore estimated the dynamics of species diversity among artiodactyl mammals in Western Europe, using statistical methods that account for the quality of preservation of the fossil record.
During the Eocene, artiodactyls constituted one of the most diverse groups of endemic mammals in Western Europe. The tropical and favorable conditions of this period allowed them to develop a variety of ecological adaptations (arboreal behavior, occasional bipedalism, amphibious behavior, and insectivory). Toward the end of the Eocene, the group’s diversity even peaked at levels equivalent to those found on the African plains today.
However, this thriving group experienced a dramatic extinction at the Eocene–Oligocene boundary. Our results show that 77% of endemic artiodactyl species went extinct during this event, and that the ecological niches vacated allowed immigrant and modern artiodactyl fauna to establish themselves, consisting mainly 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 of European endemic species. Indeed, the sudden change and decline in the type and abundance of available food resources did not give endemic species time to adapt, providing immigrant species with the opportunity to replace them.
Future studies that examine and compile fossil records of other mammal groups—such as rodents, carnivores, and perissodactyls (which today include horses, rhinoceroses, and tapirs)—will undoubtedly enhance our understanding of the major European faunal turnover that occurred 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 basic and applied research across 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
This article is republished from The Conversation under a Creative Commons license. Readthe original article.