Climate warming 56 million years ago: can the biodiversity of the past help us anticipate the future?
In light of the current major climate changes, a key question arises: how will animals, and mammals in particular, respond to future significant increases in temperature?
Rodolphe Tabuce, University of Montpellier
To provide some food for thought based on observed facts, we can look back to around 56 million years ago. At that time, two short but very intense periods of global warming coincided with unprecedented changes in the fauna of Europe. We have just published our work in the journal PNAS, which provides a better understanding of this pivotal stage in the history of mammals.
Warming favorable to mammals
The first heat peak whose consequences we have studied and summarized is called the Paleocene-Eocene Thermal Maximum (or PETM). This was a hyperthermal event, dating back 56 million years, which saw continental temperatures rise by 5 to 8°C in less than 20,000 years. Obviously, this timeframe is incomparable to the rapid rise in temperatures over the last two centuries due to human activity, but the PETM is considered by paleoclimatologists to be the best geological analogue to current warming due to its speed on a geological timescale, its magnitude, and its cause: a massive release of methane andCO2 into the atmosphere, most likely from gigantic basalt outpourings across the entire North Atlantic (Greenland, Iceland, Norway, northern United Kingdom, and Denmark).
These powerful greenhouse gases, and the resulting rise in temperatures, caused major changes to the flora and fauna in all marine and terrestrial ecosystems. In Europe, Asia, and North America, the PETM coincided with the sudden appearance of the first primates (represented today by monkeys, lemurs, and tarsiers), artiodactyls (represented today by ruminants, camels, pigs, hippopotamuses, and cetaceans), and perissodactyls (represented today by horses, zebras, tapirs, and rhinoceroses). This event therefore played a major role in creating the biodiversity we know today.
But just before this major upheaval, another shorter and less intense hyperthermic episode, known as the PETM Pre-Onset Event (or POE), occurred around 100,000 years earlier, approximately 56.1 million years ago. It is now estimated that the POE caused temperatures to rise by 2°C. Some scientists believe that this initial "heat wave" may have triggered the PETM through a cascade effect. Returning to the evolution of paleo-biodiversity, while the impact of the PETM on mammalian fauna is relatively well understood, the impact of the POE remained unknown prior to our work.
Meticulous field research in Occitania
To address this issue, we focused our research in the south of France, in the Corbières Massif (Aude department, Occitanie region), where the geological layers of the transition between the Paleocene and Eocene are numerous and very thick, giving us hope of identifying the PETM, the POE, and paleontological deposits of mammals dating from before and after the two heat peaks. In other words, our goal was to describe very clearly and objectively the direct effects of these warming events on mammalian fauna.
For several years, we have therefore been conducting multidisciplinary studies, combining the expertise of paleontologists, geochemists, climatologists, and sedimentologists. In addition, through participatory science initiatives, we have involved amateur paleontologists, naturalists, and other enthusiasts of the Corbières Massif in our field research (prospecting and paleontological excavations). Our work led to the discovery of a fauna of mammals in the commune of Albas. This fauna is perfectly dated within the very short time interval between the POE and the PETM. Dating a paleontological site more than 56 million years old with an accuracy of a few thousand years is quite remarkable. The scenarios that emerge from this, particularly those relating to the history of mammals (date of appearance of species and their geographical dispersal), are therefore very precise.
The fossil deposit discovered in Albas was dated using isotopic analysis of the organic carbon contained in the geological layers. The sedimentary rocks (limestone, marl, and sandstone) found in nature today originate from the accumulation of sediments (sand, silt, gravel, clay) deposited in superimposed layers called strata. In Albas, the sediments found are mainly marl, interspersed with small banks of limestone and sandstone. This "geological mille-feuille" can be imagined as the pages of a book: they tell us a story written in time. This time can be calculated in different ways. While archaeologists use carbon-14, geologists, paleoclimatologists, and paleontologists prefer to use, for example, the ratio between stable carbon isotopes (13C/12C). This method has two advantages: it provides information on the presence of hyperthermal events during the original deposition of sediments (the more negative the ratio between the13C/12Cisotopes, the warmer the inferred temperatures) and it allows the strata to be given a precise age, since hyperthermal events are brief and well-dated episodes. The sudden increase in 12Cin the atmosphere during hyperthermic events is explained by the rapid release of ancient organic carbon reservoirs, naturally enriched in 12C, particularly as a result of past photosynthesis by plants. Indeed, today as in the past, plants preferentially use 12C: lighter than 13C, it can be mobilized more quickly by the organism.
Thus, POE and PETM are identified by very strongly negative values of the13C/12Cratio. The power of this method is such that it can be applied both to sediments of oceanic origin and to sediments of continental origin deposited in lakes and rivers, such as in Albas. This makes it possible to compare the ages of fossil deposits very accurately on a global scale. The fauna discovered in Albas could therefore be compared with contemporary fauna, particularly in North America and Asia, in an extremely precise chronological context.
Surprising wildlife in Albas
The fauna of Albas is rich with 15 species of mammals documented by more than 160 fossils, mainly remains of teeth and jawbones. It documents rodents (the richest order of mammals today, with more than 2,000 species, including mice, rats, squirrels, guinea pigs, and hamsters), marsupials (represented today by kangaroos, koalas, and opossums), as well as primates, insectivores, and carnivores that are described as "archaic." This adjective refers to the fact that the fossil species identified have no direct relationship with the current species of primates, insectivores (such as hedgehogs, shrews, and moles), and carnivores (felines, bears, dogs, otters, etc.). Many groups of "archaic" mammals are documented in the fossil record; many appeared at the same time as the last dinosaurs of the Cretaceous period, and most became extinct during the Eocene epoch, certainly due to ecological competition with "modern" mammals, i.e., mammals directly related to current species. Many of these "modern" mammals appeared during the PETM and spread very quickly across Asia, Europe, and North America via "natural land bridges" located at high latitudes (present-day northern Greenland, Scandinavia, and the Bering Strait in Siberia). These transcontinental passageways were possible because the landscapes of the present-day Arctic were then covered with dense tropical to paratropical forests, providing "shelter and food" for mammals.
In the wake of these initial geographical dispersals, we see a diversification in the number of species among all "modern" mammals, which very quickly occupy all available habitats. Thus, in addition to the groups already mentioned (such as arboreal primates), it was during this period that the first chiroptera (or bats) adapted to flight and the first cetaceans adapted to aquatic life appeared. This is why the post-PETM period is often referred to as a key period in the history of mammals, as it corresponds to the innovative phase of their "adaptive radiation," i.e., their rapid evolution, characterized by great ecological and morphological diversity.
A discovery that changes the game
But let's go back to before the PETM, more than 100,000 years earlier, just before the EPT, during the very end of the Paleocene. At that time, we thought that European fauna consisted solely of "archaic" species that were essentially endemic because they were confined to Europe. The continent was then fairly isolated from other neighboring land masses by shallow seas.
The fauna of Albas has undermined this scenario. Indeed, it shows that "archaic" species, which are mainly endemic, coexist with, surprisingly, "modern" cosmopolitan species! Among these are rodents and marsupials, of which Albas documents the oldest European species, the first known with certainty in the Paleocene. Detailed study of the fauna of Albas reveals that the direct ancestors of most of the species discovered are of North American origin, particularly species known in the US state of Wyoming dating from before the EPT. The conclusion is simple: these mammals did not migrate from North America during the PETM as previously thought, but somewhat earlier, most likely during the POE. In contrast to the "archaic" mammals of the Paleocene and the "modern" mammals of the Eocene, we have therefore termed the mammals of Albas "precursors." These "precursor" mammals, like their "modern" cousins 100,000 years later during the PETM, reached Europe via the warm, humid forests located in what is now Greenland and Scandinavia. What a surprise to imagine American marsupials arriving in Europe via the Arctic!
Our next studies will aim to document European fauna just before the EPO in order to better understand the impacts of this hyperthermic event, which is less well known than the PETM but just as decisive for the history of mammals. Returning to our initial hypothesis—the idea of an analogy between past and future biodiversity—our research shows that the EPO enabled a large migration of American mammals to Europe thanks to a temperature increase of around 2°C. This could offer us some insights into the future of European biodiversity in the current context of similar warming.
The EDENs 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.
Rodolphe Tabuce, CNRS Research Fellow, University of Montpellier
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