Climate Warming 56 Million Years Ago: Can Lessons from the Past Inform Our Future Biodiversity?

For several years, we have therefore initiated multidisciplinary studies, combining the expertise of paleontologists, geochemists, climatologists and sedimentologists. In addition, via participatory science actions, we have involved in our field research (prospecting and paleontological excavations) amateurs in paleontology, naturalists and other enthusiasts of the Corbières massif. Our work has resulted in the discovery of a fauna of mammals in the territory of the town of Albas. This fauna is perfectly dated in the very short time interval between the Poe and the PETM. Date a paleontological site over 56 million years old with a precision of a few thousand years is simply remarkable. The scenarios that result from it, in particular those relating to the history of mammals (date of appearance of species and their geographic dispersions) are thus very precise.

The dating of the fossiliferous deposit discovered in Albas was carried out by isotopic analysis of the organic carbon contained in the geological layers. The sedimentary rocks (limestones, marls and sandstones) that we meet in the current nature come from the accumulation of sediments (sands, silts, gravel, clay) deposited in bunk layers, called strata. In Albas, the sediments encountered are mainly marls, interspersed with small benches of limestone and sandstone. We must imagine this “geological mille-feuille” as the pages of a book: they tell us a story inscribed in time. This time can be calculated in different ways. While the archaeologist will use carbon 14, the geologist, the paleoclimatologist and the paleontologist will prefer to use, for example, the relationship between the stable carbon isotopes (¹³C/¹²C). This method has a double interest: it provides the presence of hyperthermic events during the original sediment deposit (plus the ratio between isotopes ¹³C/¹²It is negative and the more hot temperatures are hot) and it gives a specific age to the strata, since hyperthermic events are short and well -dated episodes. The sudden increase in ¹²C in the atmosphere during hyperthermic events is explained by the rapid release of old organic carbon tanks, naturally enriched in ¹²C, in particular by the result of the past photosynthesis of plants. Indeed, today as in the past, plants preferentially use the ¹²C: lighter than the ¹³C, it is faster mobilized by the organization.

Thus, Poe and PETM are identified by very strongly negative values of the ratio ¹³C/¹²C. The power of this method is such that it can be applied both in the sediments of oceanic origin only in the sediments of continental origin deposited in lakes and rivers as in Albas. We can thus compare the ages of the fossiliferous deposits in a very precise manner on the scale of the whole world. The fauna discovered in Albas could therefore be compared to contemporary fauna, in particular North America and Asia in an extremely precise chronological context.

Surprising fauna in Albas

Albas fauna is rich in 15 species of mammals documented by more than 160 fossils, mainly remains of teeth and mandibles. It documents rodents (the richest order of current mammals, with more than 2000 species, including mice, rats, squirrels, guinea pigs, hamsters), marsupials (represented today by kangaroos, koalas and sarigues), but also primates, insectivores and carnivorers that are called “archaic”. This adjective refers to the fact that the fossil species identified have no direct kinship with the current species of primates, insectivores (such as hedgehogs, musicians and taupes) and carnivores (felines, bear, dogs, otters, etc.). In the fossil register, many groups of “archaic” mammals are documented; Many appear at the same time as the last dinosaurs of the Cretaceous and most of them go out during the Eocene, certainly in the face of an ecological competition with the “modern” mammals, that is to say mammals having a direct kinship with current species. Many of these “modern” mammals appear during the PETM and disperse very quickly in Asia, Europe and North America via “natural terrestrial bridges” located in high latitude (current North Greenland, Scandinavia and Strait of Bering in Siberia). These transcontinental routes are possible because the landscapes of the current Arctic are then covered with tropical dense forests with para-tropical, ensuring the “gîte and the cover” in mammals.

In the wake of these first geographic dispersions, we are witnessing a diversification of the number of species in all “modern” mammals which will very quickly occupy all the available living environments. Thus, in addition to the already mentioned groups (such as arboreal primates), it is during this period that the first chiropts (or bats) suitable for the flight and the first cetaceans adapted to aquatic life appear. It is for this reason that we often describe the post-petite period of key period in the history of mammals because it corresponds to the innovative phase of their “adaptive radiation”, that is to say to their rapid evolution, characterized by a great ecological and morphological diversity.

A discovery that changes the scenarios

But back before the PETM, more than 100,000 years earlier, just before the Poe, during the very end of the Paleocene. At that time, we thought that European fauna consisted of species only “archaic” and essentially endemic because they are confined to Europe. The continent is then quite isolated from other continental masses bordering by shallow seas.

The Albas fauna undermined this scenario. Indeed, it sees essentially endemic “archaic” species with, and this is the surprise, cosmopolitan “modern” species! Among these, the rodents and marsupials of which Albas documents the oldest European species, the first known with certainty in the Paleocene. The detailed study of Albas's fauna reveals that the direct ancestors of most of the discovered species bear witness to a North American origin, and in particular within species known in the American state of Wyoming dated before the Poe. The conclusion is simple: these mammals did not migrate from North America during PETM as we thought before, but a little earlier, most likely during the Poe. In contrast to the “archaic” mammals of the Paleocene and “modern” Eocene, we therefore qualified the mammals of Albas as “precursors”. These “precursors” mammals, like their “modern” cousins 100,000 years later at PETM, reached Europe via hot and humid forests located on current 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 POE in order to better understand the impacts that this hyperthermic event, less known than PETM, but just as decisive for the history of mammals. To return to our starting hypothesis – the idea of an analogy between past biodiversity and that of the future – we must remember from our research that the POE has enabled a great migration of American mammals to Europe thanks to an increase in temperatures of approximately 2 ° C. This could offer us avenues for reflection on the future of European biodiversity in the current context of similar warming.

The Edens project is supported by the National Research Agency (ANR), which finances research on projects in France. Its mission is to support and promote the development of fundamental and finalized research in all disciplines, and to strengthen dialogue between science and society. To find out more, see the ANR website.