Climate Warming 56 Million Years Ago: Can the Biodiversity of the Past Help Us Anticipate the Future?
In light of the major climate changes we are currently experiencing, a key question arises: How will animals—and mammals in particular—respond to significant future temperature increases?
Rodolphe Tabuce, University of Montpellier
To provide food for thought based on observed facts, we can look back to the past, about 56 million years ago. At that time, two brief but very intense periods of global warming coincided with unprecedented changes in Europe’s fauna. We have just published our research in the journal PNAS, which provides a better understanding of this pivotal stage in the history of mammals.
Warming Conditions Favorable to Mammals
The first heat wave whose consequences we have studied and summarized is known as the Paleocene-Eocene Thermal Maximum (or PETM). This was a hyperthermal event, dating back 56 million years, during which continental temperatures rose by 5 to 8 °C in less than 20,000 years. Obviously, this timeframe is incomparable to the rapid rise in temperatures over the past two centuries due to human activities, but the PETM is considered by paleoclimatologists to be the best geological analog to current warming because of its speed on a geological timescale, its magnitude, and its cause: a massive release of methane andCO₂ into the atmosphere, most likely resulting from gigantic basaltic eruptions across the entire region of the present-day North Atlantic (Greenland, Iceland, Norway, the northern United Kingdom, and Denmark).
These powerful greenhouse gases, and the resulting rise in temperatures, caused upheavals in the flora and fauna of 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, contributing in part to the biodiversity we see today.
But just before this major upheaval, another shorter and less intense hyperthermal episode—known as the PETM Pre-Onset Event (or POE)—occurred about 100,000 years earlier, around 56.1 million years ago. It is now estimated that the POE caused a 2 °C rise in temperatures. Some scientists believe that this initial “heat wave” may have triggered the PETM through a cascade effect. Returning to the evolution of paleobiodiversity, while the impact of the PETM on mammalian faunas is relatively well understood, the impact of the POE remained unknown prior to our research.
Thorough field research in Occitanie
To address this issue, we focused our research on the south of France, in the Corbières Massif (Aude department, Occitanie region), where the geological strata marking the transition between the Paleocene and Eocene are numerous and very thick, raising the hope of identifying the PETM, the POE, and paleontological mammal sites dating from before and after the two temperature peaks. In other words, our goal was to describe very clearly and objectively the direct effects of these warming events on mammalian faunas.
For several years, we have therefore conducted multidisciplinary studies, combining the expertise of paleontologists, geochemists, climatologists, and sedimentologists. In addition, through citizen science initiatives, we have involved paleontology enthusiasts, naturalists, and other enthusiasts of the Corbières Massif in our field research (paleontological surveys and excavations). Our work led to the discovery of a mammalian fauna within the municipality of Albas. This fauna is precisely dated to 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 simply remarkable. The resulting scenarios—particularly those related to the history of mammals (the date of species’ emergence and their geographic dispersal)—are thus highly precise.
The fossil-bearing deposit discovered at Albas was dated using isotopic analysis of the organic carbon contained in the geological layers. The sedimentary rocks (limestone, marl, and sandstone) found in the natural environment today result from the accumulation of sediments (sand, silt, gravel, and clay) deposited in superimposed layers, called strata. In Albas, the sediments found are mainly marl, interspersed with small beds of limestone and sandstone. Think of this “geological mille-feuille” as the pages of a book: they tell us a story inscribed in time. This time can be calculated in various ways. While archaeologists use carbon-14 dating, geologists, paleoclimatologists, and paleontologists prefer to use, for example, the ratio of stable carbon isotopes (13C/12C). This method serves a dual purpose: it provides information on the presence of hyperthermal events during the original deposition of the sediments (the more negative the13C/12Cisotope ratio, the higher the inferred temperatures) and it allows for the precise dating of the strata, since hyperthermal events are brief and well-dated episodes. The sudden increase in 12Cin the atmosphere during hyperthermal events is explained by the rapid release of ancient reservoirs of organic carbon, which are naturally enriched in 12C—particularly as a result of past photosynthesis by plants. In fact, today as in the past, plants preferentially use 12C: being lighter than 13C, it can be mobilized more quickly by the organism.
Thus, the POE and PETM are identified by very strongly negative values of the13C/12Cratio. This method is so powerful that it can be applied to both oceanic sediments and continental sediments deposited in lakes and rivers, such as those at Albas. This allows for very precise comparisons of the ages of fossil-bearing deposits on a global scale. The fauna discovered at Albas has therefore been compared to contemporary faunas, particularly those of North America and Asia, within an extremely precise chronological context.
Surprising Wildlife in Albas
The fauna of Albas includes 15 species of mammals documented by more than 160 fossils, primarily remains of teeth and lower jaws. It includes rodents (the most diverse order of extant mammals, with more than 2,000 species, including mice, rats, squirrels, guinea pigs, and hamsters), marsupials (represented today by kangaroos, koalas, and possums), as well as primates, insectivores, and carnivores that are described as “archaic.” This adjective refers to the fact that the identified fossil species have no direct evolutionary relationship with extant species of primates, insectivores (such as hedgehogs, shrews, and moles), and carnivores (cats, bears, dogs, otters, etc.). The fossil record documents numerous groups of “archaic” mammals; many appeared at the same time as the last dinosaurs of the Cretaceous, and most went extinct during the Eocene, likely due to ecological competition with “modern” mammals—that is, mammals directly related to extant species. Many of these “modern” mammals appeared during the PETM and spread very rapidly 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 what is now the Arctic were then covered by dense tropical and subtropical forests, providing “shelter and sustenance” for the mammals.
In the wake of these initial geographic dispersals, we see a diversification in the number of species among all “modern” mammals, which very quickly came to occupy all available habitats. Thus, in addition to the groups already mentioned (such as arboreal primates), it was during this period that the first chiropterans (or bats) adapted for flight and the first cetaceans adapted for aquatic life appeared. For this reason, the post-PETM period is often described as a pivotal period in the history of mammals, as it corresponds to the innovative phase of their “adaptive radiation”—that is, 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 POE, during the very end of the Paleocene. At that time, we believed that European faunas consisted solely of “archaic” species that were essentially endemic, as they were confined to Europe. The continent was then fairly isolated from other neighboring landmasses by shallow seas.
The fauna of Albas has upended this scenario. Indeed, it features “archaic” species—mostly endemic—coexisting with, surprisingly, “modern” cosmopolitan species! Among these are rodents and marsupials, for which Albas documents the oldest European species—the first known with certainty from the Paleocene. A detailed study of the Albas fauna reveals that the direct ancestors of most of the discovered species point to a North American origin, specifically among species known from the U.S. state of Wyoming dating from before the POE. 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 Albas mammals “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 upcoming studies will aim to document European faunas just before the POE in order to better understand the impacts that this hyperthermal event—less well known than the PETM but just as pivotal to the history of mammals—may have had. Returning to our initial hypothesis—the idea of an analogy between past and future biodiversity—our research shows that the POE enabled a large-scale migration of American mammals to Europe due to a temperature rise of approximately 2 °C. This could offer 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 basic and applied research across 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
This article is republished from The Conversation under a Creative Commons license. Readthe original article.