Where barren arid lands extend today, 80 million years ago a lush ecosystem with characteristically "succulent" features thrived, researchers report: Plant fossils and geological evidence show that in the Cretaceous, extensive tropical forests grew in what is now northeastern Africa, with plant communities already strong in today's rainforests resembled. This sheds light on the previously unclear origins of these important ecosystems, say the scientists.
Where it is warm and humid all year round on our planet, nature has unfolded its potential almost explosively: Characterized by majestic giant trees and an amazingly diverse flora and fauna, the rainforests form the most species-rich habitats on the earth's surface. In the course of evolution, the typical features of these tropical ecosystems have obviously emerged. But how and when did this development begin? "At what time in the history of the earth these biodiversity hotspots developed is a matter of scientific debate," says lead author Clément Coiffard from Freie Universität Berlin.
Since when do typical rainforests exist?
Evidence from the evolutionary history of certain plants typical of today's rainforests suggests that these ecosystems already existed 100 million years ago. But solid evidence is rare: "So far, no corresponding fossil evidence has been identified that is older than about 70 million years," says Coiffard. Coiffard and his colleagues have now devoted themselves to "deeper" research into the developmental history of typical rainforest ecosystems by examining an extensive collection of fossil finds stored in the archive of the Museum für Naturkunde Berlin.
The material was collected in the Quseir Formation in southern Egypt in 1984 and 1987 and has so far only been subjected to a rough examination. Coiffard and his colleagues have now examined the finds in detail again. In doing so, they compared the characteristics of the approximately 80 million-year-old sedimentary rock with the geological properties of today's deposits from different ecosystems. Above all, however, they examined the diversity of species and the taxonomic composition of the flora based on the fossil traces of plants.
Concerning the geological basis of the site, Coiffard explains: "The sediments of the Quseir Formation are characterized by wet habitats with a warm-humid tropical climate". Previous work that focused on the material examined had already identified fossil plant species at this site. But the extensive findings of the new study now provide a much more comprehensive picture of the Cretaceous vegetation at the site: "Our current study of the 21,361 fossil specimens led to the identification of 70 distinguishable plant types," says senior author Dieter Uhl from the Senckenberg Research Institute and Natural History Museum in Frankfurt.
Characteristic Findings
Specifically, the team was able to identify 54 dicotyledonous plant species, 22 flowering plants, eleven monocotyledonous plants, four fern species and one tree species on the basis of characteristic fine structures. "So we see here both a great diversity of flora and a composition of vegetation that is similar to that of today's rainforests," says Uhl. Certain plants were particularly significant, emphasizes the paleobotanist: "For example, fossil evidence of the arum family, which we find mainly in the tropics today".
The team also found scattered traces of fossil charcoal in the sediments, an indication of fires in the area. This may seem strange when it comes to the topic of "rainforests". But co-author Haytham El-Atfy from the University of Tübingen explains: “We also know of fires in today’s rainforests. In addition, in the Cretaceous period, the global temperatures and at times the oxygen content of the atmosphere were significantly higher than today, which could have encouraged fires," says the scientist.
According to the researchers, the plant fossils examined and the sediments surrounding them reflect all the properties that can also be found in today's tropical rainforests: It is becoming apparent that a tropical climate with annual mean temperatures of 26 to 33 degrees Celsius prevailed, and predominantly humid conditions and no distinct seasons. In addition, the Cretaceous ecosystem apparently already had a characteristically diverse flora. "We therefore assume that there were already extensive tropical rainforests in north-eastern Africa in the heyday of the dinosaurs, around 80 million years ago," summarizes Coiffard.
Source: Senckenberg Research Institute and Natural History Museums, specialist article: Biogeosciences, doi: https://doi.org/10.5194/bg-20-1145-2023