Fossils from the key groups that were used to reveal the Eocene–Oligocene extinction in Africa. Primate fossils on the left; carnivorous Hyaenodont, top right; rodent, bottom right. These fossils were found in Egypt's Fayum Depression and are kept at the Duke Lemur Centers Division for Fossil Primates. Credit: Matt Borths at Duke University Lemur Center
Sixty-three per cent. This is the percentage of mammal species that disappeared from Africa and Arabian Peninsulas around 30 million years ago. It was the result of Earth's climate changing from wet to cold. We are just now learning more.
A new study, published in the journal Communications Biology this week, examines a previously unknown extinction event that occurred after the transition between geological periods known as the Eocene or Oligocene.
This time period saw dramatic climate change. The Earth became cooler, the ice sheets expanded, sea level dropped, forests changed to grasslands and carbon dioxide became scarcer. This is a reverse picture of today. Nearly two-thirds (or more) of all species in Europe and Asia were extinct at the time.
It was possible that African mammals may have escaped unharmed. The mild climate of Africa and its proximity to the Equator may have helped buffer against the worst effects of the cooling trend.
Researchers have now shown that African mammals are just as affected by the colder climate as their counterparts from Europe and Asia, thanks to a large collection of fossils at Duke Lemur Center Division of Fossil Primates. Elwyn Simons, a Duke ecologist who searched for fossils in the Egyptian desert for over a decade, was responsible for the collection.
Researchers from Egypt, England, and the United States examined fossils from five mammal groups. These included a group of extinct carnivores called Hyaenodonts, two rodent groups, the anomalures and the hystricognaths. There were also two primate groups, the Strepsirrhines (lemurs, and lorises) and our own ancestors the anthropoids (apes, monkeys).
The team was able gather data from hundreds of fossils at multiple locations in Africa and build evolutionary trees. This includes pinpointing when new lineages emerged and stamping the first and last appearances of each species.
The results showed that all five mammal species suffered massive losses at the Eocene–Oligocene border.
Dorien de Vries (a postdoctoral researcher at University of Salford) said, "It was really a reset button." He is also the lead author of this paper.
These groups begin to appear again in fossil records after a few million year. However, they have a new look. After the great extinction event, the fossil species that are found in the Oligocene reappear later on are different from those that were previously discovered.
Dental CT scans reveal that mammal teeth were less diverse in the Oligocene extinction periods. This is an example showing the three-dimensional shape of a lower molar from a fossil anomaluroid rodent. Credit: Dorien De Vries, University of Salford
Steven Heritage, co-author of this paper and Researcher and Digital Preparator at Duke University’s DLCDFP, stated that "it's quite clear that there was a large extinction event and then a recovery phase."
These animals' teeth are the best way to see what is going on. The evidence is in the teeth of these animals. Molar teeth can reveal a lot about how a mammal eats which in turn tells us a lot more about their environment.
Different teeth were found in the primates and rodents that resurfaced after just a few millions years. These new species ate different foods and lived in different environments.
De Vries stated, "We see a significant loss in tooth variety, and then there is a recovery period with dental shapes and adaptations."
Matt Borths (curator of Duke University's DLCDFP) said, "Extinction can be interesting in that way." He was also coauthor of the paper. It kills things but also offers new ecological opportunities to the lineages that are able to survive in this new world.
The Eocene–Oligocene boundary was responsible for the decline in diversity and subsequent recovery. Most lineages became extinct but a few survived. These surviving lines became more diverse over the following several million years.
Erik R. Seiffert is Professor and Chair of Integrative Anatomical Sciences at Keck School of Medical Sciences of the University of Southern California. He was a former student of Simons and co-author of this paper. "The ancestral tooth shape dictated what was possible for later dietary diversification."
Seiffert said, "There's an interesting tale about the role that bottleneck played in our early evolutionary history." "We were very close to not existing if our monkey-like ancestors hadn't died out 30 million years ago. They didn't.
These few remaining mammals faced many challenges, not just a rapidly changing climate. East Africa was hit hard by extreme weather events such as super volcanic eruptions and flood basaltsenormous volcanoes, which covered large areas with molten rocks. The Arabian Peninsula was also separated from East Africa at this time, opening up the Red Sea to the Gulf of Aden.
Borths stated that "we lost a lot diversity at the Eocene–Oligocene border." Borths stated that the species that survived had sufficient tools to withstand this climate change.
Hesham Sallam co-author of this paper and founder of the Mansoura Universität Vertebrate Palaeontology Centre in Egypt, said that "Climate change through geological times have shaped the evolution tree of life." The best way to find out how climate change will impact ecological systems is to collect evidence from the past.
Continue reading Ancient teeth from Peru suggest that now-extinct monkeys crossed Atlantic From Africa
Information: Dorien De Vries et.al, Widespread loss in mammalian lineage, dietary diversity, and early Oligocene Afro-Arabia, Communications Biology (2021). Information from Communications Biology Dorien de Vries and colleagues, Widespread loss in mammalian lineage, and dietary diversity during the early Oligocene period of Afro-Arabia (2021). DOI: 10.1038/s42003-021-02707-9
