Tue. Aug 9th, 2022

Fossils of African fauna

Fossils from the central groups are used to reveal the Eocene-Oligocene extinction in Africa with primates on the left, the carnivorous hyaenodont, top right, rodents, bottom right. These fossils are from the Fayum Depression in Egypt and are stored at the Duke Lemur Center’s Division of Fossil Primates. Credit: Matt Borths, Duke University Lemur Center

Fossils from the Duke collection reveal a previously unknown mass extinction in Africa.

Sixty-three percent. It is the proportion of mammalian species that disappeared from Africa and the Arabian Peninsula about 30 million years ago after the Earth’s climate changed from swamp to icy. But we’ll only find out now.

Summary of decades of work, a new study published this week in the journal Communication Biology reports on a previously undocumented extinction event that followed the transition between the geological periods called the Eocene and the Oligocene.

This period was marked by dramatic climate change. In an inverted picture of what is happening today, the earth became cooler, ice caps expanded, sea levels dropped, forests began to shift to grasslands, and carbon dioxide became scarce. Nearly two-thirds of the species known in Europe and Asia at the time became extinct.

African mammals may be thought to have escaped unharmed. Africa’s mild climate and proximity to the equator could have been a buffer from the worst of this period’s cooling trend.

Mammalian teeth CT scans

Dental CT scans show that mammalian teeth became less diverse during the early Oligocene eradication events. Here is an example of the three-dimensional tooth shape of a lower molar of a fossil anomaluroid rodent. Credit: Dorien de Vries, University of Salford

Now, thanks in large part to a large collection of fossils housed in the Duke Lemur Center Division of Fossil Primates (DLCDFP), researchers have shown that despite their relatively warm environment, African mammals were as affected as those from Europe. and Asia. The collection was the life work of the late Elwyn Simons of Duke, who for decades scoured Egyptian deserts for fossils.

The team, made up of researchers from the United States, England and Egypt, looked at fossils from five mammal groups: a group of extinct carnivores called hyaenodonts, two rodent groups, the anomalors (scaly squirrels) and hystricognaths (a group that includes hedgehogs and naked moles) and two primate groups, strepsirrhines (lemurs and lorises), and our very own ancestors, the anthropoids (monkeys and apes).

By collecting data on hundreds of fossils from several locations in Africa, the team was able to build evolutionary trees for these groups, determine when new genera branched off, and time-stamp the first and last known occurrences of each species.

Their results show that all five mammalian groups suffered large losses around the eocene-oligocene boundary.

“It was a real reset button,” said Dorien de Vries, a postdoctoral fellow at the University of Salford and lead author of the paper.

After a few million years, these groups begin to reappear in the fossil record, but with a new look. The fossil species that reappear later in the Oligocene, after the great extinction, are not the same as those found before.

“It’s very clear that there was a huge eradication event, and then a recovery period,” said Steven Heritage, researcher and digital preparation at Duke University’s DLCDFP and co-author of the paper.

The evidence is in the teeth of these animals. Molar teeth can tell a lot about what a mammal eats, which in turn tells a lot about their environment.

Rodents and primates that reappeared after a few million years had different teeth. These were new species that ate different things and had different habitats.

“We are seeing a huge loss in dental diversity and then a recovery period with new tooth shapes and new adjustments,” de Vries said.

“Extermination is interesting that way,” said Matt Borths, curator of Duke University’s DLCDFP and co-author of the paper. “It kills things, but it also opens up new ecological opportunities for the genera that survive into this new world.”

This decline in diversity followed by a recovery confirms that the boundary between eocene and oligocene functioned as an evolutionary bottleneck: most genera became extinct, but a few survived. Over the next several million years, these surviving lines diversified.

“In our anthropoid ancestors, diversity comes to almost nothing about 30 million years ago, leaving them with a single tooth type,” said Erik R. Seiffert, professor and chairman of the Department of Integrative Anatomical Sciences at the Keck School of Medicine at the University of Southern California. , a former graduate student at Simons and senior co-author of the paper. “The tooth shape of this ancestor determined what was possible in terms of later dietary diversification.”

“There’s an interesting story about the role of this bottleneck in our own early evolutionary history,” Seiffert said. “We came pretty close to never existing if our monkey-like ancestors were extinct 30 million years ago. Fortunately, they did not. ”

A rapidly changing climate was not the only challenge these few surviving types of mammals faced. As temperatures dropped, East Africa was plunged by a series of major geological events, such as volcanic super-eruptions and flood basalts – huge eruptions that covered vast expanses of molten rock. It was also at that time that the Arabian Peninsula separated from East Africa and opened the Red Sea and the Gulf of Aden.

“We lost a lot of diversity at the eocene-oligocene frontier,” Borths said. “But the species that survived apparently had enough of a toolbox to sustain through this fluctuating climate.”

“Climate change over geological time has shaped the evolutionary tree of life,” said Hesham Sallam, founder of the Mansoura University Vertebrate Paleontology Center in Egypt and co-author of the paper. “Gathering evidence from the past is the easiest way to learn about how climate change will affect ecological systems.”

Reference: “Widespread Loss of Mammalian Genealogy and Dietary Diversity in the Early Oligocene of Afro-Arabia” by Dorien de Vries, Steven Heritage, Matthew R. Borths, Hesham M. Sallam, and Erik R. Seiffert, October 7, 2021, Communication Biology.
DOI: 10.1038 / s42003-021-02707-9

Funding for this study came from The Leakey Foundation, US National Science Foundation (BSC-1824745 to DD. And DBI-1612062 to MRB) and the Natural Environment Research Council (NERC NE / T000341 / 1). Fieldwork in the Fayum Depression, Egypt, and digital cure of Fayum fossils were supported by the US National Science Foundation (BCS-0416164, BCS-0819186 and BCS-1231288) as well as Gordon and Ann Getty and The Leakey Foundation. Micro-CT scanning was partially supported by NSF grant DBI-1458192, DBI-2023087 and IMLS grant MA-245704-OMS-20.

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