A mass extinction event occurred 30M years ago and scientists have only just learned about it

Fossils of the key groups used to unveil the Eocene-Oligocene extinction in Africa with primates on the left, the carnivorous hyaenodont, upper right, rodent, lower right. These fossils are from the Fayum Depression in Egypt. (Matt Borths)

TORONTO -

Climate change wiped out nearly two thirds of the mammal species in Africa and the Arabian Peninsula in a mass extinction event 30 million years ago that researchers have only just learned about now.

According to a new study, published in the journal Communications Biology, the Earth’s climate dramatically shifted during the transition between the geological periods called the Eocene and Oligocene.

The Earth grew colder, ice sheets expanded, sea levels dropped, forests changed to grasslands, and carbon dioxide became scarce, the researchers said. In Asia and Europe, nearly two-thirds of the species known at that time went extinct.

Mammals in Africa, however, were thought to have been largely spared due to the continent’s mild climate and proximity to the equator.

As it turns out, that was not the case.

By analyzing a large collection of fossils housed at the Duke Lemur Center Division of Fossil Primates (DLCDFP) in Durham, N.C., researchers have been able to show that mammals in Africa and the Arabian Peninsula were just as affected by climate change as those in Asia and Europe.

To reach this conclusion, the team comprised of researchers from the U.S., England, and Egypt studied fossils from five mammal groups:

• A group of extinct carnivores called hyaenodonts;
• two rodent groups;
• a group of scaly-tail squirrels called anomalures;
• the hystricognaths group (which includes porcupines and naked mole rats)
• a group called strepsirrhines (lemurs and lorises); and
• humans’ ancestors, the anthropoids (apes and monkeys).

The researchers gathered data on hundreds of fossils from multiple sites in Africa in order to construct evolutionary trees for these five different groups and pinpoint when new lineages branched out and time-stamp each species’ first and last known appearance.

They discovered that all five mammal groups suffered huge losses around the period in question.

“It was a real reset button,” Dorien de Vries, the study’s lead author and a postdoctoral researcher at the University of Salford, said in a press release.

By reset, he means they were able to show how these mammal groups started popping up again in the fossil records a few million years after the mass extinction event; however, with one major caveat.

The fossil species that appeared in the Oligocene period were different from their earlier ancestors.

According to the study, the evidence for this was in the species’ teeth.

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

The rodents and primates that emerged later had different teeth, which led the researchers to conclude they were new species that ate different things and had different habitats.

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

The researchers described this period of decline in diversity followed by a recovery as a sort of “evolutionary bottleneck,” in which most lineages went extinct and only a few survived.

Erik Seiffert, a professor and chair of the Department of Integrative Anatomical Sciences at the Keck School of Medicine of the University of Southern California and a senior co-author of the paper, said diversity in humans’ anthropoid ancestors was reduced to a single tooth type after the extinction event.

“There’s an interesting story about the role of that bottleneck in our own early evolutionary history,” Seiffert said.

“We came pretty close to never existing, if our monkey-like ancestors had gone extinct 30 million years ago. Luckily they didn’t.”