Two thirds of all mammals vanished from Africa and the Arabian Peninsula around 30 million years ago, when the climate on Earth changed from swampy to icy. But we are only finding out about this mass extinction now. Researchers examined hundreds of fossils from multiple sites in Africa, built evolutionary trees, and pinpointed each species’ first and last known appearances. The climate shift ‘was a real reset button.’

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

That time period was marked by dramatic climate change. In a reverse image of what is happening today, the Earth grew cooler, ice sheets expanded, sea levels dropped, forests started changing to grasslands, and carbon dioxide became scarce. Nearly two-thirds of the species known in Europe and Asia at that time went extinct.

African mammals were thought to have possibly escaped unscathed. Africa’s mild climate and proximity to the Equator could have been a buffer from the worst of that period’s cooling trend.

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

The team, comprising researchers from the United States, England, and Egypt, looked at fossils of five mammal groups: a group of extinct carnivores called hyaenodonts, two rodent groups, the anomalures (scaly-tail squirrels) and the hystricognaths (a group that includes porcupines and naked mole rats), and two primate groups, the strepsirrhines (lemurs and lorises), and our very own ancestors, the anthropoids (apes and monkeys).

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Materials provided by Duke University. Original written by Marie Claire Chelini. Note: Content may be edited for style and length.