Nearly 63% of Afro-Arabian mammalian species went extinct approximately 30 million years ago (Oligocene epoch), after Earth’s climate shifted from swampy to icy.
A Pleistocene landscape. Image credit: Roman Uchytel, via the Netherlands Institute of Ecology.
In a new study, University of Salford’s Dr. Dorien de Vries and colleagues looked at fossils of five mammal groups: (i) a group of extinct carnivores called hyaenodonts; two rodent groups: (ii) the anomalures (scaly-tail squirrels) and (iii) the hystricognaths (a group that includes porcupines and naked mole rats); and two primate groups: (iv) the strepsirrhines (lemurs and lorises) and (v) the anthropoids (apes and monkeys).
By gathering data on hundreds of fossils from multiple sites in Africa, they were able to build evolutionary trees for these groups, pinpointing when new lineages branched out and time-stamping each species’ first and last known appearances.
Their results show that all five mammal groups suffered huge losses around the Eocene-Oligocene boundary.
“It was a real reset button. After a few million years, these groups start popping up again in the fossil record, but with a new look,” Dr. de Vries said.
“The fossil species that re-appear later in the Oligocene, after the big extinction event, are not the same as those that were found before.”
“It’s very clear that there was a huge extinction event, and then a recovery period,” said Dr. Steven Heritage, a researcher at Stony Brook University and the Duke Lemur Center Museum of Natural History.
The evidence is in the teeth of these animals. Molar teeth can tell a lot about what a mammal eats, which in turns tells a lot about their environment.
The rodents and primates that reappeared after a few million years had different teeth. These were new species, who ate different things, and had different habitats.
“We see a huge loss in tooth diversity, and then a recovery period with new dental shapes and new adaptations,” Dr. de Vries said.
“Extinction is interesting in that way. It kills things, but it also opens up new ecological opportunities for the lineages that survive into this new world,” said Dr. Matt Borths, curator of the Duke Lemur Center Museum of Natural History.
This decline in diversity followed by a recovery confirms that the Eocene-Oligocene boundary acted as an evolutionary bottleneck: most lineages went extinct, but a few survived.
Over the next several millions of years, these surviving lines diversified.
“In our anthropoid ancestors, diversity bottoms out to almost nothing around 30 million years ago, leaving them with a single tooth type,” said Professor Erik Seiffert, a researcher in the Keck School of Medicine at the University of Southern California, the Duke Lemur Center Museum of Natural History, and the Department of Mammalogy at the Natural History Museum of Los Angeles County.
“That ancestral tooth shape determined what was possible in terms of later dietary diversification.”
“There’s an interesting story about the role of that bottleneck in our own early evolutionary history.”
“We came pretty close to never existing, if our monkey-like ancestors had gone extinct 30 million years ago. Luckily they didn’t.”
The study was published in the journal Communications Biology.
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D. de Vries et al. 2021. Widespread loss of mammalian lineage and dietary diversity in the early Oligocene of Afro-Arabia. Commun Biol 4, 1172; doi: 10.1038/s42003-021-02707-9