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The end of the Eocene around 33 million years ago marks a time of great change on Earth. In a slow reversal of what we see today, temperatures have dropped and glaciers have stretched their icy fingers toward the equator.
The loss of life across the Asian continent was profound. But Africa’s biodiversity, sheltered by the heat of the tropics, seems to be spared from colossal changes. Or so we thought.
According to a study recently published by a team of researchers from across the United States, we just weren’t looking at the fossil record the right way.
Research suggests that far from thriving on this cold change, mammals of the Arabian Peninsula and the African continent have seen significant declines, with nearly two-thirds of their maximum diversity having disappeared 30 million years ago.
It is not clear what precipitated each loss, although with widespread temperature fluctuations and intense volcanic activity shaking the region, there are plenty of possibilities.
Whatever the cause of the loss, the ecological niches left open by extinction did not remain vacant for long.
“It’s very clear that there was a huge extinction event and then a recovery period,” said Duke University biologist Steven Heritage.
Much of what we know about climate change during the transition from the Eocene to the next epoch, the Oligocene, comes from analyzes of changes in oxygen isotopes in sediment cores excavated from the ocean floor.
Matching them with various other clues about sea level fluctuations and evidence for glacier growth gives us a general picture of how our planet has evolved as a whole.
Still, the signs at more local levels can be a bit patchy, relying more on modeling and careful examination of fossils that appear sporadically here and there.
Recordings taken on land can provide a mixed picture, so it’s no surprise that there has been some debate about the impact of global cooling on masses near the equator.
On the one hand, there is evidence from animals such as the ancestral parents of modern lemurs that have gone extinct from northeastern Africa. Yet other studies suggest that Africa has experienced virtually no environmental change, if any.
The fossil record can be difficult to interpret due to its tendency to be rather patchwork. Not all species leave their carefully preserved remains in one convenient place, but with the right analytical tools, researchers can still extract a wealth of information from a handful of bones.
The team collected data on fossils representing five groups of mammals, including carnivores called hyaenodonts, two groups of rodents, squirrel-like anomalies, and two groups of primates – one occupied by our own ancestors.
From these samples, the researchers constructed a family tree representing the time of known appearances and losses for each. Statistical tools could then give scientists a better idea of when losses were large enough in given areas to be linked to global events.
By examining characteristics within related groups, the researchers were also able to see how species diversified to fill the niches released by the lost animals.
Take the teeth of an animal for example. Subtle differences in their forms over a long period of time can tell us how quickly species adapted to a new abundant food source.
“We see a huge loss of tooth diversity, then a period of recovery with new tooth shapes and new adaptations,” says lead author Dorien de Vries of the University of Salford.
By the way, our own primate ancestors seem to be among the hardest hit. The diversity of anthropoid teeth 30 million years ago has fallen to virtually nothing. It was so bad that only one type of tooth morphology remained, limiting the types of food their descendants could eat.
Bottlenecks like these are common throughout the history of evolution. Knowing how species respond to them could be vital, given the strain we place on so many ecosystems around the world today.
Somehow, this tooth design got us through. Without it, our species might never have seen the light of day.
“It was a real reset button,” says de Vries.
This research was published in Communications biology.
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