The end of the Eocene, about 33 million years ago, marks a time of great change on Earth. In a slow reversal of what we see today, temperatures have fallen and glaciers have stretched their icy fingers toward the equator.
The loss of life was profound throughout the entire Asian continent. But Africa’s biodiversity, protected by the warmth of the tropics, appears unaffected by the dramatic changes. Or so we thought.
According to a study recently published by a team of researchers from across the United States, we weren’t looking at the fossil record in the right way.
The research indicates that mammals in Arabia and across the African continent, which were far from thriving during this cold change, experienced a dramatic decline, with nearly two-thirds of their maximum diversity disappearing 30 million years ago.
It’s not clear exactly what led to each loss, but despite the widespread fluctuations in temperature and intense volcanic activity rocking the region, there is no shortage of opportunities.
Whatever the cause of the loss, the ecological niches left by the extinction were not empty for long.
“It’s very clear that there was a massive extinction and then a recovery period,” said Duke University biologist Stephen Heritage.
Much of what we know about climate change in the transition from the Eocene to the following epoch, the Oligocene, comes from analyzes of changes in oxygen isotopes in sediment cores extracted from the ocean floor.
By matching this evidence with many other evidence about sea level fluctuations and evidence for glacier growth, we get a comprehensive picture of how our planet as a whole is changing.
However, the markings at the local level can be a bit patchy, relying more on modeling and careful examination of the fossils that appear sporadically here and there.
Data from Earth can paint a mixed picture, so it’s not surprising that there is debate about the ultimate impact of global cooling on masses near the equator.
From the trend there is evidence that they are animals like the ancestors of modern lemurs that are disappearing from northeastern Africa. Yet other studies indicate that Africa has seen little or no environmental change.
Interpretation of fossil records can be difficult due to their tendency to be somewhat incomplete. Not all species leave their remains neatly preserved in a convenient location, but with the right analytical tools, researchers can still get a range of information from just a handful of bones.
The team collected data on fossils representing five groups of mammals, including carnivores called Hinodonts, two groups of rodents resembling a squirrel anomaly, and two groups of primates — one inhabited by our ancestors.
From these samples, the researchers generated a family tree showing the timing of manifestations and known losses for each. Statistical tools can then give scientists a better idea of when losses in certain regions were large enough to correlate with global events.
By looking at traits within related groups, researchers can also see how species diversified to fill niches vacated by lost animals.
Take, for example, the teeth of an animal. Subtle differences in their shape over a long period of time can tell us how quickly a species adapts to a new abundant food source.
“We see a significant loss of tooth diversity and then a recovery period with new tooth shapes and new adaptations,” said lead author Dorian de Vries of the University of Salford.
Besides, our most important ancestors seem to be among the hardest hit. Diversity in human teeth has decreased to nearly 30 million years. It was so bad that there was only one type of dental morphology left, limiting the type of food their grandchildren could eat.
Such bottlenecks are common throughout the evolutionary record. Knowing how species respond to them can be vital given the pressures we place on many ecosystems around the world today.
In a sense, the dental design succeeded. Otherwise, our species would not have seen the light of day.
“It was a real reset button,” says Davies.
This research was published in Communication biology.
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