The end of the Eocene (~33.9 million years ago) was a rough time for many species. It was a period marked by global cooling and drops in sea level, and in Eurasia, these global environmental impacts led to a mass extinction of marine organisms, plants, and land animals known in Europe as the Grande Coupure. But it appeared as though mammals living in Africa and the Arabian Peninsula were spared, presumably thanks to their location near the equator’s warmth. Not so, says a paper published on October 7, in Communications Biology. Careful review of the fossil record from these areas revealed a mass extinction of at least five groups of mammals that coincided with the mass die-offs elsewhere.
In a Nature Ecology and Evolution perspective piece about the paper, study coauthor and evolutionary biologist Erik Seiffert writes how the team generated “lineages through time” plots based on previously built, time-scaled phylogenetic trees of Afro-Arabian mammals. Seiffert says this “revealed that there had been rapid diversification of lineages during the late Eocene, followed by a major loss of lineage diversity that started at the Eocene-Oligocene boundary and continued through the early Oligocene.” All of the mammal groups for which the research team had time-scaled information showed this trend.
The scientists also examined a collection of dental fossils from Egypt and stored at the Duke Lemur Center Division of Fossil Primates (DLCDFP) from five mammal groups: extinct carnivores called hyaenodonts, two rodent groups (anomalures and hystricognaths), and two primate groups—the strepsirrhines, which includes lemurs, their close relatives, and their ancestors, and the anthropoids, the lineage that led to humans. The researchers looked at lower teeth to extract information about the animals’ diets and, in turn, the environment where they lived. “We see a huge loss in tooth diversity, and then a recovery period with new dental shapes and new adaptations,” lead author and University of Salford archaeologist Dorien de Vries says in a press release about the paper.
Seiffert explains in the release that diversity in modern humans’ anthropoid ancestors “bottoms out to almost nothing around 30 million years ago, leaving them with a single tooth type,” and that this “ancestral tooth shape determined what was possible in terms of later dietary diversification.”
Paleontologist Chris Beard, who was not involved with this study, tells Inside Science that he does not find the results particularly surprising. Though, he notes that “this result could be viewed as counterintuitive, especially for arboreal primates and rodents” whose species evolution would likely be supported by the forest patches of the early Oligocene. “So, either the assumption of increasingly patchy conditions across the Eocene-Oligocene boundary is wrong, or the macroevolutionary response to it was different than expected.”
Seiffert offers some thoughts in his Nature Ecology and Evolution piece as to why these mammals went extinct, suggesting global cooling followed by persistent volcanic activity drastically changed the landscape and environment, making it unsuitable over a span of several millions of years. He writes that the team wants to expand the dataset in the future using the same methods, but “the real challenge is to expand the search for Oligocene mammals in Africa and Arabia, so that we can more thoroughly evaluate the nature of this previously ‘hidden’ extinction event.”
Coauthor and paleontologist Hesham Sallam notes in the press release that this is yet another example of how climatic changes “have shaped the evolutionary tree of life,” adding that “Collecting evidence from the past is the easiest way to learn about how climate change will affect ecological systems.”