About 34 million years ago, the “lost extinction” in Africa wiped out most of the primates, rodents and predators that preyed on these two groups. The species disappeared in a slow wave of extinction that spanned millions of years and went unnoticed by scientists – until now.
This previously unseen extinction connects two geological eras: the Eocene (55.8-33.9 million years ago) and the Oligocene (33.9-23 million years ago). When the greenhouse climate of the Eocene began to shift toward the glacial temperatures characteristic of the Oligocene, sea levels dropped, the Antarctic ice sheet rose, and about two-thirds of all animal species in Europe and Asia became extinct.
But researchers believed that life in Africa escaped this fate, and that animals there were protected from the worst effects of the cooling climate by their proximity to the equator. The few African fossils from that period have given scientists few clues as to what actually happened to the continent’s animals as the Earth cooled; a new look at animal lineages has recently shown that climate change at the end of the Eocene also dealt a devastating blow to African mammal life.
Using hundreds of fossils spanning tens of millions of years, from the mid-Eocene to the Oligocene, scientists have reconstructed the evolutionary chronology in the family trees of five groups of African mammals. The researchers focused on two groups of primates, two groups of rodents and one group of extinct predators known as hyenadonts (“hyena teeth”) that preyed on rodents and primates, according to the new study.
“Africa doesn’t have the same fossil density as other continents,” said study co-author Eric Seiffert, professor and chair of the Department of Integrative Anatomical Sciences at the Keck School of Medicine at the University of Southern California in Los Angeles. “So we had to find a way to extract as much information as possible, which is why we used this fairly novel approach,” Seiffert told Live Science.
The authors used what fossils they had to track species diversity and loss over time in those animal groups. As they did, patterns began to emerge, showing that around 34 million years ago, a cooling Earth lopped off entire branches of those mammals’ family trees. Species diversity didn’t drop abruptly, as is often the case in global mass extinction events. Rather, the decline happened over millions of years, until 63% of the species in those mammal groups had disappeared.
“Over the course of 4 million years, we see this gradual slow loss of all of the lineages that had been present in the late Eocene,” Seiffert said. “The biggest trough of that lineage diversity curve really bottoms out at 30 million years ago, and then starts to pick back up around 28 million years ago.”
Dental CT scans show that mammal teeth became less diverse during the early Oligocene extinction events. Here is an example of the three-dimensional tooth shape of a lower molar of a fossil anomaluroid rodent.
When those groups began to diversify again, many of the new species had evolved new traits that weren’t present in species that came before the extinctions, according to the study. For example, rodent and primate species that emerged during the Oligocene had different tooth shapes than their extinct cousins, hinting that these animals were adapted to survive in different ecosystems than their predecessors experienced.
“Extinction is interesting in that way,” study co-author Matt Borths, curator of Duke Lemur Center Division of Fossil Primates, said in a statement. “It kills things, but it also opens up new ecological opportunities for the lineages that survive into this new world.”
Was it global cooling that extinguished those African mammals? While that was probably a factor, other evidence from Africa and the Arabian peninsula from around 31 million years ago suggests that unusually active volcanoes may have posed another insurmountable challenge to their survival, Seiffert said.
“All this volcanic activity that would ultimately lead to the rising and development of the Ethiopian highlands, it started around 31 million years ago with some really dramatic volcanic super-eruptions,” he said. “That part of eastern Africa was continually being altered by these volcanic events. If not necessarily causing extinctions, those constant changes to the environment may have been at least delaying diversification in some of these lineages.”
The findings were published Oct. 7 in the journal Communications Biology.
