Climate Extinction Makes Mammals’ Teeth Unusual

The ground is warmer 34 million years ago — and even more wonkier. The supercontinent of Pangea is divided. Dinosaurs are long gone. But Antarctica has forests without glaciers. Other continents seem to have scratched and wiped out versions of themselves today. Mammals are everywhere — especially monkeys and rats. “From New York to Los Angeles, up to Canada, circling trees all over the place,” Seiffert told North American monkeys. “But when this climate event happened 34 million years ago, they all disappeared.”

Some scientists believe that the level of carbon dioxide in the atmosphere has dropped to a critical threshold, causing the average air temperature to drop and Antarctica to freeze. The more sunlight that illuminates the more ice made temperatures lower even more. The transition from the Eocene to the Oligocene has been described as a transition from “greenhouse” to “ice house.”

Then, in Africa, the situation is much different. About 31 million years ago, volcanoes near the equator, in present-day Ethiopia, erupted with devastating toxic fumes and relentless flooding of molten basalt.

Fossil records in North America, Europe, and Asia are very well established during the 11 million years before and after these events. Scientists can count fossils that show what animals existed before the climate cooled, and what after, and figure out what disappeared. However, Seiffert says, “at this point in time, Africa’s fossil record is really useless.” That discrepancy bothered him, so his team tried to parse the relationships between any fossil records they had.

For their study, Seiffert and de Vries focused on a family tree that dates back 76 million years, when monkeys and rats were scattered. In particular, they studied the teeth of two suborders in mice (hystricognath and anomaluroid) and two suborders in primates (strepsirrhine and anthropoid). These clades produce extant species such as capybara, scaly-tailed flying squirrels, lemurs — and us.

The researchers decided to reconstruct the phylogeny — or family tree of evolutionary relationships — in these groups from 56 million to 15 million years ago. Using the teeth as a guide for “who’s who,” they painted branches between the lines from fossils found in the late Eocene to their descendants who survived until the Miocene, about 20 million years ago. When they were finished, a huge gap arose: Descendants from the Miocene descended from a unusually small fragment of earlier mammals. The researchers found that 63 percent of the generations that existed in the late Eocene never overcame the next period. About 30 million years ago, they concluded, these species must have become extinct, thanks to their changing environment. “There is no other explanation,” Seiffert said. “They’re probably exhausted.”

The diversity of the species gives the team a big picture view of how many species are lost in the changing climate, but not how different those species are from each other — in other words, how many. anatomical diversity is also extinct. For example, says de Vries, imagine a scenario in which two species of birds become extinct. Those two species may be very similar, or they may be very different in terms of body types, genetics, or ecological niches. “If you have a hummingbird and a flamingo, that’s a lot different than if you have a dove and a dove,” he said.

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