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They discover new fossils, but they also bite

A small group of paleontologists recently discovered 10 species of ancient mammals previously unknown to science. But they had a huge number of helpers at their dig site: thousands of tiny ants.

The ancient mammals, described in a study published in May by the Rochester Institute of Vertebrate Paleontology, include a pocket mouse that weighed less than a light bulb, a rat-sized relative of the mountain beaver and an ancestor of rats kangaroos.

The study sheds new light on the diversity of mammals that existed in North America about 33 to 35 million years ago, when the climate was changing dramatically. It also pays a rare tribute to the insects that collected the fossils and makes the case for continued scientific collaboration between paleontologists and harvester ants, with whom they have a long love-hate relationship.

“They’re not fantastic when they bite you,” said Samantha Hopkins, a professor of earth sciences at the University of Oregon who was not involved in the study. “But I have to appreciate them because they make my job a lot easier.”

Most species of harvester ants live in underground burrows that are found under mounds of dirt.

Harvester ants fortify these mounds by covering them with chunks of rock and other tough materials. Ants are known to travel more than 100 feet from their burrows and dig 6 feet underground in search of materials that help secure their mounds.

This material includes fossils, particularly in the badlands of Wyoming, Nebraska, and South Dakota, where fossils are abundant and can be found in soft soils. Harvester ants can carry materials 10 to 50 times their body weight, although they do not weigh much, so the heaviest fossil they can collect weighs less than the average pill.

Given these size constraints, harvester anthills are hotspots for what scientists call microvertebrate fossils, which are fossils of animals too small to see without a microscope. For more than a century, scientists like Hopkins have been scraping the sediments of harvester anthills for these fossils, making it easier to find large numbers of fossilized mammalian teeth without spending hours in the field sifting through sand. and the earth.

In 2015, an amateur fossil hunter from Sioux County in the northwest corner of Nebraska noticed a staggering array of fossilized teeth and jawbones atop anthills on his property. He began sending samples to Clint Boyd, a senior paleontologist with the North Dakota Geological Survey. Over the years the samples kept coming in and by 2020 Boyd had over 6,000 identifiable specimens.

With the help of Bill Korth, a research associate at the Rochester Museum & Science Center in New York, and a few other paleontologists, Boyd was able to identify dozens of species in the collection, as well as 10 new species.

These new species included Cedromus modicus, a relative of modern squirrels that had only been around for a few million years, as well as Yoderimys massarae, the smallest member of a long-extinct group of rodents known as the Eomyidae. The beaver’s relative, Costepeiromys attasorus, was named for the species of harvester ant that discovered it.

According to Boyd, naming the species after its insect collaborators was the least he could do. “They are amazing little ants,” he said.

Based on the location and age of the rocks surrounding the anthills, researchers believe the fossils date to the late Eocene and early Oligocene. During this time, the Earth’s climate was cooling considerably. Understanding the true extent of mammalian diversity during and after this time will help scientists better predict how today’s mammals might respond to climate change.

“It’s not enough to look at the big things,” Hopkins said. “Small mammals could be the canaries of the coal mine.”

Luckily, there are still boxes and boxes of anthill fossils that Boyd and his colleagues still have to go through, and more are coming along.

“We haven’t done enough even with everything we’ve done,” Boyd said. “There is still so much to learn.”

This article originally appeared in The New York Times.