A small group of paleontologists recently discovered 10 species of ancient mammals previously unknown to science. But they had a huge number of helpers in their dig: thousands of tiny ants.
The ancient mammals, described in a study published in May by the Rochester Institute of Vertebrate Paleontology, included a pocket mouse that weighed less than a light bulb, a rat-sized mountain beaver relative and an ancestor of kangaroo rats.
The study sheds new light on the diversity of mammals that existed in North America about 33 million to 35 million years ago, when the climate changed dramatically. It’s also a rare tribute to the insects that collected the fossils and strongly advocates continued scientific collaboration between paleontologists and harvest ants, with whom they have long had a love-hate relationship.
“They’re not fantastic if 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 types of harvester ants live in underground burrows that sit beneath mounds of soil.
Harvest ants enhance these mounds by covering them with bits of rock and other tough materials. The ants are known to travel more than thirty feet from their burrow, digging two feet underground in search of materials that help secure their mounds.
That material includes fossils, especially in the badlands of Wyoming, Nebraska, and South Dakota, where fossils are abundant and found in loose soil. Harvest ants can carry materials 10 to 50 times the weight of their bodies, although they don’t weigh very much, so the heaviest fossil they can collect weighs less than the average pill.
Given these limited dimensions, the harvester ant hills are hot spots for what scientists call microvertebrate fossils, which are animal fossils too small to see without a microscope. For more than a century, scientists such as Dr. Hopkins scraped sediment from the sides of the harvester’s anthills in search of these fossils, making it easier to find large numbers of fossilized mammal teeth without spending hours in the field digging through sand and dirt.
In 2015, an amateur fossil hunter in Sioux County, Nebraska’s northwest corner, spotted a staggering number of fossilized teeth and jawbones atop the ant hills on his property. He began sending samples to Clint Boyd, a senior paleontologist with the North Dakota Geological Survey. Over the years, the monsters kept coming, and by 2020, Dr. Boyd more than 6,000 identifiable copies.
With the help of Bill Korth, a research associate at the Rochester Museum & Science Center in New York, and a few other paleontologists, Dr. Boyd identify dozens of species within the collection, as well as 10 new species.
These new species include Cedromus modicus† a relative of modern squirrels that existed only a few million years, as well as Yoderimys massarae† the smallest member of a long-extinct group of rodents known as Eomyidae. The beaver relative, Costepeiromys attasorus† was named in honor of the harvest ant species that discovered it.
According to Dr. Boyd was the least he could do to name the species after his insect workers. “They’re great little ants,” he said.
Based on the location and age of the rocks around the ant mounds, the researchers estimate that the fossils date from the late Eocene and early Oligocene. At that time, the Earth’s climate cooled dramatically. By understanding the true extent of mammalian diversity during and after that time, scientists can better predict how mammals might respond to a changing climate today.
“It’s not enough to just look at the big things,” said Dr. Hopkins. “The small mammals may be the canaries in the coal mine.”
Fortunately, there are still boxes and boxes of ant hill fossils where Dr. Boyd and his colleagues have yet to go through it, with more to emerge.
“We haven’t done enough, even with how much we’ve done,” said Dr. boyd. “There is still so much to learn.”