In the western United States, the industrious insects known as harvest ants are often cast as pests. These ants collect seeds and live in large sediment mounds, and they can deliver nasty stings to creatures they perceive as threats. A mound can last for decades and, to the chagrin of some property owners, the land is protectively plucked from vegetation up to 10 meters away.
But as these ants build their mounds, they do something remarkable: they act as the world’s smallest fossil collectors.
The colonies covered their mounds with a half-inch thick layer of small, bead-sized rocks, possibly to protect the structures from wind and water erosion. To find material for this covering, the ants venture more than thirty meters down the hill. In addition to bits of gravel, they collect all the small fossils and archaeological artifacts they come across.
The scientific wealth these ants can accumulate is staggering. Examining 19 anthills on a site in Nebraska, researchers recently found more than 6,000 microfossils — each no more than a few millimeters wide — of ancient mammals. The specimens contain small teeth and jaw fragments that represent nine new species of rodents and a new species of insectivorous shrew-like animals.
The fossil migration, recently described in the scientific journal paludicola, also includes the teeth of primates, old cousins of rabbits, and an unidentified bat species. As small as these teeth are, their shape provides a wealth of information, including where the teeth fall on the mammalian tree of life.
“It gives us this concentrated source of fossils that would otherwise take a lot of effort to dig through rocks … or just years and years of crawling around on hands and knees hoping to find something loose,” said study co-author Clint Boyd , a paleontologist with the North Dakota Geological Survey in Bismarck.
And thanks to the ants’ efforts, researchers can use these fossils to better understand what happened in North America about 34 million years ago, an evolutionarily significant period that marked the end of the Eocene epoch and the beginning of the Oligocene. During this time, the planet entered a prolonged cooling period, which caused some species to become extinct and ecosystems across the old Earth were rearranged.
“Power hills are like the best friends of archaeologists and paleontologists,” said National Geographic researcher Benjamin Schoville, an archaeologist at Australia’s University of Queensland who was not involved in the study.
Little fossil hunters
The ignorant talent of ants for fossil hunting has been known to scientists for more than a century. In an 1896 publication on fossil sites in the western US, paleontologist John Bell Hatcher advised collectors to visit the local anthills regularly, “because they will almost always yield a fair number of mammalian teeth.” Hatcher’s preferred method of picking teeth – sieving the sediment with a sieve – seems to have worked well. He boasted that he regularly found 200 to 300 individual teeth and jaw fragments on just one mound.
As well documented as the ants’ behavior is, it has adopted the flavor of folklore — widely understood but not systematically studied. However, the few studies done to date have shown that harvester ants can pick up some notable specimens.
In 2009, a team led by Schoville published the results of observations it made of 812 anthills in Nebraska. Of these, nearly a fifth had small flakes of stone, perhaps debris left by Native Americans who grind stones into tools or projectile points. “Some debris from human habitation is just represented by those little artifacts,” he says.
The study also showed how far the ants will travel. In one experiment, Schoville’s team arranged beads in concentric rings around several mounds. The farthest of these rings was 48 meters (157 ft) from the peaks. To Schoville’s surprise, ants near one of the mounds brought back beads from that distant distance—about the equivalent of a human foraging seven miles from home.
Comb the plains
The new study, also conducted in Nebraska, came about through the persistent efforts of the Gulotta family, who own the ranch where the study mounds are located.
Marco Gulotta, Sr., an avid amateur fossil collector, knew that harvester ant mounds could yield small teeth and bones. Along with his sons Mel and Marco Jr. Gulotta collected gallons of gravel from the outer layers of the hills, used a sieve to separate the material, and searched the pebbles for ancient remains. Gulotta then began posting photos of his finds on the Fossil Forum, an online community of paleo enthusiasts.
Boyd and his colleague Deborah Anderson, a paleontologist at St. Norbert College in De Pere, Wisconsin, saw the reports and contacted Gulotta to persuade him to send Anderson some of the microfossils. From there, the project snowballed, with Anderson, Boyd and Bill Korth of the Rochester Institute of Vertebrate Paleontology in New York working together to view thousands of tiny remains.
Through it all, the researchers worked closely with the Gulotta family. In the fall of 2020, Boyd visited the ranch to catalog the locations of the anthills using GPS. The Gulottas donated the thousands of microfossils analyzed in the study to the South Dakota School of Mines and Technology, where they will be available for future researchers.
“Sometimes, you know, some people see a bit of antagonism between academic paleontologists and landowners when it comes to fossils,” Boyd says. “But this is a great example of how we can all work together and do important scientific research.”
The Prehistoric Midwest
When these fossils were originally formed — between 37 million and 32 million years ago — the Great Plains of what is now the central U.S. were warmer, wetter and more forested, says Korth, the lead author of the new study. The fossils therefore capture a small slice of mammal life in this cramped environment.
Many of the remains are believed to have come from predatory feces after the small animals had been eaten and digested. Once buried, the teeth and bone fragments are fossilized – and they are remarkably well preserved.
These fossils, each just a few millimeters in diameter, not only contain 10 new species of small mammals, they also complement the biology of known creatures and reveal never-before-seen types of teeth from several extinct rodents. “There were some known species of two or three specimens, of which we now have thirty to forty specimens,” Korth says.
The harvest ants collect gravel pieces of a certain size, so only fossils of that size will reach their mounds. “They’re not going to build a perfectly curated museum building,” Schoville says.
Still, thanks to GPS coordinates and knowledge of the topography, Korth and Boyd’s team was able to determine the specific rock layers each ant hill sampled. By keeping track of the types of fossils found on each mound, the researchers were able to estimate when different species appeared and disappeared in the site’s rock layers. With this information, the team was then able to deduce which rock strata in this part of Nebraska marked the end of the Eocene and the beginning of the Oligocene Era about 34 million years ago.
Much to the researchers’ delight, their ant-based estimate matches an estimate made 13 years earlier using another method — meaning ant hills could provide an independent way to refine the boundaries of geologic time. That’s all the more reason to look at harvest ants as human partners in paleontology.
“You walk through landscapes in search of fossils,” says Schoville. “You should also look for anthills along the way.”