Across the western United States, the industrious insects known as harvester ants are often cast as pests. These ants gather 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 up to 30 feet away is protectively picked clean of vegetation.

But as these ants construct their mounds, they do something remarkable: act as the world’s smallest fossil collectors.

The colonies clad their mounds with a half-inch-thick layer of small rocks about the size of beads, possibly to protect the structures from wind and water erosion. To find material for this cladding, the ants venture