Just north of the town of Gallup, New Mexico, is a hill of olive-colored sandstone. One late spring afternoon paleontologist Stephen Hasiotis walks up its grassy apron, crosses over onto bare rock, and loses his composure. Oh man, oh man, he mutters. Look at all this.
He kneels by a stub of white rock—one of many—that just barely pushes through the darker stone around it. Its surface is not the smooth, featureless face you’d expect from an exposure to wind and rain; rather it shows a mass of fine tangles, of tubes branching into more tubes or tying themselves off in blobs. The rock looks as if someone had patiently modeled it before it hardened, some 155 million years ago. And in fact, according to Hasiotis, someone did. Termites, he says. This was all done by termites.
Originally this hill was a sand dune in a desert; when the climate turned damper, a stabilizing soil buried the dune and eventually formed a hard brown mudstone that now sits like a cap on top of the sandstone hill. Geologists who have visited the hill over the years assumed that the strange patches of white rock on the slopes were formed by lightning, which, in striking the sand, fused the grains into columns of a mineral known as fulgurite. But in 1995 a group of geologists noted the intricate texture of these white rocks—which fulgurite doesn’t have—and decided they needed to call in Hasiotis.
Hasiotis is a rare sort of paleontologist: he searches the land for evidence of animals that are unlikely to have left behind any fossilized remains. He looks for the leavings of invertebrates—such as insects, spiders, crustaceans, and worms—which, from a fossil hunter’s perspective, are just made of the wrong stuff. Some are soft and pulpy; others have exoskeletons made of protein known as chitin. Chitin’s a good source of nutrition for other insects and soil critters, so the bodies break down relatively fast, Hasiotis points out. As a result, the fossil record gives paleontologists a skewed vision of the history of life on land. We know that today invertebrates are staggeringly diverse, with perhaps 5 million species of insects alone (mammals number only 4,000), and that they are essential cogs in the machinery of ecosystems: they pollinate plants, break down organic matter, help create soils, and alter the composition of the atmosphere. Presumably, terrestrial invertebrates were just as important tens or hundreds of millions of years ago, but without fossils their history is difficult to reconstruct. Still, it’s not impossible: while invertebrates may not leave bones behind, they do leave permanent marks on the land in the form of trails, tunnels, nests, burrows, and other cryptic inscriptions. Recognizing these traces is a craft that only a few scientists have mastered. They are known as ichnologists—from the Greek ichnos, for track. Hasiotis is, in a sense, a paleontological tracker.
When he first came to this hill in 1995, he could see right away that the white rocks bore the signs of ancient termite activity. In semiarid regions colonies of termites routinely set up nests around the roots of a tree or shrub. They dig out tunnels and chambers around the plant and use chewed-up wood and their own droppings to line the walls. The mound becomes a kind of insect castle, with chambers dedicated to specific purposes: some are filled with eggs, others with waste or corpses or the fungus the termites harvest for food. As the colony’s population increases to a million or beyond, workers dig out more and more rooms, until eventually they build a tower up to 30 feet tall; underground, their networks may stretch more than 100 feet.
