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.
Now, as he climbs the hill, Hasiotis points out the clues that tell him these rocks were once such termite nests. He picks up loose hunks of rock lying on the sandstone that have the dribbly look of melted candle wax, and he indicates the tunnels and the pancake-shaped fungus gardens. He traces his finger over broken corridors, indicating the hair-thin walls that the termites made in the sand—material so tough that it is still visible after 155 million years. This stuff is like termite concrete, Hasiotis says.
Because this is only the third time Hasiotis has climbed the hill, he is seeing much of it for the first time. I still cannot believe it. I still cannot freaking believe it, he says as he stares around him. Here’s a place you could come back to for ten years and not see everything. The farther he walks, the more astounding the termite nests become. One is so wide that Hasiotis—a big man with the body of a bouncer—can’t get his arms around it. Another stretches along the slope of the hillside for ten feet, twisting and branching, before diving into the ground.
Hasiotis scrambles up to the mudstone cap and finds a path down to the other, as-yet-unseen side of the hill. Dozens of mounds are strewn here as well. This is so sick! he shouts. A ten-foot-tall hunk of termite nest buttresses a sandstone spire. Another rivals a redwood stump in its girth. These are the biggest fossilized termite mounds ever found—and 60 million years older than the oldest fossil of an actual termite. Yet they are only a fraction of their original size. When they were inhabited by living termites, they would have reached all the way to the surface of the ground, which is marked today by the mudstone cap. Hasiotis glances downhill at the mounds and then up to the summit. I’d guess that some of these were 170 feet long.
Sometimes Hasiotis describes ichnology as a kind of animal archeology, and there could be no better example than this hill outside Gallup. It’s a termite city, he says as he walks among the towers and broken rubble. It’s like we’re in The Planet of the Apes, when Charlton Heston walks through the ruins of New York City. But here it’s this great termite civilization 155 million years old.
Hasiotis never had much doubt about what he should do with his life. I knew from the time I could pick up rocks I wanted to be a geologist, he says. Growing up outside Rochester, New York, he would pick through the trashed stone thrown out by local mineral shops and would prospect the local rock exposures for fossils. I’d get everything that was drizzling out of the creek beds. At the University of Buffalo he majored in geology, and in 1983 he traveled to Utah for a field course. There the history of the planet was hung like a tapestry from the mesa walls, and he swore he’d find a way to work there.
He found the way in graduate school. For his master’s project, he chose to examine the mystery of some 220-million-year-old rocks in southwestern Utah. For 50 years scientists had noticed that some of these rocks were riddled with holes a few inches across. Sometimes the interiors of the holes were filled with sediment that had hardened into long, tube-shaped casts. Because the holes could be found in rock that had formed from sediments and soils along rivers, some researchers concluded that they had been made by lungfish. In Africa today some species of this fish survive droughts by digging chambers in which they fashion mucus-lined cocoons. Given that the evolutionary roots of lungfish reach back 400 million years, it was reasonable to suppose that they might have been sleeping in Utah 220 million years ago.
So I went out to find the lungfish, says Hasiotis. Starting in 1987, he spent years at dozens of sites; for months at a time he would be alone, climbing crumbling rock faces in 110-degree heat to measure the holes and casts. Yet as hard as he looked, he found no lungfish. I didn’t find any fossils in the bottoms of the burrows, didn’t find any kind of teeth, didn’t even find scales. He took a more careful look at burrows he knew for sure had been made by lungfish—ones in which fossils had been found—and ones made by living fish. As a rule, they took the shape of three-foot-long smooth-walled cylinders that terminated in a bulb where the lungfish curled up. But the holes that Hasiotis was looking at in Utah were different: they were as deep as 12 feet, scarred by crisscrossing lines, and often branched into chambers or looped back up to the surface.
Hasiotis began to search the literature for information about other burrowing animals and eventually discovered that there was one—the crayfish—that made exactly the sorts of burrow he was seeing. His adviser was skeptical, though—at the time, the oldest crayfish fossil known was only 135 million years old, 85 million years younger than the burrows in Utah. So Hasiotis spent months studying living crayfish, pouring plaster into their burrows on riverbanks and digging out the casts. They were identical to his fossil burrows, down to the scars on the walls made by the modern crayfish with their tails, claws, and legs.
To Hasiotis these traces were as distinctive as an actual fossil, but still he met with resistance. The problem is that for a lot of people, the minute you find a fossil, that’s the golden spike. You drive that spike and cement it in, and people want it to stay there. As he tried to prove his claim, his master’s project stretched out for a dangerously long time, and it was only on his last field season in 1989 that he found an actual crayfish fossil in the bottom of a burrow. I picked it up and remember saying, ‘Dear God, my prayers are answered.’ Since then Hasiotis has found thousands of fossils of crayfish bodies and has found crayfish burrows dating back 300 million years.
As Hasiotis began trying to interpret crayfish burrows, he learned as much as he could about ichnology. Only in the past 50 years has it become a real science, as paleontologists have studied the abundant traces left by living animals on the seafloor—clams digging into sediment, worms slithering across the surface, sea urchins dragging themselves through the muck. A trace fossil usually cannot tell you which species made it, just as a footprint can’t easily tell you the name of the person who left it, but it is often distinctive to a particular group of animals. Just as important, traces can tell you about the place where they were made. Intertidal zones, where nutrients run deep into the sediment, are dominated by vertical burrows, for example, but the farther out to sea you go, horizontal burrows become more common, until you reach ocean basins, where animals hardly mark the seafloor at all. As conditions change—as oxygen levels drop or rise, for example—animals will shift around on the seafloor, and the traces they leave behind record the changes. Researchers have decoded marine ichnology so well that it’s now a standard tool for oil exploration and reconstructing environmental changes.
But Hasiotis also learned that ichnologists have done very little work with life on land. They always refer to anything on continents as just ‘non-marine,’ he says. Terrestrial ichnology is actually far more complex: the invertebrates in a forest leave marks completely different from those in a beach dune or a bog. Here, Hasiotis decided, was something meaty enough to make his Ph.D. project: he would move beyond crayfish to build a general system for using trace fossils to understand terrestrial environments. Nobody was doing this, because it was too much of a mess. I thought, ‘Okay, at least I won’t have too much competition.’
He began studying at the University of Colorado in 1991, but to pay his way through school he worked part-time at the U.S. Geological Survey in Denver. There he prepared fossils and cast molds for museums and the usgs; when he had free time he went up and down the halls, picking the brains of the usgs scientists. By pure chance two of them, sedimentologist Russell Dubiel and paleontologist Thomas Bown, were the scientists who had suggested that the Utah holes were lungfish burrows. Apparently they didn’t mind that Hasiotis had proved them wrong—they were soon helping him look for trace fossils and embarking on a long-term exploration of the trace-rich but relatively unstudied Petrified Forest National Park in Arizona.
In the southern part of the park lies a series of bare mesas called the Tepees, striped with purples, grays, and reds. Hasiotis scrambles up his favorite, which he calls the Battleship. From time to time he stops in front of 220-million-year-old slabs and boulders that to the untrained eye look as if they are covered by nothing more than scuffs and cracks. To Hasiotis, though, they are seething with vanished life. Here is the five-toed footprint of a phytosaur, a 30-foot-long crocodile-shaped reptile. Over there is a crescent with ripples of stone around its edges—probably made by the snout of a four-foot-long amphibian as it dug into the mud to find clams. Snails once ambled peacefully through the wet sediments that eventually became these boulders, their shells raising distinctive ridges along the troughs of their trails. Caddis fly larvae lodged themselves in a stream bottom in hard cones they built with tiny stones that still speckle the rock. Crayfish dug burrows everywhere. The trail of a big millipede is still visible, its two sets of legs leaving a distinct double trail of pockmarks. Hasiotis spies the trail of a horseshoe crab as it ambled down a creek bottom, stopping occasionally to graze on algae and then taking a hard right turn. Years later this patch of land must have dried out, because on the same slab, overlying the older aquatic animal traces, Hasiotis can see the marks of Triassic beetles that dug through dry dirt.
In this quick climb up the Battleship, Hasiotis has rebuilt an ecosystem without finding a single bone. Beetles, worms, and other small invertebrates fed on algae and organic matter; they were eaten by crayfish, which were eaten by the amphibians, which were eaten by phytosaurs. People have wondered why there are so many big vertebrate fossils found around here, he says. Well, they had an excellent menu to choose from. And by studying traces like these more closely, Hasiotis can tell a lot about the physical environments that supported such food webs. Crayfish, for example, dig burrows from the surface down to the water table, and side tunnels out to nearby streams. As the climate shifts and the water table rises and falls, later generations of crayfish will build new burrows to new depths, and long after the crayfish have vanished, their burrows stay behind to mark the fluctuating climate. Ants, termites, and other invertebrates are just as fussy about where they build their homes.
It’s no longer ‘Oh, this is a crayfish’ and ‘This is an ant,’ says Hasiotis. It’s ‘This is a crayfish, and it tells you the water table was five feet down or less, and so the precipitation must have been very high or the land was very low lying.’ Then if you tie it in with everything else in the traces, you can tell if the climate was highly seasonal, maybe to the point of being monsoonal. It’s a powerful tool. One of Hasiotis’s biggest finds is in the northern end of the park, in a remote tree-trunk-filled valley called the Black Forest. In 1994 two of Hasiotis’s associates happened to notice some peculiar glassy bulbs lodged in a petrified log they were sitting on. When Hasiotis came to investigate, he recognized the bulbs as the fossilized remains of a bees’ nest.
Discovering 220-million-year-old bees was even more disconcerting than finding crayfish of the same age. The evolution of crayfish had always been murky, but bees seemed to have a very clear history. We know that insects evolved from a crustacean-like ancestor at least 420 million years ago and that the most primitive forms were probably much like today’s silverfish. They were almost certainly loners, interacting with other members of their species only to mate. Today, however, the world is home to thousands of insect species that live in societies rivaling our own in complexity. Principal among these are the myriad species of bees, wasps, ants, and termites, many of which live in colonies dominated by queens and divided into castes of soldiers, workers, and drones. Together they build and maintain labyrinthine nests, hunt for food, and defend their colony from invaders. All these insects are grouped within two orders: bees, wasps, and ants, which all probably descend from a single ancestor, are part of the order Hymenoptera; termites, meanwhile, evolved from other, roachlike ancestors and belong to the order Isoptera. Each of these groups evolved its social behavior independently.
Although insects are 420 million years old, the oldest direct fossils of social species date back only 100 million years—to about the time when flowering plants became widespread. Since many hymenopteran insects live off flowering plants—such as honeybees, which feed on pollen—the concurrence of their oldest fossils suggested a real link. Hymenopterans, it was thought, might well have coevolved with flowering plants, and this new food supply allowed them to organize themselves into complex societies. Termites, which eat wood, might have arisen at about the same time because the success of flowering plants created an abundance of lumber that could support their colonies.
Yet here in the Petrified Forest, Hasiotis had recognized a bees’ nest 120 million years older than the oldest bee fossil, testament to a social organization that arose long before the rise of flowering plants. The nest has a main tunnel that starts in a knot of the tree and travels for a few feet inside the trunk. Sets of cells branch off the tunnel in a propeller array, each the size of a black-eyed pea and the shape of a little baseball bat. It’s the same structure used by some living species of sweat bees, a widespread family named for their fondness for perspiration. Recent chemical analysis shows that the cells contain some of the organic compounds found in beeswax.
Since discovering the bees’ nest, Hasiotis and his co-workers have found trace fossils of every major group of social insect far older than the oldest fossil of their bodies. Elsewhere in the Petrified Forest they have found 220-million-year-old nests that bees built not in trees but in soil. Their architecture matches hives made by other sweat bee species, with the cells huddled close in a primitive honeycomb arrangement. The researchers have found termite nests of the same age—not the gigantic towers that appeared 65 million years later north of Gallup, but nests the size and shape of baseballs, with spiraling ramps inside. They are remarkably like the nests of termites today in South American rain forests. Hasiotis has also found 150-million-year-old ant nests in Utah. Ants build chamber-and-corridor structures similar to those of termite nests, but while termites line their walls with their homemade concrete, ants simply dig out spaces and reinforce the walls by hammering against them with their heads.
These ancient insect traces are so abundant that Hasiotis is often staggered by the way they leap out at him. While hiking in the Petrified Forest, he wanted to show off what he suspected was another phytosaur footprint. But as he climbed up a ridge toward it, he stopped for a moment by a sandstone boulder studded with a dozen dark peanut-shaped lumps. These, Hasiotis explained, were 220-million-year-old wasp cocoons, which are 120 million years older than the oldest wasp fossil. He discovered these particular cocoons in 1995, and now he stepped back to take another picture of the boulder. Sometimes you walk up a creek and you hear a humming, he said as he focused, and you wonder if you hit a nest. But it’s all wasps in the ground, digging holes for their eggs. They’re not highly social, but they are gregarious. They like to be with each other. It’s sort of like row houses.
As he advanced the film between shots he glanced at the ground. There was a peanut-shaped rock resting by his foot. That’s one too, he said, picking it up. Here’s another one, he said, bending down again. They must have weathered out of the rock. See, you can even make out the weave of the silk. He pointed to the fine lines on the cocoon. Still crouching, Hasiotis found a third cocoon and then began to scan around his feet. He realized that he was actually standing on a pavement of cocoons, some as small as rice grains, others as big as walnuts. Some incoherent profanities came out of his mouth—for two years he had hiked past this rock, oblivious to his trampling over thousands of the oldest wasp cocoons on Earth. Now he could do little more than say, Oh, Lord, and start stuffing his pockets.
Hasiotis has made a special study of wasp cocoons through history. In his lab at the University of Colorado he has thousands of their fossils, ranging through their tenure on Earth—some are 220 million years old, others 175 million, 75 million, 60 million, 50 million, 40 million, 30 million, and 3 million. He can match them all with cocoons made by living species; they all fall into distinct categories of size, shape, and weave. In fact, most of the invertebrate fossil traces that Hasiotis has studied are identical to forms made today. Evolution does not seem to have been a gradual, continual sculptor in these cases. Rather, social insects hit on a set of behaviors—such as building certain kinds of nests and cocoons—far earlier than paleontologists previously thought and then held on to them without much change for hundreds of millions of years. It’s something you hate to touch, says Hasiotis, because when people talk about evolution, it’s always how things are constantly changing and always getting better. But these monuments to the existence of insects say no—their behavior was almost identical to what it is now.
Social insects, Hasiotis suspects, didn’t need flowering plants to start living the way they do now. If you don’t have flowering plants around, there are still tons of other plants around—lots of gymnosperms and ginkgoes and cycads and other weird sorts of plants that had developed cones and fruiting bodies and flowerlike structures and had saps and resins. It’s just a change of resources through time. For the most part, though, there’s a lot of stasis since there’s no reason to change at all once these organisms have reached a particular threshold of success. Crayfish have made it through every major mass extinction—the same goes for ants and termites, so they’re doing something right.
Driving back to Colorado from the Petrified Forest, Hasiotis passes through eastern Utah. Outside the town of Dinosaur he suddenly hits the brakes. His coffee mug rolls around under the pedals as he throws the car in reverse for 20 yards. My wife gets upset when I want to pull over and flip a carcass, he says. But there’s no one to stop him here. He gets out of the car and walks onto the grass along a barbed-wire fence, sheep running away through the fields beyond. The corpse of a mule deer had caught his eye.
There’s a whole ecosystem here, he says, leaning over the withered body. The mule deer looks as if it had been hit by a car a few weeks before and was then worked over by a coyote. But while the coyote managed to chew off some of the flesh, the bulk of the scavenging has been done by insects. Flies came first, laying eggs that hatched into maggots and fed on the flesh while it was still reasonably fresh. Bees and wasps landed on it to drink the fluids. Other insects visited to lay their own eggs, even as ants crawled over the corpse to hunt their larvae. Digging into the flesh, the insects created tunnels through which bacteria could flow deep into the body and further decompose it.
Ecosystems would suffer without these bugs. A dead animal is a giant canister of organic compounds, and insects are responsible for releasing its contents, returning them ultimately to the soil to nourish plants and animals. Ashes to ashes, Hasiotis says, carbon to carbon.
He picks up part of the deer’s pelvis. Crawling across it is the hairy half-inch larva of a dermestid beetle. In environments where there is little rainfall, dermestid beetles lay their eggs on a corpse’s dried flesh. When the larvae hatch, they use their powerful mandibles to chew their way through the jerky, devouring even the fur, until only bare bones remain. Even then the dermestids aren’t done: within two months they chew their way into the bone itself, sucking the marrow and using the calcium to build their exoskeletons as they mature in their cocoons, from which they emerge as purple-black adult beetles ready to fly away to another corpse. As a result, dermestid beetles are important ichnologically as well as ecologically. Of all the insects that strip down a corpse, dermestids are among the few that leave a trace on the skeleton itself.
Part of Hasiotis’s research in the past few years has been at Dinosaur National Monument, which is his destination on this ride. After another hour of driving he reaches the monument, on the Utah-Colorado border, where he is met by the monument paleontologist, Dan Chure. Together they walk into a huge hangarlike building known as the Quarry. The Quarry encloses the face of a hillside filled with dinosaur fossils. Dozens of intertwined skeletons gather in a giant frieze, a jumble of spiked stegosaurs, bipedal predators, and long-necked sauropods. Visitors can touch the bones near the bottom of the rock face, but Hasiotis and Chure want to show off some fossils higher up. They climb 20 feet up the rock face, using femurs, shoulders, and spines as their toeholds, until they reach a five-foot-long humerus. Check it out, Hasiotis says proudly.
The bone is covered with borings just like the ones on the mule deer Hasiotis had inspected along the highway. There are so many, in fact, that it looks as if someone had sprayed the fossil with bird shot. Other paleontologists noticed these holes over the years, but they dismissed them as the effects of any number of trivial events—as the imprint of excavators’ picks, perhaps, or etchings made by droplets of acid that formed in the sediment. But Hasiotis has shown that they were wrong. As he points out, the holes are either smooth circles or ellipses, which are imprints a beetle can make but not acid or a pick. And when Hasiotis put a riddled dinosaur toe under a scanning-electron microscope, he could even see scrape marks on the sides of the borings that perfectly match the ones made by the jaws of living dermestid beetles.
Once again Hasiotis catapulted an insect’s evolutionary history back into deep time: the oldest dermestid beetle fossil is 30 million years old; these borings date back 150 million years. The traces of the beetles also tell a new story of the afterlife of these dinosaurs. Because the dinosaurs at the monument were found jumbled in river sediment, paleontologists have assumed that they were killed by a flood and then piled up in a carcass jam. But if that had been their fate, there would have been no way for the beetles to make their borings. They have a very narrow range of environments. It has to be dry, hot, and arid. The carcasses were not in water, says Hasiotis.
The dinosaurs must have been suffering through a harsh drought, dropping off here and there. Carnivorous dinosaurs may have scavenged some of their meat, but they left most of it on the bone for the flies, ants, bees, and wasps. Once the flesh had dried out, the dermestid beetles arrived. They chewed the bones clean and then bored into them. During this entire time there must have been little rain—otherwise the beetles wouldn’t have been able to mark the dinosaur bones so much. Hasiotis has even found borings that overlap older ones, suggesting that two generations of dermestids managed to be born on the bones. All told, he estimates, it took four to nine months for a 30-ton dinosaur to be stripped bare. Only after that time did a flood gather up all the naked skeletons, still held together by ligaments, and sweep them into a channel.
As Hasiotis has examined the bones at Dinosaur National Monument, he has marked the ones with beetle borings with a piece of green tape. Strips of green now speckle the bone bed, marking 40 percent of the bones in the Quarry. It’s a staggering sight: we think so often of the dinosaurs as the top of the food chain of their day, but trace fossils show that these giants ended up as a beetle’s breakfast.
Hasiotis now decides to hike into the nearby foothills of Split Mountain. It is new territory for him, and he wants to do some reconnaissance. He scrambles over the tilted beds backward through time, through the Cretaceous Period, the Jurassic, the Triassic, the Permian. He picks up a scrap of anonymous fossilized bone with a beetle boring; he finds what may be half a dinosaur egg. He scares up a writhing giant centipede, and later, on a slab of marine Jurassic rock, he points out the tracks of an extinct crustacean that lived here when this was part of the ocean. He leafs through Triassic layers of shale as thin as pages from a book and finds the trails of worms. Moving into sandstone canyons, he heads back into the Permian Period, perhaps 280 million years ago, when this area was a desert bigger than the Sahara. Here the rock is almost bare of tracings, although a few beetles had explored the sand looking for food.
He is climbing a canyon toward the top of the ridge when he notices strange little rocks sitting in the sand. They look like Rice Krispies Treats, small beige nodules of rock cemented together. Hey, have you seen this before? Hasiotis asks, a little uncertain, as he picks one up. They look like a crude version of the fossilized termite mounds back in New Mexico. Hasiotis turns it over, frowning. It may be a sign of social organization, he says, but he doesn’t elaborate. He walks silently farther up the slope, picking up more of the pieces, until he reaches the source: long blocks of the stuff only a few inches thick.
Ichnology is always a game of elimination. So many things can create the same patterns in rocks. A shrimp burrow, the fossilized droppings of a giant earthworm, and the cast of a tree root covered with nodules of minerals look almost identical. An ichnologist tells them apart by clues—the pattern of wrinkling, the kind of rock they are made of, how the pieces are cemented together. It could be that the Rice Krispies rock is nothing but nodules, but he doubts it. The bits of rock seem to be attached to each other like simple chambers. If they had formed on their own, the connections would have been more random. Hasiotis has seen these kinds of rocks before, and he wonders if they are simple termite nests. If they are, they will push back social insect roots even further into the past.
But here on Split Mountain the prospect doesn’t make Hasiotis particularly happy. He drops the rock back on the ground. I almost wish we hadn’t found them, he says. It’s going to turn in my head and nag me. He begins to climb the slope again, scanning the ground and the canyon walls, with no idea what he will find before he reaches the top.
