What We Leave Behind

Fossils tell only one small part of the story

Jurassic — ended 145 million years ago

March in the Pioneer Valley is mud season. On the precipice of spring, the ground thaws into a gooey, soupy mess that engulfs truck tires, commandeers hiking trails, and swallows rubber boots with all the hunger of an enduring winter. Into this fifth season, dormant trees and torpid snakes awaken. Wild turkeys take advantage of the fresh landscape, unveiled of snow, to scratch and scrounge for young grasses and buds. Robins fill the branches once more, and those of us who are wanderers stretch our legs along the meandering paths of New England, sinking and sticking with each step. Mud season is the bridge to new life and warmer weather, and even as our tires spin fruitlessly on dirt roads or our boots are slurped into the quicksand of melting snow, we venture forth. Through it all, our tracks accumulate.


In the basement of the Beneski Museum of Natural History at Amherst College, there is a room full of footprints. The walls are lined with partially illuminated slabs of sandstone and shale (compacted silt, sand, and clay). Below a display counter, carefully arranged drawers catalog hundreds of unique specimens. The rocks are shiny and smooth or dull and rough; some feature the pockmarks of ancient raindrops, others the ripple marks of long-vanished lakes. But across each geological surface run the ancient tracks of dinosaurs.

In 1835, Edward Hitchcock—a former pastor, professor of natural history, and future president of Amherst College—followed a colleague’s urging to investigate mysterious, footprint-like markings that had been discovered on quarried flagstone in Massachusetts’ Pioneer Valley. Once a primordial swampland, by Hitchcock’s lifetime this portion of the Connecticut River Valley was nothing short of bucolic. The imprints Hitchcock found there were sharp, three-toed, and distinctly birdlike. Intrigued, he took to hunting down and categorizing samples throughout the valley until he had amassed what remains one of the most extensive collections of Jurassic tracks in the world. 

The largest tracks were a foot and a half in length; analyzing his samples, Hitchcock quickly developed the tenuous theory that they belonged to giant prehistoric birds, introduced by the creator ages before. By the time the word “dinosaur” was coined in 1842, Hitchcock had already created an elaborate taxonomy of ancient avian species, aided by the meticulous illustrations of his wife, Orra. Even as the scientific world around him was upended by early evolutionary thought and ultimately Darwin’s theory of natural selection, Hitchcock clung to his own familiar interpretation of the New England trackways and the “sandstone birds” until his death in 1864.


Tracking down Dr. Stephen Gatesy in the back room of the prep lab at the far west end of the first-floor hallway in the Beneski Museum is no easy feat. Gatesy doesn’t work for Amherst College; a few times each month, he makes the trek from his research lab at Brown University to Western Massachusetts to study the Hitchcock Ichnology Collection in person. Although we’d arranged to meet at the museum, staff were unfamiliar with his name, and no one could point me toward any labs in the building. Eventually, a determined janitor took up my cause and marched me to every possible location matching my vaguely noted meeting place. After a thorough tour of the geology department, we came face-to-face with a tall, gray-haired man who smiled knowingly and thanked my guide before introducing himself.

Tracks and traces tell us the short stories of animal lives. We are eager to read them.

Steve, as he quickly asked me to call him, is the type of congenial scientist who will readily spend two hours of his day talking you patiently through his research and the subtler details of a sparsely labeled paleontology exhibit. His passion for tracks arose at an early age, sparked by the plaster casting materials his father would bring home from his dentistry job in suburban New Jersey. 

“I still have a deer track that we cast fifty years ago,” Steve told me. “My parents let me try taxidermy on roadkill squirrels and birds that had hit the window. I was given space in an old freezer for specimens, and I often cleaned skulls and reassembled skeletons.” Today, he channels his passion for the natural world into his work in ichnology, the study of paleontological tracks and traces. Although Steve considers himself primarily a biologist, his research centers on the evolution of locomotion, studied through fossils; by analyzing ancient tracks alongside the modern motor patterns of living birds, he is starting to understand how dinosaurs once walked Earth, especially in the disorienting substrate of collapsing mud.

Mud, after all, fossilizes motion. Hitchcock took each footprint at face value, classifying a new species for each unique number, girth, and placement of toes. When Steve looks at a track, however, he sees a fraction of a complex timeline. Indistinct footprints that other paleontologists might view as flawed or distorted impart to Steve a wealth of information about the movement of a dinosaur. Depending on the composition of the substrate the footprint was formed in and the sedimentary layer from which it was harvested, a track might reveal any of a hundred different instants in an individual step. Without access to live dinosaurs to observe, Steve has turned to some of their closest living relatives for further research: birds. Specifically, the helmeted guineafowl—African poultry with a horn-like bone protruding from its bald head. They are strikingly reptilian in appearance and, as Steve says, “loud, obnoxious, and full of personality.” But they are easy enough to procure and train, so for Steve’s research purposes, they are exceptional model dinosaurs.

Guinea fowl also like to walk, so Steve’s team designs runways for them, collecting X-rays and electromyograms of their bones and muscles as they move. The birds are placed in mediums ranging from sticky, yogurty mixtures to millions of poppy seeds, and motion analysis of the resulting video is used to inform the team’s virtual simulations. Particle by particle, layer by layer, they reproduce the motion of the mud and sand as it moves out of the way of the bird’s foot, collapses around it, and lifts back up. Working backward, they begin to understand how a fossil was formed. 

These are the features Steve sees that Hitchcock couldn’t: the thin, wiggly toe marks where the ground has flowed and squelched inward, the bulbous heel where the foot moves upward through the substrate, and the signature protrusion of mud that lifts off the surface as the dinosaur moves toward its next step. Steve’s goal is to understand the evolution of motion, but more and more his work has become about correcting the misperceptions of his scientific predecessors. Many of the tracks cataloged at Beneski and other museums have been shaped irreparably by the biases and little mistakes of those who discovered them. Protrusions have been chiseled off, toe markings have been cleared out, and the layers of history once housed in the fossil have been disturbed, even erased. Surface-level tracks, clear and crisp, are boring to Steve. It’s the weird ones that tell a story. “It’s good to explain it to somebody, because I realize how far we’ve come,” Steve says, near the end of our interview, pointing out a forward exit hole on a track that he and his team had glossed over for years. “It’s that evolving perspective that I really like. Your eyes are different every time.”

Cretaceousended 66 million years ago

When you enter the Beneski Museum, you are greeted by a towering mastodon. Monstrous tusks shoot out from above the skeleton’s mouth, and it is hard not to imagine prehistoric beasts roaming the manicured lawns of the Amherst College campus. Below the mastodon, a small sign graphs extinction rates over the course of the past 525 million years. “Organisms disappear continuously at a low or background rate, but during each of the five largest mass extinctions, more than 35% of the extant marine genera became extinct. After Sepkowski, J.J. (1998).” It is an impressive display, but my eyes stick on the name at the end of the sign: it is still spelled wrong, as it has been since I first visited the museum in the fall of 2007 on a college tour with my mom. My mom—Carol Sepkoski—is proud of her brother Jack’s work; if she sees his name in a museum, she makes it a point to tell the curator who she is. She certainly made it a point to alert the docent about that invasive “w.”

I think of Uncle Jack often. More than once, I have googled “Jack Sepkoski obituary” on a whim. Perhaps I’m looking for an uncle I had only begun to know by age eight, and sometimes I think I will find more of him in the pages of Paleobiology or Nature. The obituaries talk about his kindness, his intelligence, his humor, his passion for punk rock, and his sudden heart failure. But mostly they describe in detail his contributions to the field of paleontology. In the New York Times, paleontologist Dr. Douglas Erwin noted, “Most of us aren’t going to be remembered in a hundred years’ time, but Jack will, because he changed the way we think about the fossil record and the history of life.”

When my mom talks about her brother, it is about his younger days. In one story, Jack and his friend build a rocket in the garage. Upon launch, the rocket goes awry and blasts into the friend’s armpit, leaving a crater that would last a lifetime. Nature does not mention these contributions to aerospace engineering. In another, he is in the middle of the final exam of his Ph.D. at Harvard, when someone bursts into Memorial Hall to tell him that his wife is in labor. The school wants to send a proctor to the hospital with him to finish his exam, but Uncle Jack ignores this and races out of the hall to hail a cab. His academic career, and his son, David, turned out all right.

Twenty years after his passing, I’ve found myself following my uncle’s footsteps to Harvard. I have taken his doctoral thesis out of the Cabot Science Library under the guise of doing research for a science writing course, but I am really just hoping to hold a trace of his story for a moment. At 563 pages, the tome elicits stares as I lug all seven pounds of it through the streets of Cambridge. I watch at least one passerby eye the spine in my arms, attempting to make out the title of the volume. If he did catch it, it is unlikely he made much sense of the words: “Dresbachian (Upper Cambrian) Stratigraphy in Montana, Wyoming, and South Dakota.” At home, I turn carefully to the first page, listening to the crinkle of a cover that has likely not been opened in years. A piece of paper pasted inside marks the volume as the property of Harvard University. “This Thesis by . . . Joseph John Sepkowski.” I have to read it again to be sure, amused by the small mistakes that seem to trail his memory. The rest of the book is written in barely decipherable geological code, and the best I can do is appreciate the pictures of western mountain contours and rocky outcroppings and savor the acknowledgments.

What I remember is Jack’s vast collection of bizarre scientific toys, stuffed animals, and comic books. His house was covered in dinosaurs and dog hair from Ronnie the rotund golden retriever mutt. Sometimes he smelled of smoke. He and my aunt, Christine, a fellow paleontologist, lived by a seashell-laden cove in Rhode Island. Often, he would quiz me and my brother on unusual animal facts or tell us the tale of some prehistoric beast. What history remembers, however, is the mark Jack left behind on the scientific field: his classification of the “Big Five” extinction events. By analyzing thousands upon thousands of marine fossils, Jack and his colleagues formulated a hypothesis about the periodicity of mass extinction. According to their research, major extinction events have occurred regularly throughout time, at intervals nearly 26 million years apart—possibly, they postulated, in relation to galactic forces. During each of the five mass extinctions, biodiversity plummeted as over 70 percent of all species were lost. The seminal graph that resulted from their research is still tattooed across the paleontological landscape and my cousin David’s arm.

Holocene present day

Today, biodiversity is once again in rapid decline worldwide. According to an extensive assessment published by the United Nations in 2019, over the past century, extinction rates have increased sharply, and the abundance of land-based native species has dropped by 20 percent. In the last fifty years alone, global vertebrate populations have shrunk by an average of 60 percent. The pattern is eerily similar to the previous Big Five extinctions. This time, there is no asteroid to blame; human interference with Earth’s climate and natural resources is wreaking havoc on the species around us. Scientists increasingly believe we are living through a sixth extinction. Amidst such loss, rare conservation success stories provide some critical hope. The American beaver, hunted to the edge of extinction in the early 1800s, has managed to claw its way back from the brink.

Beavers are the most prolific natural architects of the American landscape; their dams, lodges, and forest management have shaped the ecosystems we see today. Using their ever-replenishing incisors, beavers prune forest growth, opening up the canopies as they fell and debark tree trunks for the nutritious cambium tissue layer inside. Branches, logs, rocks, and plant matter are compiled into mud-bound dams, which terrace streams and ponds to deepen the waterways and flood shallower banks. Drowned tree roots die off, further opening the undergrowth and providing homes for insects and birds in their rotting wood. Captive silt and decaying plant material make for rich soil and improved water quality. As beavers chew their way around the wetlands, biodiversity—a necessity for a resilient planet—flourishes.


On an unseasonably warm valley morning in April, I drive two hours west from Cambridge to meet up with veteran tracker John McCarter and a dozen other amateur naturalists in a small pull-off by the East Branch Swift River in central Massachusetts. I’d met John once before, on a bobcat-tracking expedition several Februaries back for professional development as an outdoor educator. So when, as I dug my way through Steve Gatesy’s research on Jurassic ichnites, my thoughts kept returning to the ephemeral nature of bobcat tracks on snow, I’d called Walnut Hill Tracking and signed up for the next class—beaver tracking

John has the scruffy look you might expect from someone who spends much of his time exploring the woods as a professional tracker. He wears a camo hoodie that matches the seat covers of his steel-gray Toyota Tundra and the outfits of three of the other men on the trip. As we gear up and gather in the parking lot, John snaps a pair of gaiters over the tops of his boots and expounds on his strategies for tick evasion (DEET, permethrin-treated coveralls, gaiters). Later, we hear about the case of Lyme disease that kept him out of work for a year and a half, and I begin to understand the unusual gravity of his tick preamble.

The rest of the crew have traveled from across New England for John’s workshop on wetland exploration. They are farmers and biologists and retired nature lovers looking for an afternoon adventure in the Quabbin Valley. We don’t look much alike, one donning head-to-toe hunting gear, another in a cerulean cardigan and neckerchief. But something in the wild energy of the landscape has drawn us all together. Someone excitedly recalls another expedition where they came across a bobcat’s cache, the ribs of an unlucky beaver splayed open in the snow. As John says, tracks and traces tell us the short stories of animal lives. We are eager to read them.

What other stories do we lose through time when all we leave behind submits to the yielding and wild earth?

As we descend into the forest, there is a salient temperature drop. Patches of ice still cling to the shady ground. John points out the recent scars on the base of a tree from the roughly thawing winter, and the forest floor, cleared of debris by an influx of rainwater the previous week. Minutes into the trek, John stops abruptly and begins backtracking through the woods without explanation, scanning along the river as we follow in close pursuit. He stops again, gesturing across the water. “There’s a trail worn into the slope over there, and it goes up at a 45-degree angle to the top of the ridge.” I peer through the low branches and can just make out a darker length of ground along the opposite bank. “That’s an otter trail. It’s exceptional how pronounced it is.”

We continue along the river. The high canopy opens up and the lower trees and brambles thicken. I adopt a defensive position—head low, hands up—to keep the branches out of my eyes as we bushwhack single-file toward Pottapaug Pond. John stops us again at an unassuming glob of mud and roots along the water’s edge. It’s a scent mound, he tells us. In addition to their coveted fur, beavers were hunted for castoreum, an oily secretion said to smell like vanilla and appropriated by humans for use in perfumes, desserts, and cigarettes. One by one, like a religious rite, we approach the mound, kneeling to get a good whiff of beaver scent markings. I carefully position my nose as close to the mud as possible, without touching it; I smell motor oil (which John says is the scent of male beavers’ anal secretions), but the sweet smell of castoreum eludes me. Along a thin trail leading to the scent mound, we had found otter scat, packed with fish scales and crushed bone. “I feel very strongly that there is communication between the otters and beavers,” John told us. The trails and clearings we find throughout the day seem to affirm this; we see traces of otter, beaver, coyote, fox, bobcat, squirrel, and raccoon. “Who walks here? Pretty much everybody.” 

By the afternoon, I have taken to following John closely, since he is prone to setting off without warning and rarely slows for the rest of the group. I’m up front with a handful of others when we arrive at the first bank lodge, a cavern dug underwater into the rise of the shore. Quietly, he motions for us to step out onto the earthen roof. “Watch the water.” A jet of bubbles shoots out from the edge of the bank. One. Two. Three. With the last bubbles, I catch a glimpse of a dark beaver body torpedoing through the water.

John is not concerned with seeing the animal, but with making sense of what is left behind. Through deep observation, he teaches amateur trackers to understand animal movement by analyzing tracks and traces. “Ultimately,” John wrote me after the trip, “the tracker sees that he is not an outside observer sneaking a peek at a mysterious realm, but another mammal inexorably woven into the tapestry. Then the knowledge that what we do to the Earth, we do to ourselves is inescapable. That seems pretty important to me.”


In the spring of 1997, John McCarter crossed a beaver dam over Spriggy Brook and found himself on someone else’s trail. He treaded swiftly, softly, onto a lobe of land protruding into the Quabbin Reservoir, where he spotted a distinct mound of dirt and leaf litter. Kneeling close beside it, he noticed the volcanic opening on top, and carefully uncovered a large pile of scat. John had spent years studying bobcat traces throughout the New England forests. He prodded the mound with a stick, analyzing it. He pulled out his tape measure, noting the length and diameter. This scat was segmented, tapered, dark, and dense: all telltale signs of feline excrement. The only puzzle was the size. Logically, John knew there were no mountain lions in central Massachusetts; the eastern mountain lion was widely believed to be extinct. Still, the mound was bigger than that of any bobcat he had ever seen. Perplexed, he continued on, following the cat’s trail of scat to a downed hemlock sapling. The tree had uprooted and blown over, and beneath its bright young needles was what John immediately recognized as a cache. Felines, like many carnivores, hoard their excess food in covered stores to return to for a later meal. Bobcats will pull leaves and soil over the remains, scratching at the ground until they are satisfied with their handiwork and have sufficiently marked their property. But the swipe marks around this cache site were expansive; the reach, made with the cat’s front feet, was far too long for a bobcat, and the claw marks were wide-set, at least an inch apart from one another. However unlikely the circumstances, John was sure now that he was following a mountain lion. 

The cache beyond Spriggy Brook was filled with dense russet fur, partially devoured innards, and two halves of a skull, wrenched apart with impressive force—the unfortunate fate of an unsuspecting beaver. As John bagged a sample of the scat and began to re-cover the cache, he felt a sudden tingling on the back of his neck. It was a feeling he’d felt only once before, an instant before getting attacked from behind, in his past life as a social worker. He sprang up and looked around at the empty woods. Somewhere in the shadowy undergrowth, he was sure, the mountain lion was choosing whether to lunge.

John returned to the site of the mountain lion cache for photos the following week. The beaver remains had been revisited, and on top of the mound, where John’s hand had been just days before, sat a fresh mound of scat. The DNA testing took more than a year to complete. Newspapers that had interviewed him about the incident left out details and mixed up timelines. A biologist for the state accused John of staging the site by bringing in a cooler of captive cougar scat. Eventually, John badgered the Wildlife Conservation Society into writing up an official report verifying the scat as belonging to a mountain lion. Then, in 2011, another mountain lion was tracked through the Quabbin Valley woods and down to Connecticut, where it died on the highway after being hit by a car. John believes still other sightings in New England have been covered up entirely to avoid increased conservation regulations and a potentially devastating impact on tourism. Or perhaps, like Hitchcock to his sandstone birds, many people are more inclined to cling to their own certainties than to accept the implications of new evidence. Whatever the population range of mountain lions may be, it is clear that there is much more to the story than the few discovered traces.


John shows us the width of a beaver’s individual incisor on the markings of a felled tree (“beavers are not that great at directional felling”). He teaches us to recognize the leaf-matter patterns of an otter roll. He helps us distinguish bobcat scat from fox (“a fox couldn’t defecate like that in its wildest dreams”). Somehow, we follow beaver trails and dams for six hours without seeing a single footprint, the sloppy substrate of beaver scent mounds holding only the indistinct swipes of passing paws. The valley has eased out of mud season and into spring proper. 

As we make our way back to the trailhead and gather up our belongings, John stops me to apologize. “Next time we’ll find tracks.” I grin wearily and shake his hand, calculating eras of geological time as I tramp across the parking lot to my car, leaving my own wake of muddy footprints. I wonder whose traces will be immortalized in the indurated substrate of history before the next mud season rolls around. In this sixth extinction, perhaps these will become the trackways that tell our stories to distant eras—each intricate narrative of beaver, mountain lion, paleontologist, or writer reduced to a series of imperfect footprints. What other stories do we lose through time when all we leave behind submits to the yielding and wild earth?

About the Author

Molly Sepkoski St. Clair

Molly Sepkoski St. Clair is a writer and educator living in Somerville, Massachusetts. She studied psychology and creative writing at Vassar College and holds a master’s degree from the Harvard Graduate School of Education.

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