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A Trove of Ice Age Fossils Buried in a Wyoming Cave Is Rewriting Our Understanding of Prehistoric Animals
At a site known as Natural Trap Cave, a team of scientists are rappelling down to uncover the secrets of what the Earth was like during the Pleistocene
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Julie Meachen, an expert in Ice Age megafauna, skirts a dirt ledge called “the Saddle” deep inside a remarkable hollow in northern Wyoming known as Natural Trap Cave. She stops to observe Megan Hormell, a graduate student at Des Moines University and one of several volunteers who have rappelled into the cave on a nylon line dangling from the entrance 80 feet overhead. Sunlight streams through a jagged skylight to illuminate a bell-shaped chamber 140 feet in diameter. I sit to one side of the Saddle as remains of Ice Age mammals slowly emerge into the light. Like the scientists scattered over the subterranean hillside, I wear warm clothing, boots and a caving helmet. My cumbersome climbing gear rests, along with everyone else’s, on a tarp.
Aboveground, on the western flank of the Bighorn Mountains, less than two miles from the Montana border, it’s a scorching July day. Down here, the temperature hovers at a near-constant 42 degrees Fahrenheit, shifting only a few degrees with seasonal heat waves or blizzards. Bundled in thick coveralls and fleece jackets, the researchers wield trowels with gloved hands. It is the next-to-last-day of a two-week expedition. The 22 members on this 2024 dig include seasoned scientists and students, undergraduate to postdoctoral, along with expert cave explorers who ensure the safety and smooth operation of delicate work at the bottom of a deadly pit. The cave’s name, Natural Trap, is literal: Over the past 150,000 years, countless animals have fallen into it.
When approached from the relatively flat plain to the east, the pit appears suddenly beneath a small rise. You can see how tens of thousands of years ago, carnivores and the herbivores they chased could miss the black shadow until it was too late.
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This article is a selection from the June 2025 issue of Smithsonian magazine
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Perhaps because they were running, or because of their relative weights and body shapes, certain animals fell more horizontally, landing on the cave’s floor, well beyond the 75-foot perimeter of the entrance, and dying on impact. As the debris pile grew over millennia, some animals that fell rolled or bounced, creating the broad field of bones that fills the chamber today. The bones of at least one Miracinonyx, an extinct cat commonly known as the American cheetah, stayed together: Its intact skeleton was observed on a ledge deep inside the cave in 2023.
Each soil layer has a name related to its color and composition. Hormell works in the Cobbly Gray, lower than Blocky Brown and higher than Rocky Cheetah. Others work farther downslope, each focused on small finds. For example, Andrew Grass, a geoscientist digging 30 feet away, has compared the teeth of extinct species of American horses from different periods within the pit, using them to build a temporal road map of changing tooth morphologies. He has just extracted a new equid molar to add to the mix.
Setting aside her pick, Hormell cleans an exposed bone with the sort of small paintbrush normally used on interior trim. A foot or so to her right, a bony plate the size of an iPad angles upward from the dirt: a scapula from an animal closely related to present-day bighorn sheep. A male, if it matches the skull fragment that Tony Hotchner, another graduate student, found just yesterday. Possibly 23,000 years old, based on the dating of previous finds within the Cobbly Gray.
“This is another vert,” Hormell tells Meachen, using paleontological shorthand for “vertebra.”
Meachen nods. “After you get the vert out, I would take this little wall down.” She indicates a jumble of packed rocks and dirt to the left of the scapula.
“Yeah, the wall has to go, but there’s ribs in the wall,” Hormell says.
“OK. Do your best to start taking it down, take that little nubbin out, and then worry about the scapula,” Meachen says. “That’s a beautiful scapula, and I’d love to get it out.”
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These workers are not hunting future museum displays. Instead, by documenting subtle changes within animal species over time, they seek clues to extreme climate changes of the past. And Natural Trap Cave provides an astoundingly well-suited resource for the purpose, holding a largely unbroken record of mammal lineages going back tens of thousands of years. “It’s an amazing place,” says Kirk Johnson, a paleontologist and the director of the Smithsonian’s National Museum of Natural History, who visited the cave himself 25 years ago. “It is a single site, and the time range over which it was open is long, so it has the potential to show change over time—as long as the individual skeletons are dated.” That’s why Meachen and her team are focusing on the past 39,000 years, a time starting in the Late Pleistocene that coincides with the most carefully dated bone-baring layers inside the cave, deposits that were laid down during a continuous cycle of glaciers advancing and retreating. “We’re going to see if we can accurately model the dry and wet cycles at the site” during that period, Meachen says. This sort of climate analysis—based on slow changes in animal teeth, as well as their mass and other attributes at a single site—has never been published in the scientific literature.
Grass’ examination of horse teeth, for example, helps us better understand animals that have been regularly found in the pit, including two similar genera of native American horses. Grass found that the enamel patterns on what is called the occlusal, or biting, surface of the tooth are different for each species. Researchers Lillian Spencer and Eric Scott also studied the ways horse teeth and those of bighorn sheep were worn down differently from different foods, and concluded that their diets were much like those of the wildebeests and zebras of the modern African Serengeti. There, the wildebeests come through first and eat new, softer vegetation, while zebras come later, eating the tougher, older grass. The rate of tooth wear on horses and sheep over time can point toward wetter or drier climates, changes that can be confirmed by plant DNA preserved near animal remains. Another project by Thomas Minckley, a geophysicist at the University of Wyoming, not only tracked shifting vegetation, by extracting carbon isotopes and pollen from native plants (sagebrush, junipers, daisies, grasses and members of the genus Amaranthus) from specific layers in the mound; it also created a new tool to date soil and fossils within a particular section, simultaneously showing whether the climate was wetter or drier when each layer was laid down. A combination of pollen counts and carbonate values suggests times of relatively wet summers within the heavy glaciation period starting as early as 150,000 years ago. This generally forced the same plants to survive amid steppe-like, cold and dry conditions. A dozen of these and other groundbreaking studies were published in a special issue of the journal Quaternary International in 2023, but in an essay introducing the issue, Meachen and a colleague wrote that the volume was “only the beginning,” and that through many future studies and analyses, Natural Trap Cave “will continue to change our view of the Pleistocene world.”
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Beyond dating and comparing finds, Meachen and her colleagues are connecting the remains of individual species inside the cave to a long-lasting migration pathway up on the surface—a pathway that, it’s now clear, helped shape the world population of mammals today. “We know that we’re seeing migration of different species tied to changing climates,” she says. “But can we better understand how these climates affected them? Do we see patterns? What kind of patterns might they indicate for modern animals in a drying climate?”
Answering these questions requires a diverse set of tools that were not yet invented when the site was first researched in the 1960s. Now, by casting a wide net across disciplines, Meachen and her colleagues are bringing assorted bits of evidence into sharp focus.
Earth’s past 2.5 billion years included five known glaciation periods, when global temperatures plummeted and vast ice sheets advanced across the planet, starting long before Natural Trap Cave began to form. The Last Glacial Maximum ran from about 19,000 years ago to 29,000 years ago. When the Last Glacial Maximum began, an ice sheet covered most of the continent, reaching from the Arctic to present-
day Baffin Bay in the east and southern Montana in the west. A narrower ice sheet ran down the West Coast. Sometimes this sheet expanded eastward to merge with the other, but in warmer times the glaciers parted, opening the Ice-Free Corridor.
At least 15 miles wide—and sometimes much wider—at its American end, this verdant freeway for migrating mammals lasted thousands of years at a stretch. Before the Last Glacial Maximum, large animals traveled in both directions. Mammoths and mastodons came from Asia to North America; the first horses and camels, smaller versions of those living today, left their native North America for Asia, Africa and Europe. As the animals migrated between continents, they interbred. “We are finding, through ancient DNA, that many of the species at Natural Trap Cave are hybrids between North American and Eurasian populations,” Meachen explains.
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When a colder climate expanded the glaciers, closing the Ice-Free Corridor, descendants were stuck on separate ends for about 15,000 years before it reopened, allowing cross-migration to resume. Then the glaciers melted further, raising sea levels that flooded the Bering Strait—blocking intercontinental migration once again. Both when the corridor was open and when it was closed, large animals moved by the thousands directly through the area around Natural Trap.
Wyoming cave explorers first rappelled into Natural Trap Cave in the late 1960s. In 1971, Lawrence Loendorf, an archaeologist who had heard that there were bones in the cave, collected a few mammal fossils and a red jasper knife at the base of the pit. Other than the knife and a wooden artifact found in a pack rat’s nest, thought to be the shaft of an atlatl dart-throwing tool, no human artifacts or remains were ever found there. Shortly after this discovery, the Bureau of Land Management, which controls the site, built a metal grate across the pit entrance to protect animals and people from falling in, as well as to prevent looting. A lockable gate at the grate’s center allowed permitted scientific access.
Loendorf, returning in 1972 with paleontologist Carol Jo Rushin, excavated a 10-by-10-foot pit inside the cave, where they found a surprising wealth of Pleistocene species. The paleontologist Larry Martin, who led multiple subsequent excavations in the ’70s and early ’80s, declared it would take “at least 50 years” to uncover the secrets of Natural Trap. But in 1985, fearful that continued digging might harm the fossil trove, the Bureau of Land Management closed the cave until new paleontological methods and tools became available. It remained closed for three decades—until Meachen came to the site in 2014.
“As a child, I always loved mammals,” Meachen recalls. “I was not a dinosaur kid at all. I liked the taxidermied animals and the mummies.” Raised in Chicago until she was 5 years old, she visited the Field Museum with her parents often. In high school in Florida, where her family had moved for her mother’s job, she worked in a veterinarian’s office. As an undergraduate at the University of Florida, she enrolled in a paleontology course that led to a job normally held by graduate students in the university’s extensive paleontology collection. “I got to open drawers and look at these cool bones from animals that had been dead for hundreds of thousands to millions of years,” she says. “I was transfixed.”
She went on to earn her PhD in ecology and evolutionary biology at the University of California, Los Angeles. While there, she conducted research at the nearby La Brea Tar Pits and became familiar with Late Pleistocene fauna. When she joined Des Moines University, as a professor of anatomy, she began looking for a project that could extend her knowledge—and the world’s—of her chosen period of study.
By 2014, groundbreaking research into the geology of cave formation, based on findings at a site just ten miles south, had helped explain the origins of Natural Trap, which only underlined the site’s uniqueness. The cave’s first passages were carved by rainwater some 300 million years ago. More than 200 million years later, the Bighorn Mountains emerged, lifting the landscape to its present elevation. Another round of creation—this time from rock-dissolving microbial acids, carried upward by rising spring water—expanded the ancient caves from below and made new ones, with large chimneys climbing toward the surface. When the roof of the largest cavity collapsed, it exposed a pit in the middle of a migratory pathway, essentially creating this perfectly placed and protected vault of intercontinental paleontology.
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Still, the cave presented unusual research challenges. As Pleistocene animals fell in, periodic flooding washed away some remains, creating occasional gaps in the fossil record of the large excavation mound. “The sediment deposition gets really complex,” David M. Lovelace, a geoscientist at the University of Wisconsin-Madison, who led a comprehensive study of the cave’s stratigraphy, explains. “Sediment that’s washing in can leave little pockets of deposition in one area. Then the inlet or the stream will shift slightly, so it deposits in another area.” Sometimes a fresh stream cuts through older sediment to drop in new surface material. “It literally forms almost a new cave through the old sediment, so you can deposit younger material under older, previously existing material. The complexities become outstanding.”
The team’s remarkable success at precise dating, in turn, allows Meachen and other researchers to prioritize fossils within the best-dated deposits—and in this way track slower evolutionary changes, the kind found between layers, where thousands of years of evidence may have washed downhill.
Since 2014, each team member has been taught vertical rope technique by Juan Laden, a New Zealand climbing whiz who moved to Lander, Wyoming, in the 1970s. In addition to caving around the world, Laden works as an arborist, leads mountain search and rescue, sails extensively, and is an accomplished photographer.
I have rappelled into deep cave systems for more than 40 years, but like everyone else allowed into Natural Trap, I first have to prove to Laden that I can climb 100 feet up a nylon rope about as thick as a finger. I take my “exam” up on the surface, near the cave’s entrance, where Laden has installed a cranelike metal contraption of his own design on the back of an old pickup truck, with a pulley that allows him to gradually lower any length of rope while the climber remains a few feet off the ground.
As I make my way to the truck, my metal ascenders clanking against one another, Laden notes my dated gear. Most modern cavers use the European “frog” ascent system, but I cling to an arrangement called a “rope-walker,” designed in the 1960s for deep pits in Alabama and Mexico. I huff and puff 50 feet up the line until Laden is finally satisfied.
“That’s enough,” he says at last, lowering me to the dusty ground.
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When it’s time to enter the cave, another team member, Doug Warner, inspects my descending gear, an equally dated rappel rack, a U-shaped piece of metal loaded with “brake bars” to control my rate of descent. Then I stand at the edge of the metal grate, trying not to stare at the darkness below.
Warner surprises me with a whistle. Even more surprising, the whistle is answered by another, from a few feet inside the cave. He points out a Say’s phoebe—a smallish gray bird with a brilliant yellow belly. She flutters about her nest in a precarious niche above the pit, trying to nudge her fledglings into flight, occasionally popping up through the grate to whistle at Warner. “We’ve been talking for several days now,” he says, checking that my carabiners are locked. Then he pronounces me safe to descend.
Scientists entering the cave clip into a safety line before walking across the grating to the metal “door” built in 1971. A regular extension ladder pokes up through the opening. Still clipped into the safety, I climb ten feet down to a flat ledge, noting the excited bird and her peeping chicks. I walk to the edge, where the main descent line dangles free. I attach my rack to the main line, unclip from the safety and step backward into space.
Descending, I note another ledge, this one holding enormous nests of sticks built by raptors before the gate was installed. Then the walls recede.
Below me, a shaft of sunlight illuminates a bright circle of the floor. Researchers move about like so many ants on an anthill. A caver at the bottom helps pull me over to unclip outside of the excavation zone, and I remove my climbing gear.
When I join the researchers underground, Hormell is extracting bighorn bits while others dig elsewhere about the room. As the day progresses, the circle of light creeps across the floor. (When it hits a certain boulder, I’m told, it’s lunchtime.) B.D. Voss, a geology education doctoral student, hands Meachen a small bone, seeking her opinion. She agrees that it’s modern—not a fossil. “Maybe a pack rat.”
Voss drops the bone into a plastic five-gallon bucket. Seeing that it is nearly full, Voss carries the bucket to a designated dump pile, a growing mound that holds 50 years’ worth of excavation discards. Voss then returns to help Hormell prize out a few more vertebrae and ribs as they work their way toward the bighorn scapula, which they finally pull from the Cobbly Gray. Months later, lab work will reveal that what had appeared in the cave to be a nearly complete skeleton was actually parts from several sheep.
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At day’s end, team members emerge. Most carry packs with carefully labeled specimen bags. When it’s my turn, I rope-walk up about 25 feet before stopping to catch my breath. Hanging in the air, I see beyond the mound to the darkness of the larger cave reaching into the distance. The scope of all I see from my elevated view—separate layers of excavation blocked off with orange flags, still-untouched zones on the mound, the enticing entrance to the forbidden lower chamber—suggests that many decades of research remain at Natural Trap Cave. “I honestly think the work is endless,” Meachen says later. “It certainly will outlast my career. I see no end to the questions we can ask about this site.”
I emerge to a freshening breeze. Support cavers who remain near the entrance are gathered beneath a large blue tarp, which now flutters in the afternoon winds. I join exiting team members under the tarp as they pull sports drinks from a cooler, discussing the day’s work.
Across Bighorn Canyon I see the twin peaks of the Pryor Mountains, home to about 120 free-roaming mustangs, European descendants of the extinct Eohippus. One horse from either the genus Equus or Haringtonhippus (both now extinct) contributed a tooth to today’s findings. Only careful lab work will identify the horse’s correct genus, species and age. As the sun dips farther, we head toward camp, ambling through fragrant junipers on our way to the cook tent.
“It’s roast and potatoes tonight!” shouts John Logsdon, the project’s longtime operations manager and chief cook (and since 2023, Meachen’s spouse). “Everybody can have a potato! We have too many, so eat up.”
Hormell, an experienced cook, assists in serving. We eat on paper plates, camp chairs crowded beneath shade that moves with the setting sun. A short walk away, the empty tents of the team’s camp rattle in the hot wind.
“I get to take a shower in four days,” Logsdon says wistfully, doling out potatoes.
“That’s about five days too late,” retorts a caver in the food line.
Beyond the sheep and horse, the day’s finds include part of a wolverine and an American cheetah petrosal bone. A marble-like bone of the inner ear, essential to balance, the petrosal is the hardest bone in all vertebrates, save for tooth enamel. Its hardness makes it ideal for preserving ancient DNA, so team members always celebrate a new petrosal.
The next morning, on the year’s final day of digging, they will cheer once more as Hotchner discovers a mammoth vertebra. Each preserved tooth, vertebra and neural fiber presents a thread in Natural Trap Cave’s evolutionary tapestry, helping to identify species and provide evidence of incremental changes in bone density, food sources or habitat.
Meachen says her next project will look at aridity through time at Natural Trap Cave. Funding could be tricky; she now worries that support from the National Science Foundation, a crucial source, may vanish entirely. “My fingers and toes are crossed.” But if she can proceed, she says, “We’ll be able to determine how large and eventually small mammals respond to relatively rapid drying events.” And clues to how some species survived climatic changes in the prehistoric past could prove vital as the planet grows rapidly warmer and drier.
“One thing I’ve learned about Late Pleistocene megafauna is that they were pretty resilient,” Meachen says—right until some yet-to-be-discovered climate threshold was crossed, causing mass death. “It took a lot of things simultaneously to kill them off. But once that cascade happened, it couldn’t be stopped. I guess that’s the scariest part for fauna today. They’ve been through a lot, but once you hit the perfect balance of low population sizes and habitat loss, it’s too late. I hope that lesson isn’t lost on us.”