Last October, Kirk Johnson got the call: A bulldozer driver named Jesse Steele had unearthed ribs, vertebrae, and a tusk belonging to a young, female Colombian mammoth while excavating a dam at Ziegler Reservoir near the Snowmass Village ski resort. Johnson, curator and vice president for research and collections at the Denver Museum of Nature and Science, felt the familiar excitement. For the career paleontologist and geologist, mammoth discoveries are not unusual, with at least partial remains surfacing every several years. The finds are wonderful, but, Johnson says, “When a construction worker calls me up and says, ‘I think I found a mammoth bone,’ I say, ‘Yeah, you probably did.’ ”
Still, each discovery demands investigation, so the next day, he dispatched a crew to plan for further exploration in Snowmass. About 10 days later, Johnson visited the site himself, along with colleagues Steven Holen and Ian Miller. Expecting little more than the usual handful of bones, the three researchers didn’t even bring tools.
They laid out what had been found inside a building at the reservoir. As they examined the remains, Holen noticed another mammoth leg bone, clearly from a larger specimen. From that moment, nothing about the dig would be routine.
Most Ice Age sites yield one animal; finding two is rare. The Ziegler site went from rare to one-of-a-kind before Holen could put down the second bone. While Johnson and his peers were still mulling the magnitude of the dual find, a bulldozer unearthed a third leg bone in a different part of the reservoir.
A few days later, the three men returned to Denver, still abuzz from the excitement of the unique find. The next morning, workers in charge of excavating the reservoir uncovered more tusks and a tooth. One of them took a photo of the tooth and sent it to Johnson, who saw immediately it wasn’t from one of the mammoths. This tooth belonged to a mastodon, and it sparked a frenzy of questions about what on earth had happened at Ziegler.
In 2009, when two boys digging in the dirt near Ken-Caryl Ranch unearthed a partial jawbone and tusk, it was hailed as the most significant mastodon find in Colorado. Remains from the ancient elephantine creatures are littered throughout North America, from Alaska to Honduras, but they’ve almost never been found in Colorado; the Ken-Caryl bones were only the third discovery of mastodon fossils in the state. At the time, Holen told the Denver Post it was “the first good record of an American mastodon in Colorado.”
Nestled high in the mountains outside of Aspen, however, the Ziegler find showed that mastodons have a far more expansive history in Colorado than anyone had realized. Within a few weeks of the discovery, the museum had installed a crew at the site while construction workers excavated the dam, carefully avoiding what they thought was a single site. By the start of ski season (and the end of digging season) several weeks later, museum crews would identify 28 different piles of fossils at Ziegler, yielding almost 600 bones from as many as 10 individual mastodons—including 16 tusks and the first mastodon skull ever unearthed in Colorado. Some of Ziegler’s skeletons are more than 50 percent complete, and not all the bones are from the prehistoric proboscideans. The ancient lake is teeming with fossils: The remains of at least five species of large Ice Age animals are preserved at Ziegler, including a Jefferson’s ground sloth the size of an ox, a small deer-like animal, and several huge Ice Age bison with horns spanning six feet across.
With any prehistoric discovery, there’s potential for hyperbole. But the numbers alone make the Ziegler site spectacular, and the mastodons are its crown jewels. It’s long been known that mammoths once roamed Colorado, even at elevation, but mastodons have never been found this high in North America—Ziegler Reservoir sits at almost 8,400 feet. Stranger still, Johnson says, the two are never found together.
To the layperson, mammoths and mastodons aren’t much different. Essentially, they’re both big, furry elephants. But finding them together is like discovering panda and polar bear remains in the same spot—they’re both bears, but they wouldn’t live in the same place at the same time. Mastodons and mammoths overlap in geologic time, but their diets—and therefore their habitats—are simply too disparate to find them side-by-side. Although they’re both vegetarians, the smaller mastodons were forest dwellers, while the larger mammoths were grassland grazers.
The orientation of the two species at Ziegler added to the mystery because the remnants of different time periods are stacked like pancakes below the lake. The mastodons lie in the lowest, oldest layers, embedded with trees. The mammoths reside at the top, where no trees remain, suggesting the tree line must have fallen and the climate cooled somewhere in between. “We’re seeing something that no one’s seen before,” Johnson says. It’s “the one place in North America where you can tell the story of ecosystems changing through time at high elevation.”
If the discovery of the elephantine specimens isn’t manna enough, consider this: The ancient plants and animals in the site are nearly pristine. Buried in peat and clay, the bones haven’t even petrified, giving scientists a shot at extracting DNA, à la Jurassic Park. “The preservation is staggeringly good,” Johnson says. “The leaves are still green. The trees still have bark on them. And the insects are still iridescent.”
The unnatural pairing of mammoths and mastodons is only part of the puzzle at Ziegler. Why did so many creatures meet their end in this one spot? The answer may lie in the age and the location of the ancient lake. Lakes are what Johnson calls an “attractive nuisance” for animals; they feed, drink, and hunt at the water’s edge and occasionally drown or fall through the ice in winter. Over time, sediment carried in by rainfall or tributary streams fill a lake, trapping the remains.
Most lakes fill up in a few thousand years, leaving only a small number of fossils. Ziegler is confounding because it trapped so many. What’s more, sitting at nearly 8,400 feet with no tributaries running into it, Ziegler has no obvious source of sediment. Johnson speculates that eons of dust storms, along with earthquakes that freed debris from the walls of the lake, may be the culprits. The amazing preservation of such a variety of plants and animals—of widely varied ages—left even the experts scratching their heads, until they looked at a geological map of the area. It shows that the boundaries of this tiny lake were pushed into place by a moraine—a steamroller of earth and rock propelled by a glacier that carved its way through the landscape and deposited the incongruent remains. This particular steamroller is thought to be 130,000 to 150,000 years old, and the unique conditions mean hundreds of animals could be preserved in the lake.
It’s only a theory, Johnson says, but so far, it’s the prevailing one. As researchers learn more, Johnson expects “a jillion” more hypotheses. None of them will change the most important thing we already know about Ziegler: “Something happened here,” he says. “Whether it’s a crime scene or a cemetery, there are a bunch of cool things going on.”
Scientists will return to Snowmass in May to finish excavating the dam site before the reservoir is filled. Time is on their side because only one of Ziegler’s 11.5 acres will be disturbed to build the dam before filling it in with water. And that, Johnson says, is the best way to preserve whatever else lies beneath. Finding a series of sites at this elevation with such well-preserved remains makes it, Johnson says, the most noteworthy Ice Age find ever in Colorado. It’s potentially akin to the La Brea Tar Pits, whose asphaltlike sediment has entombed millions of Ice Age fossils and attracted about that many visitors, intrigued by the notion of a real-world Land of the Lost nestled in the heart of Los Angeles.
The Colorado mastodons are already exhibiting similar box-office appeal. The Denver Museum of Nature and Science has dubbed the ongoing excavation “The Snowmastodon Project.” Last year, the fossils attracted 3,000 visitors on the only day they were on display. And in Snowmass Village, the town has established a “Tusk Force” to help plot the future, which might include a museum. The town council has already declared Widespread Panic’s “Big Wooly Mammoth” the official town song. Locals have nicknamed the mastodon “Snowy,” and she’s become the unofficial mascot for Snowmass Village, the ski-town-cum-Ice-Age wonder that now sports banners bearing her likeness under the slogan, “I dig Snowmass.” And it’s all thanks to a bulldozer driver who had a keen eye and a keener sense of history. “We’ve really seen it resonate more than we ever expected,” Johnson says. “And what’s so amazing is it wouldn’t have happened if Jesse Steele hadn’t looked over the blade.”
Abigail Eagye is a freelance writer in Carbondale. E-mail her at firstname.lastname@example.org.