Portal to the Pleistocene: Amazing finds in Snowmass
November 12, 2010
SNOWMASS VILLAGE – In the waning light of an October afternoon, a bulldozer operator turned up a new era in Snowmass Village.Jutting from the peat, the giant bones of a Columbian mammoth opened a window to the Ice Age that has astounded scientists and captivated just about everyone else, turning the town on its ear.In the four-plus weeks since that initial discovery, unassuming Ziegler Reservoir has emerged as one of the most important paleontological sites in Colorado. Consider this: there had been only three recorded findings of mastodon bones in Colorado before Ziegler. By the middle of last week, evidence of as many as 10 of the prehistoric beasts had been unearthed, including the first-ever discovery of a mastodon skull in Colorado. No mammoths had ever been discovered at high altitude in the state before, and no site had yielded evidence of both mammals in the same place.Though they co-existed during the last Ice Age, the fossil finds so far at Ziegler suggest the mammoths inhabited the area more recently than did the mastodons. Mammoths were grazing animals, more closely related to elephants than the smaller mastodons, which fed on leaves and branches.The bones of at least three Ice Age bison, a Jefferson’s ground sloth – another first in Colorado – and a small deer have joined the tally, along with a small reptile of some sort and a mouse-like animal.”Every time we dig, we find stuff,” said Dr. Kirk Johnson, chief curator and vice president of research and collections at the Denver Museum of Nature & Science. The museum assembled a crew of staff and volunteers, and summoned an assortment of amazed experts to consult on the treasure trove of bones being pulled from an ancient lake bed.With winter bearing down, the team was expected to pull out by Nov. 14, leaving until spring whatever else might be exhumed from the drained reservoir, which the Snowmass Water and Sanitation District is in the process of expanding.The dizzying excavation and daily reports of astonishing discoveries will give way this winter to preserving more than 200 bone specimens and piecing together the story they tell.
The last Ice Age, generally defined as occurring from 2.5 million to about 12,000 years ago, spelled the end of the road for many of Earth’s large mammals. Scientists continue to debate the cause of their extinction, according to Dr. Dan Fisher, an expert on mastodons from the University of Michigan who was called to Snowmass Village. The Ziegler finds could help yield new clues in the quest to solve that mystery, he said.By the end of the Pleistocene Epoch, many present-day species of animals existed, according to Fisher.”But in addition were larger species that made for a diverse fauna,” he said. Only about 30 percent of those large animals – ones that weighed more than 100 pounds – remain. The effects of climate change and human activity are but two theories about what happened to beasts such as the mammoth and mastodon.”That’s something on which there’s a lot of ongoing debate,” he said. “You know, sites like this can help provide those answers.”Adding to the significance of the Ziegler discoveries are the state of the bones. Buried beneath clay, within a deep layer of peat composed of the decayed plant matter of an ancient marsh, or in the silt below the peat, they did not fossilize (or turn to stone, in other words). Because they are more than 10,000 years old, they are considered fossils, but the bones still contain protein and collagen, according to Dr. Steve Holen, the museum’s curator of archeology and its mammoth expert.Scientists will attempt to extract DNA, which provides vital insight into the evolution of an animal, he said. It helps scientists trace the relationship, for example, between the Jefferson’s ground sloth, an unusual, ox-sized animal with huge front claws, and modern tree sloths. The Jefferson’s ground sloth, incidentally, is named after the third U.S. president, a paleontology enthusiast who read a paper to the American Philosophical Society in 1797 on the bones of an animal found in West Virginia. It was later named Megalonyx jeffersoni, in his honor.The mammoth and mastodon tusks collected at the site are also revealing. Johnson compared them to fingerprints – no two sets are the same. Their rings tell scientists the age of the animal. Gender, and even whether or not a female gave birth, can be determined from examination of the tusks, he said. Because the two tusks of an animal spiral in different directions at the tips, pairs can be matched together, allowing scientists to determine how many different animals they represent. Their shape distinguishes mammoth tusks from those of mastodons.”The tusks tell an amazing life history of the animal,” Johnson said.What has not yet been found at the reservoir is evidence of predators, human or otherwise. Modern humans were living in the era of the mammoth and mastodon, Johnson said, and scientists have kept a keen watch for any hint of people connected to the animal finds at Ziegler.”We’re hyper-alert to anything that would suggest that,” Johnson said. “You don’t want to suggest human involvement unless you have definitive evidence. We have none of that.”Still, in the tent that was erected over the initial mammoth find, crews have carefully sifted through the damp peat in search of items like spear points. Such a discovery would elevate the site to a new level of importance, according to Johnson.”Being an archeologist, in my mind, the big question is, were humans here?” said Holen, whose research has focused on the Clovis people, the earliest known North American culture, and sites where humans and mammoths were present.Most certainly, according to Fisher, certain predators were in the vicinity. It’s possible that prehistoric carnivores such as saber-toothed cats, dire wolves and the short-faced bear (think grizzly, but two to three times larger) roamed the area, he said.Predators are a much rarer find because there were fewer of them, just as there are fewer cougars than elk today, Johnson explained.(To watch a simulated battle between a short-faced bear and a Jefferson’s ground sloth, go to http://www.youtube.com/watch?v=dhJ4dSPtfi0.)
While giant tusks, teeth as large as a man’s hand and a bison head and horns measuring more than 6 feet across get all the headlines, guys like Johnson are reveling in the Ziegler mud.Heavy machinery at the soggy site peeled off a foot or two of surface muck, about 5 feet of clay, 5 feet of fertile peat and 5 to 8 feet of silt, all sitting on a glacial deposit of gravel. The first mammoth, a juvenile that scientists believe was a female, was found in the peat layer; other remains have been discovered down as far as the gravel, suggesting they died in a different time period.Last week, paleoecologists from the U.S. Geological Survey visited the site to help museum officials assess the age of the different layers. Sediment samples collected for radiocarbon dating indicate the lower level of sediment dates back at least 43,000 years.”Ziegler Reservoir is a curious thing – basically, it’s a lake formed at the top of a hill,” Johnson said.It likely filled in far more slowly than would a lake at the bottom of a valley. Scientists are now estimating the layers of sediment range from 120,000 to 10,000 years old, the most recent layer being closest to the surface.”If that’s true, then the sediments of the lake could tell the story of the last glaciation and the last deglaciation. That’s rare,” Johnson said.In geologic terms, the last Ice Age was practically yesterday, and the animals being unearthed at Ziegler Reservoir were inhabitants of the valley until just recently. By comparison, a dinosaur such as, say, a Stegosaurus, walked the Earth some 150 million years ago.”If you’d been in Colorado 16,000 years ago, there would have been 10 different Glacier National Parks,” Johnson said. The U-shaped Maroon Creek Valley (take a look at it the next time you’re at the top of Buttermilk) is a classic glacial valley, carved by the ice, he said.The mountains above Snowmass Village, including the ski area, were in place when mammoths and their prehistoric counterparts were heading to the watering hole that would eventually become Ziegler Reservoir. The mammals that have been uncovered there were herbivores, and the lake that slowly became a peat bog preserved evidence of the plants they were eating, along with some of the animals themselves.Prehistoric logs have been recovered, along with samples of distinctly chewed wood that suggests the work of beavers. Samples of spruce, fir and assorted other vegetation have been collected, along with insects and leaves that still hold their waxy exterior. Snails, seeds and pollen grains, also preserved in the peat, have been collected for analysis. Plants of a colder, moister climate likely ringed the lake some 12,000 years ago.The soft peat breaks easily into layers that reveal ubiquitous plant matter – mostly long, narrow leaves – probably from rushes that covered the lake as it became the shallow bog.”You can actually peel plants right off the mud,” Johnson said.It was in that marsh that at least one mammoth apparently collapsed and was buried for 12,000 years or so, until a bulldozer dug it up.”The geologic fact of lakes is, they fill up. That gives us fossil joy. We love it when stuff gets buried,” Johnson said.The single, scattered bones found in the deeper layers may have been from animals that died on the shoreline much earlier, and were scavenged. Bones eventually tumbled down an underwater slope, Johnson theorized. A 7-foot mastodon tusk that was recently recovered came out of what was clearly slide debris, he said.From the bones, the plant material and the present-day landscape, scientists can re-create a picture of what the area looked like as the last Ice Age ended, said Johnson, who intends to commission a freelance artist to tackle the project.”We can build a legitimate, plausible reconstruction. We’ll paint this landscape,” he firstname.lastname@example.org