Earthquake ID’d as prime suspect in paleontological whodunnit at Snowmass Village, Colorado
by Allen Best
The paleontological dig at Snowmass Village, set to end this weekend, has turned up everything from an ancient camel to the massive bones and joints of a plant-eating creature called the Jefferson’s ground sloth, which was about the size of a modern ox or grizzly bear. Remains of 20 different kinds of ice age animals have been found.
But salient among the 4,517 fossils excavated as of Wednesday morning is the number of mastodons, particularly juveniles. And the chief suspect, says Dr. Kirk Johnson, the lead detective in this paleontological whodunit, is an earthquake.
“What is so curious about this site is that we are regularly finding parts of small animals,” said Johnson, a paleontologist and chief curator of the Denver Museum of Nature and Science, in an interview on June 29.
“Every day we are finding parts of small animals, and that is just kind of unusual. As a general rule, animals grow up pretty fast. A mammal lives about 70 years, and it’s full grown by age 20 or so. To see a lot of animals that are one to three to five years old, or even just six months old, is just really unusual.”
The ages, he went on to explain, can be partly determined by conditions of teeth. Some of the teeth have been small and with no evidence of wear, suggesting they were not yet eating vegetation, but instead still were drinking their mother’s milk.
“If you think about the human population, babies are not all that large a part of the overall population. It makes you wonder why aren’t these animals maturing?”
(For images, go to the Museum of Nature & Science website athttp://www.dmns.org/press-room/press-kits/the-snowmastodon-project/snowmastodon-project-images
In the lower levels of the ancient lakebed, most of the bones are of mastodons, a creature somewhat like the elephants of today. Mastodons mostly disappeared near the end of the last ice age, 10,000 to 13,000 years ago. Also found at the site are the bones of a Columbian mammoth, another species similar to the elephants of today. It also became extinct in what scientists call the Pleistocene die-off after the last ice age.
Scientists believe the remains of fossils, trees, leaves, and insects were deposited more than 40,000 to 50,000 years ago, as radiocarbon dating has proven useless. The technique is useful only on more recent remains. But they think the lakebed can be no older than 150,000 years. That’s when a massive glacier shoved bits of soil and rock into a mounded ridge, called a terminal moraine, on which the lakebed was then formed.
Earthquakes could explain why so the bones of many juvenile mastodons came to reside at the bottom of this ancient lakebed.
The bones of the mastodons are rarely complete. Instead, they’re spread out. The dispersal suggests that after the animals died, the bodies decomposed, the tendons and ligaments first, allowing legs, vertebrae and other bones to come apart.
Within perhaps six months, the bones were submerged, the lack of oxygen slowing the decay. In one case a tusk was still white last fall when it was retrieved from the mud.
The favored hypothesis, Johnson said, is that a herd of mastodons was crossing the lake when an earthquake struck. The earthquake liquefied the sediments, creating a quick sand. Quick sand, contrary to the movies, does not swallow people. It can, however, prevent somebody from moving. Imprisoned by the mud, the mastodons eventually died.
Liquefaction of soil is a well-understood process in some areas as a result of earthquakes. “If you live in San Francisco, you don’t want to be along the bay itself if an earthquake happens,” Johnson explained.
“The idea is that you would have a whole herd of animals together, and then something kills them together.”
And then, something else happened, and relatively soon after their deaths. Perhaps triggered by another earthquake, a landslide occurred from the neighboring hillside, which probably was taller and steeper than it is today. The bones were submerged, explaining why they were in such good shape.
Johnson said the scientists don’t yet have the evidence to support this hypothesis. One piece of evidence will come from study of the mastodon tusks. Like trees, they display growth cycles, revealing even the season of the year when the animal died. If the tusks reveal identical seasons for many of the tusks, they likely had a common killer.
Study of sediments will also reveal whether an earthquake caused creation of quick sand.
Another hypothesis emerges as scientists continue their work in mapping the locations of the fossils.
Johnson’s 50-person dig team – which has been aided by 37 experts from the United States, Canada, Spain and England – is using various high-tech tools to help sort out answers to these and other puzzles. One team, from the Colorado Water Science Center, is using ground-laser scans of selected fossils, which will allow scientists to reconstruct parts of the site with high-resolution 3D models. These scans are particularly useful for mapping areas where many fossils occur together.
One small story that the 3D modeling may help explain is the partial mastodon skeleton that was founded interlocked with driftwood logs up to 35 feet long along the ancient shoreline. Some 50 logs have been found along the shoreline of the ancient lake.
Other pieces of evidence will be smaller, even microscopic. Scientists from the U.S. Geological Survey have been drilling 20 to 30 feet into the sediment and extracting cores two inches in diameter. These cores will be studied first in Denver, then moved to the University of Minnesota for permanent storage. There, the National Lacustrine Core Facility stores high-quality sediment cores from lakes around the world and make them available for research.
“Sediment cores are a very important way for us to sample the complete sequence of lake sediments and preserve them for future research,” said Johnson.
“They are a critical piece of the science that can be archived and studied for climate information such as temperature changes and drought.”
As evidenced by the logs, scientists know the site was below timberline for at least part of the 100,000-year period. But pollen residue of alpine plants indicate the site was above treeline. Elevation of the find is just below 9,000 feet, but treeline today is 11,500 to 12,000 feet.
Johnson has called the site the best high-elevation paleontological site in North America, if not the world. Being above treeline distinguishes it from other sites, such as along the Front Range urban corridor of Colorado, where the elevation is 5,000 to 6,000 feet. Even during periods of glaciation, said Johnson, they were below treeline.
The excavation in the mud and peat was triggered by the discovery of a mastodon bone last October by an alert bulldozer operator shaping the natural lake into what will become Ziegler Reservoir. With frantic digging last fall before winter set in, and then again since mid-May, a team assembled by the Denver Museum of Nature and Science has sifted two acres of the ancient lake, which they believe covered up to 12 acres.
Curators at the museum, which has formal responsibility for the paleontological excavation, are deliberately leaving most of the remaining 10 acres alone, partly so that a later generation of scientists, armed with new insights and tools, can return. But Snowmass Village needs the site as a reservoir for an emergency backup for new real estate development, and construction crews will resume their work in July. Scientists believe the water will not harm the remaining paleontological deposits.
Johnson and other scientists are not sure how much of the period between 40,000 and 150,000 years ago is represented in the lake deposits. What is known is that glaciers dominated the high mountains for much of that period, sandwiching a warm phase within.
After the deposits in the lake, glaciers advanced one more time, receding just after humans may have first arrived in North America. Most archaeologists are comfortable with human evidence in North America starting 14,000 years ago – too recent for this particular site, at least so far.
“One of the real strong benefits of this site will be that it will help people understand the broad patterns of the ice ages and put it into the context of modern climate change,” said Johnson.
Allen Best publishes Mountain Town News. He can be reached at 303.463.8630.