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Dartmouth scientists participate in rare dinosaur track investigation

Posted 11/21/00

Recently discovered dinosaur tracks in northern Wyoming are helping a team of researchers that includes Dartmouth earth scientists Gary D. Johnson and Kate Keller piece together a more accurate picture of life 167 million years ago.

The research team, which presented its results in two papers at the annual meeting of the Geological Society of America, and includes scientists from Indiana University, the University of Colorado at Boulder and Indiana-Purdue University, is studying several occurrences of dinosaur trace fossils located in sedimentary strata in northern Wyoming's Bighorn Basin.

Separated by multiple layers of sediment, the dinosaur trackways represent a several-million-year record of dinosaur presence in that area, said Johnson, Professor of Earth Sciences. The two trackways date from the Middle Jurassic period, about 167 million years ago. The most recently discovered trackway, located beneath the other, is approximately three million years older than the other trackway.

Using techniques familiar to forensic scientists, the Dartmouth researchers are learning more about how the dinosaurs lived. While body fossils such as bones and teeth tell scientists about the physical characteristics of dinosaurs, footprints and other trace fossils give scientists clues about dinosaur behavior. For example, the trackways, together representing thousands of tracks, potentially could tell the researchers whether the animals were just passing through the area or staying for an extended time, how large a group traveled together, and what their feeding strategy was, among other things, said Johnson.

Of particular importance, the most recently discovered trackways, in the Gypsum Spring Formation rock stratum, offer evidence that some of the dinosaurs inhabiting the tidal flat of the Bighorn Basin might have been swimmers.

The age difference in the tracks makes it possible for the researchers to better understand the complex environmental changes in the Middle Jurassic period. Previously scientists had believed that this region of north-central Wyoming was mostly marine in character at that time, with the rock sequences providing only minimal evidence of terrestrial habitats. However, the younger trackways found in the rock stratum known as the Sundance Formation demonstrate that there were significant episodes of coastal tidal flat habitat.

The older trackways, by contrast, suggest parts of the region only three million years prior, while representing a similar tidal flat condition, also demonstrate dinosaur traces that were developed during high tide conditions. In some areas the track-bearing surface consists of grooves that appear to be the remains of scratch marks made by dinosaurs whose feet briefly touched a muddy bottom while they were swimming. The groove marks are consistent with the general size and spacing of the terrestrial dinosaur tracks found elsewhere in the formation.

Few examples of dinosaur trackways from the Middle Jurassic period are known worldwide, making the discovery of these two separate occurrences even more significant. For the Middle Jurassic period in the United States, dinosaurian discoveries previously were limited to Utah. The discovery of hundreds of dinosaur tracks within the older Gypsum Spring Formation and thousands of tracks in the Sundance Formation contributes significantly to knowledge of the geographic distribution of dinosaurs in North America during this time.

The research team uncovered the first dinosaur trackway in 1997 and has continued to study various related sites since then. Dartmouth undergraduate students have participated in mapping several of the trackways. While the group continued its research on the Sundance trackways, a visitor to the area in 1999 discovered what he believed to be isolated tracks in the underlying Gypsum Spring Formation. Later, team members discovered more extensive outcroppings of these trace fossils throughout the region.

Unlike the Sundance tracks that preserve only the three toe imprints and rarely the heel imprint of these bipedal dinosaur's feet, many examples of toe and heel impressions are in the Gypsum Spring trackways. As a result, estimates of a dinosaur's speed based on foot size and stride can be made. Estimates of dinosaur speeds up to 9.2 kilometers per hour have been calculated.

Johnson's team dated marine fossils preserved above and below the trackways to pinpoint the age of the track site to within a million years. Volcanic ash found in the rock was dated by measuring the decay of radioactive isotopes at Dartmouth's geochronology lab under the direction of Jim Aronson and Lorin Amidon.

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