BLOOMINGTON, Ind. -- The dinosaur record of the Middle Jurassic period (159-187 million years ago) is considered sparse worldwide, with relatively little known about dinosaurs from this period. However, recent discoveries of the most extensive Middle Jurassic dinosaur tracksites in North America are changing that.
In 1997, near the town of Shell in the Bighorn Basin of northern Wyoming, Indiana University geologist Erik Kvale found extensive dinosaur track-bearing deposits in 167 million-year-old rock in the Sundance Formation that was previously thought to have been totally underwater during the time when dinosaurs lived.
Now Kvale and collaborators report the presence of an even older, more extensive dinosaur track-bearing deposit in the Bighorn Basin. The scientists presented their results today (Nov. 16) at the annual convention of the Geological Society of America in Reno, Nev.
The new discovery is in a meter-thick layer of rock in the Gypsum Spring Formation. Estimated to be 170 million years old, this newly discovered layer preserves evidence that dinosaurs that inhabited this part of Wyoming may have been swimmers.
The Gypsum Spring Dinosaur Tracksite was first discovered in 1999 by Walter Parrs Jr., a New York City resident visiting a local ranch. It includes impressions made by land-dwelling two-legged dinosaurs that were small- to medium-sized, comparable to those found in the younger Sundance Formation. Some of the tracks were made by carnivorous dinosaurs called theropods.
Outcrops containing Gypsum Spring tracks occur sporadically over a 2000-square-kilometer area. In some areas the track-bearing surface consists entirely 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 have a size and spacing consistent with terrestrial dinosaur tracks found elsewhere in the Gypsum Spring Formation.
Unlike the Sundance tracks that preserve only the three toes and rarely the heel of the dinosaur's foot, many examples of toe and heel impressions have been found in the Gypsum Spring trackways. As a result, estimates of a dinosaur's speed based on foot size and stride can be made for these dinosaurs. Estimates of dinosaur speeds up to 9.2 kilometers per hour have been calculated.
Interestingly, the researchers believe that algal and bacterial mats that once covered the tidal flats inhabited by these animals may have helped in preserving their tracks over millions of years. Such microbial mats are present on many of today's beaches and tidal flats. Within minutes to hours after the dinosaurs walked across a tidal flat, a thin microbial mat covered their tracks. This stabilized the tracks and prevented erosion of the track-bearing surface by wind or waves until it was buried by other sediments and eventually hardened into rock.
For the Middle Jurassic period in the United States, reptilian discoveries had been limited to Utah. These include: (1) the skeleton of one land-dwelling primitive crocodile-like reptile; (2) dinosaur tracks in the formation called the Entrada, which is several million years younger; and (3) a few small dinosaur tracks from the Sundance equivalent called the Carmel Formation. Therefore, the existence of abundant dinosaur tracks within the older Gypsum Spring Formation and Sundance Formation contributes significantly to knowledge of the geographic distribution of dinosaurs in North America during this time.
The original 1997 discovery in the Sundance Formation resulted in the establishment of the Red Gulch Dinosaur Tracksite on public lands administered by the Department of the Interior's Bureau of Land Management. The Red Gulch site is a 40-acre area currently being developed by BLM as a dinosaur educational site accessible to the public.
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