NEW RARE MIDDLE JURASSIC DINOSAUR TRACKSITE FOUND IN NORTHERN WYOMING
Erik P. Kvale 1
Gary D. Johnson 2
Debra L. Mickelson 3
Kate Keller 2
1. Department of Geological Sciences, Indiana Geological Survey, Indiana University
Bloomington, IN, USA
2. Department of Earth Sciences, Dartmouth College, Hanover, NH, USA
3. Department of Earth Sciences, University of Colorado at Boulder, Boulder, CO, USA
The Middle Jurassic (159 – 187 million years ago) dinosaur record is considered sparse worldwide with relatively little known about dinosaurs from this period. However, recent discoveries in the Bighorn Basin of northern Wyoming are changing that with the discovery of the most extensive Middle Jurassic dinosaur tracksites in North America. In 1997, near the town of Shell, researchers found extensive dinosaur track-bearing deposits in 167 million year old rock once thought to have been totally marine. Now the same researchers report the presence of an even older more extensive dinosaur track-bearing layer. The new discovery occurs in a one-meter thick interval 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 original 1997 discovery in the Sundance Formation of north-central Wyoming has resulted in the establishment of the Red Gulch Dinosaur Tracksite. This site, on public lands administered by the Department of the Interior, Bureau of Land Management (BLM), is a 40-acre area currently being developed by the BLM to become a dinosaur educational site accessible to the public.
First discovered in 1999 by Walter Parrs, Jr., a tourist staying at a local western guest ranch, the newly discovered Gypsum Spring Dinosaur Tracksite includes the impressions of land-dwelling (terrestrial) small- to medium-sized three-toed two-legged dinosaurs comparable in size to those found in the younger Sundance Formation. Some of the tracks can be identified as having been made by carnivorous dinosaurs (theropods). Outcrops containing Gypsum Spring tracks occur sporadically over a 2000-square-kilometer area. In some areas the track-bearing surface consists entirely of two to three subparallel grooves up to 8 cm long spaced approximately 3-4 cm apart that appear to be the remains of scratch marks made by buoyed dinosaurs whose feet briefly touched a muddy bottom while swimming. The groove marks are of a size and spacing that is consistent with the terrestrial tracks found elsewhere in the Gypsum Spring Formation.
Unlike the younger Sundance tracks that preserve only the three toes and only rarely the heels 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 dinosaur speed based on foot size and animal stride can be made for these older dinosaurs. Estimates of dinosaur speeds up to 9.2 kilometers/hour have been calculated.
Interestingly, the researchers believe that lowly bacterial mats that once covered the tidal flats inhabited by these animals may have been responsible for preserving these tracks over millions of years. Within minutes to hours of the dinosaurs walking across the tidal flat a thin covering of a greenish-bacterial mat covered the tracks. Such bacterial mats are present on many of today’s beaches and tidal flats. This bacterial mat stabilized the tracks and prevented the erosion of the track-bearing surface by wind or waves until they were buried by other sediments and eventually hardened into rock.
For the Middle Jurassic time period of the U.S., the reptilian discoveries had been restricted to Utah. These include: (1) the skeleton of one land-dwelling primitive crocodile-like reptile; (2) dinosaur tracks in the several million year younger formation called the Entrada; and (3) a few small dinosaur tracks from the Sundance equivalent unit called the Carmel Formation. Therefore, the existence of abundant dinosaur tracks within the older Gypsum Spring Formation and Sundance Formation contributes significantly to the knowledge concerning the geographic distribution of dinosaurs in North America during this time.
The researchers are presenting their research results on November 16th at the annual convention of the Geological Society of America in Reno. Kvale can be contacted from Nov. 11th – November 17th at the Reno Hilton (guest contact at 775-789-2000) and at the Indiana Geological Survey (812-855-1324) after the 17th. Johnson and Keller can be reached at (603) 646-2371. Mickelson can be reached at the Reno Hilton from Nov. 11th – November 17th at the Reno Hilton (guest contact at 775-789-2000) and at (303) 722-9995 after the 17th.