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.