TERRESTRIAL VERTEBRATE TRACKWAYS OF THE EARLY JURASSIC NUGGET FORMATION AT DINOSAUR NATIONAL MONUMENT, UTAH

A track site in the Lower Jurassic Nugget Formation within Dinosaur National Monument, Utah, preserves numerous tracks and trackways. The large site located near Cub Creek within Dinosaur National Monument is approximately 70 meters by 20 meters of track-bearing surface. The tracks are preserved on sands beneath the massive eolian dune sands of the Nugget Formation. Tracks are preserved on two different layers separated by about 10cm. There are an estimated 300 tracks at this locality, with a track density of about 3 or 4 tracks per square on average. The size of the tracks ranges from approximately 10 to 25 cm wide. Most of the tracks are poorly preserved and probable undertracks, but there are many well preserved tracks in which pushup rims and toe impressions are evident and manus and pes can be distinguished. Some trackways can be distinguished in the field. Well preserved tracks and trackways can be identified as cheirothere tracks. The tracks and trackways do not appear to have a common orientation, indicating that the track makers were walking in different directions.

Another track site located north of the Carnegie quarry within Dinosaur National Monument is in approximately the same stratigraphic position as the Cub Creek site. Track density is much lower at this site and several trackways are easily distinguished, including a well preserved cheirothere trackway which extends for 14 meters.

Analysis of maps of the site near Cub Creek should make it possible to recognize additional and longer trackways at that site and determine the orientation of trackways. And, together with the long cheirothere trackway mapped at the other site, will provide information about the stride length and gait of the cheirothere track maker.