A LOWER JURASSIC EUBRONTES-DOMINATED TRACKSITE IN THE NAVAJO FORMATION,EASTERN UTAH: PROPOSED PROTOCOLS FOR UNRAVELLING TRACKSITE HISTORY

The Mail Station Dinosaur Tracksite (MSDT), on Bureau of Land Management land in San Juan County, Utah, is one of the largest and best-preserved Lower Jurassic dinosaur tracksites in the western USA. Located stratigraphically near the top of the Navajo Formation (Glen Canyon Group) the site represents an interdune playa composed of horizontally bedded dolomitic sandstone. The assemblage is dominated by the large theropod track morphotype Eubrontes, mostly preserved as ‘elite’ tracks superimposed on a background of poorly preserved small theropod tracks including Grallator.

Mapping of the site reveals at least 100 mostly well-preserved tracks representing at least 24 trackways with highly variable orientations. The lack of parallel orientations suggestive of gregarious behavior, or shore parallel movement indicate that most trackmakers crossed a small playa area individually at different times. At least four trackways, mostly the deeper ones, provide evidence of running individuals, two of which attained estimated speeds in excess of the current “world record” estimate (42.8 km/hr) claimed for a Cretaceous theropod from Texas.

Unlike many dinosaur tracksites on BLM land that are made accessible and interpreted for the general public the MSDT is being used as an educational outdoor laboratory or “paleocamp,” for instructing young students (“citizen scientists”) in ichnology, photogrammetry and stewardship of fossils on public lands. For this and other reasons ongoing investigation of the site by professional paleontologists incentivizes conscientious attempts to unravel the sequence of track registration events with greater care than is usually afforded sites represented by maps which misleadingly imply a simplified “instant-in-time” episode of track making rather than a time transgressive series of events: i.e., fossil tracksites accumulated tracks “through time.” Thus, ichnologists, need tracksite analysis protocols that unravel sequences of events in relative time. Biogenic traces (such as overlapping trackways) and sedimentological evidence, such as across-site changes in track depth in trackways made at an-instant-in- time by a single individual, provide information on trackmaking activity and substrate conditions hitherto mostly overlooked in tracksite investigations.