AN OCCURRENCE OF REMARKABLY ABUNDANT BRASILICHNIUM TRACKS (NUGGET SANDSTONE, EARLY JURASSIC, DINOSAUR NATIONAL MONUMENT) AND THEIR ENVIRONMENTAL CONTEXT

The fauna of the Early Jurassic sand sea in the western US is poorly known. Body fossils are extremely rare, and much of what is known about animals inhabiting this harsh ecosystem is based on trace fossils. Dinosaur footprints, which dominate the ichnofauna, are known primarily from interdunal deposits. A site within Dinosaur National Monument (DNM), UT, provides insight into the rare components of the fauna.

Brasilichnium tracks occur on five different bedding plane surfaces within a crossbed set of dune sands. At the largest exposed surface (measuring 4m x 10m) over 300 Brasilichnium tracks are preserved. Average track width is 10mm. Tracks show push up rims along the heel impression and, rarely, show 4 toe impressions. The trackway of a large, unidentified vertebrate traverses the dune slope and cuts across the Brasilichnium trackways. At another horizon, the Brasilichnium tracks occur in association with two large scorpion trackways that indicate a leg spread of 10 cm. At all horizons, most tracks, both vertebrate and invertebrate, indicate the track makers were travelling up dune lee faces.

The eolian cross bed laminae that bear the tracks can be traced downslope into horizontally laminated sand of the lowest 2m of that set of cross beds. This interval rests directly upon and truncates the large eolian cross beds of the set below. It consists of thin alternating red and yellow laminae, and contains several, large invertebrate traces, each consisting of a system of closely-spaced vertical and horizontal burrows extending through 30cm of section. We attribute these features to colonial arthropods.

The DNM site is unusual and significant in two ways. First is the great abundance of Brasilichnium tracks on a single surface. Tracks on slip face deposits are generally considered to have been formed over a very short period of time, possibly during a single night. This suggests high population numbers for the trackmakers, probably synapsids, a group very poorly represented by body fossils in this Early Jurassic desert. Secondly, track surfaces can be traced to contemporaneous deposits at the base of the dune with evidence of invertebrate activity.