A NEW PTEROSAUR TRACKSITE FROM THE JURASSIC, SUMMERVILLE FORMATION NEAR MOAB, UTAH

Although late Jurassic Pteraichnus sites are found throughout the western United States in the Summerville Formation and equivalent deposits, until recently, none were known in the Moab region. The nearest correlative Pteraichnus localities are more than 100 km away to the NW, SW and E. Thus, the discovery of this site, in 2004, during a BLM sponsored project, is a significant addition to the previously known geographical distribution of pterosaurs.

The Pteraichnus-bearing unit is preserved as a loose talus block derived from a sandy, ripple-marked facies of the Summerville approximately 6-7 m above the Moab Member of the Entrada (Curtis), and 1-2 m below the overlying Tidwell-Morrison sequence. The tracks occur as well-preserved partial or complete natural casts of 33 pes and 17 manus prints on a single surface of approximately 0.5 m2. The site also reveals distinctive invertebrate traces (cf. Isopodichnus/Cruziana) associated with this and other surfaces. Observations made at the site in 1987 indicate that the track casts have weathered to become visible in only 17 years. This observation has important implications for conservation of the specimen.

The high density of tracks is typical of many Pteraichnus sites. The tracks have highly variable orientations and only a few partial trackways are discernable. The size range is also typical of most sites (manus length range 3.5-5.8 cm). The ratio of pes to manus in this specimen is perhaps unusual because at the majority of other Pteraichnus sites manus impressions outnumber those of the pes -due to the fact that pterosaurs were front heavy. Notable exceptions to this distribution pattern are sites with pes-only swimming trace assemblages. Several incomplete pes tracks may indicate that the trackmakers were periodically buoyant. While presence of Isopodichnus/ Cruziana traces within the same track-bearing unit indicates a subaqueous setting, water levels probably fluctuated allowing for multiple phases of trace-making activity.