WADDLING WITH PTEROSAURS: NEW PTERAICHNUS TRACKS FROM THE UPPER SUNDANCE FORMATION OF WYOMING

Numerous outcrops of the Jurassic Sundance Formation near Seminoe Reservoir in Carbon County, Wyoming contain an extensive number of pterosaur tracks and overprinted sauropod tracks. These ichnites are important to understanding the terrestrial vertebrate faunal paleoecology of the Upper Sundance Formation. Quantitative analysis and comparison of the Seminoe Reservoir pterosaur tracks and trackways with previously described pterosaur tracks from Wyoming's Alcova Reservoir area is part of a McNair/EPSCoR sponsored undergraduate research project run through the University of Wyoming Geological Museum. The yearlong project objectives were to document the newly discovered pterosaur tracks, do a comparative analysis to determine ichnotaxa, and investigate the quadrupedal motion of pterosaurs.

Over 150 pterosaur tracks are preserved as impressions and natural casts in multiple layers of fine-grained, ripple-marked, quartz sandstones in the Late Jurassic (Late Oxfordian-Early Kimmeridgian) Windy Hill Sandstone Member of the Upper Sundance Formation. Preserved in the sediments of ancient beach sands, the plantigrade, v-shaped pes prints typically display four digits, while tracks of the digitigrade manus show two short digits and one long, wing-bearing digit. Pes lengths vary from 6 to 12.8 cm. Pes widths vary from 1.8 to 4.5 cm. The deeper-impressed, manus prints range in length from 4 to 12 cm, with widths ranging from 1.8 to 5.9 cm. Most tracks compare favorably to the ichnospecies, Pteraichnus stokesi, as they are morphologically similar, found within the same stratigraphic level, and fall within the same size range. Over twelve trackways have been located, with two to six sets of pes and manus tracks preserved. The trackways are generally oriented to the northwest and northeast. Trackways display outwardly rotated, manus prints lateral to and behind the pes prints. Most trackways suggest animals of different sizes moving by quadrupedal motion in a linear direction, while track impressions higher in the section demonstrate a pattern of “trampling.”