Enigmatic Fossil Footprints from the Sundance Formation (Upper Jurassic) of Bighorn Canyon National Recreation Area, Wyoming

Abstract

The Sundance Formation (Middle-Upper Jurassic) of Wyoming is well known for pterosaur footprints. Two new partial trackways from the upper Sundance Formation of the Bighorn Canyon National Recreation Area (BICA) of north-central Wyoming are enigmatic. The trackways are preserved in rippled, flaser bedded, glauconitic sand and mud. The deposits were laid down in tidal flats, behind barrier islands, along the mesotidal Sundance Sea.

The best-preserved print of the primary trackway possesses four impressions: three shorter digits with negative rotation and an elongate, caudally-oriented mark. The primary trackway has low pace angulation. The combination of morphology and pace angulation matches neither tracks nor body fossils of horseshoe crabs, theropod dinosaurs, pterosaurs, crocodylomorphs, “lacertoids,” or mammaliforms. The secondary trackway, possibly consisting of undertracks, similarly possesses elongate caudal impressions but differs from the former by possessing four subparallel, cranially-oriented digits. These prints also do not closely resemble any of the aforementioned taxa. While the secondary trackway does not lend itself to conclusion, the primary track maker could have been either an injured, pathologic pterosaur or a pterosaurian taxon otherwise unknown from the ichnological record.

Keywords:

• Sundance
• Jurassic
• Wyoming
• Bighorn
• footprint
• limuloid
• pterosaur
• crocodylomorph
• tidal
• paralic

ACKNOWLEDGEMENTS

It is a pleasure to be able to contribute this work in honor of Dr. Bill Sarjeant, whose seminal and copious paleoichnological work paved the way for all future research. The footprints described herein were discovered during the course of research with Drs. Peter Dodson and Bill Donawick (University of Pennsylvania) involving the stratigraphic setting of a sauropod specimen from the Morrison Formation in Montana. Paleoichnological conversations with Dr. Glen Storrs (Cincinnati Museum of Natural History and Science) were enlightening. Comments on earlier versions of this manuscript by Dr. Peter Dodson, Barbara Grandstaff, and Matt Lamanna (University of Pennsylvania) were very helpful and greatly appreciated. Review comments by Debra Mickelson (University of Colorado, Boulder) and discussions with Dr. David Unwin (Museum für Naturkunde, Berlin) greatly improved the manuscript. We thank Bonnie Winslow and Jim Stabler (BICA) for their interest, understanding, and assistance in obtaining the collection permit. Partial funding for the discovery expedition was graciously provided by Emilie deHellebranth.

Notes

*Parenthetical value denotes measurement of positive relief portion of impression, which is separated from the main body of its print by a featureless gap.

^†Parenthetical value incorporates linear mark extending from end of digit impression. See text for details.

*Track incomplete;

^†Value averaged from many tracks; ? = Tracks not preserved, values unknown.

^‡ Lockley et al. (1995) did not provide explicit measurements for the lengths of the manus prints in the holotype of Pteraichnus stokesi, and did not provide a scale in their Fig. 1 for the holotype. Measurements used here are taken from their Fig. 6a (middle fig.), assuming their scale bar = 1 cm, as no scale was specified. This assumption is supported by using the same assumption for the scale provided for the figured pes print from the same trackway (Fig. 6c, bottom), which, when applied to the figured pes print, provides a length of 78.2 mm, close to the 80 mm length stated in their diagnosis for the ichnospecies. Using the diagram and scale provided by Bennett (1997), however, the pes prints average 77.9 mm while the manus prints average about 66 mm, providing a ratio of 0.85.

^¶Manus and pes prints not from same trackway and almost certainly made by different individuals, so ratio does not necessarily represent true proportions of tracks.

^§ Wright et al. (1997) do not provide specific measurements for Purbeckopus manus prints, but do state that they are all subequal to the pes in length. However, applying the scale given in their Fig. 3b, the average manus length (using tracks M1 and M2) is 151.0 mm and the average pes length is 196.5 mm, giving a ratio of 0.77. Presumably measurements from specimens discussed but not Fig. d by Wright et al. (1997) were included in drawing their stated conclusion of approximate equality.