A New Pterosaur Tracksite from the Jurassic Summerville Formation, Near Ferron, Utah

Abstract

Pterosaur tracks (cf. Pteraichnus) from the Summerville Formation of the Ferron area of central Utah add to the growing record of Pteraichnus tracksites in the Late Jurassic Summerville Formation and time-equivalent, or near time-equivalent, deposits. The site is typical in revealing high pterosaur track densities, but low ichnodiversity suggesting congregations or “flocks” of many individuals. Footprint length varies from 2.0 to 7.0 cms. The ratio of well-preserved pes:manus tracks is about 1:3.4. This reflects a bias in favor of preservation of manus tracks due to the greater weight-bearing role of the front limbs, as noted in other pterosaur track assemblages. The sample also reveals a number of well-preserved trackways including one suggestive of pes-only progression that might be associated with take off or landing, and another that shows pronounced lengthening of stride indicating acceleration.

One well-preserved medium-sized theropod trackway (Therangospodus) and other larger theropod track casts (cf. Megalosauripus) are associated with what otherwise appears to be a nearly monospecific pterosaur track assemblage. However, traces of a fifth pes digit suggest some tracks are of rhamphorynchoid rather than pterodactyloid origin, as usually inferred for Pteraichnus. The tracks occur at several horizons in a thin stratigraphic interval of ripple marked sandstones and siltstones. Overall the assemblage is similar to others found in the same time interval in the Western Interior from central and eastern Utah through central and southern Wyoming, Colorado, northeastern Arizona, and western Oklahoma. This vast “Pteraichnus ichnofacies,” with associated saurischian tracks, remains the only ichnological evidence of pre-Cretaceous pterosaurs in North America and sheds important light on the vertebrate ecology of the Summerville Formation and contiguous deposits.

Keywords:

• Pterosaur tracks
• Upper Jurassic
• Summerville Formation
• Utah

ACKNOWLEDGEMENTS

The work was conducted under Utah state permit # 02-336. The authors would like to thank Mike Getty, Scott Sampson, and Josh Smith of the University of Utah and the Museum of Natural History, William Stokes, Utah Geological Survey, Masaki Matsukawa, visiting professor at CU Denver, Ken Cart and Gretchen Minney, friends of the CU Denver Dinosaur Tracks Museum for their active help and involvement in facilitating this research. We also thank Laurie Bryant and Mike Leschin from the Bureau of Land Management. Kevin Padian, University of California, Jerry Harris, University of Pennsylvania and Steve Hasiotis, University of Kansas, provided, helpful and challenging reviews. Also a very special thanks to Katura Reynolds for her illustrations of the tracks and to Mike Stockwell of “Cranking Graphics” for his computer graphics, and to Karen Cloward and Ryan King for their engaging conversations relating to pterosaur foot morphology of Rhamphorhynchus and for field assistance.