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

Pterosaur tracks 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 reveals high pterosaur track densities, but low ichnodiversity, and variations in size frequency, 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 evidently 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 suggesting pronounced lengthening of stride indicating acceleration.

One well-preserved medium sized theropod trackway Therangospodus and other larger theropod track casts Megalosauripus are associated with what otherwise appears to be a nearly monospecific pterosaur track assemblage. Importantly, several of the pterosaur tracks bear 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 sametime 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.