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

Authors: Debra L. Mickelson a;  Martin G. Lockley b;  John Bishop c; James Kirkland d
Affiliations:   a Department of Geological Sciences, University of Colorado at Boulder, Boulder, Colorado, USA
b Geology Department, University of Colorado at Denver, Denver, Colorado, USA
c Geology Department, Marietta College, Marietta, Ohio, USA
d Utah Geological Survey, Salt Lake City, Utah, USA
DOI: 10.1080/10420940490445437
Publication Frequency: 4 issues per year
Published in: journal Ichnos, Volume 11, Issue 1 & 2 January 2004 , pages 125 - 142
Subject: Palaezoology;
Number of References: 50
Formats available: HTML (English) : PDF (English)
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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
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