An ichnotaxonomically diverse fossil tracksite with multiple Grallator trackways in the Aztec Sandstone (Jurassic) of Red Rock National Conservation Area, Las Vegas, Nevada

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Conference Proceeding

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Going LOCO: Investigations along the Lower Colorado River



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We describe and interpret a fossil tracksite in Red Rock Canyon National Conservation Area. For ichnotaxa are present, including two arthropod ichnotaxa (Octopodichnus and Paleohelcura) and two vertebrate ichnotaxa (Grallator and a possible undescribed mammloid form that we call Ichnotaxon A). The trackway surface consists of multiple bedding planes of foreset strata, totaling about 90 cm of stratigraphic thickness. The sedimentological setting was the distal, toe portion of the south-facing slip face of a massive, trough-cross-stratified, barchan-like dune that was at least 100 m wide and several tens of m tall. Five Grallator trackways are present, recording the movements of small, bipedal, carnivorous, theropod dinosaurs. The five trackways are widely separated from one another, reflecting solitary behavior. They occur on multiple bedding planes, indicating that dinosaurs repeatedly crossed this surface, over a time interval measured in years. Each Grallator trackway is oriented diagonally across the downslope direction of the dune. Calculations based on stride length and estimated hip height indicate a range of speeds and gaits, including walking, trotting, and running. The fastest speed recorded is 15.7 km/hr. These data suggest that the Grallator trackmakers were solitary hunters, and that they were traversing purposefully across the dunefield, choosing routes that were as energetically efficient as possible.