PALEOECOLOGY OF AN EARLY CRETACEOUS DINOSAUR TRACKWAY SITE, UPPER GLEN ROSE FORMATION, SATTLER, TEXAS

In the early 1980s, a surface containing more than 400 individual dinosaur tracks was exposed in the Texas Hill Country near Sattler in Comal County about 50 miles south of Austin, Texas. Only in the last few years has scientific work begun at the site. Approximately 100 million years old, the track layer is located in the upper Glen Rose Formation, a thick, fossiliferous, Lower Cretaceous, marine/marginal-marine unit that is composed largely of chalky limestones, marls and micrites. The trackway site is preserved in a micritic carbonate containing birdseyes, desiccation cracks and ladder-back ripples and is overlain by possibly bentonitic marl containing plentiful clams and plant fragments. Tracks include those made by both herbivores, thought to be Iguanodon-like dinosaurs, and carnivores, thought to be Acrocanthosaurus. Trackways exposed at the site are generally parallel and are oriented northeast, although randomly oriented tracks and trackways are also present. Track-maker identifications are based on Iguanodon and Acrocanthosaurus fossils that are known from equivalent units in southeast Oklahoma. The track site is located on the San Marcos Platform, a small carbonate platform developed on northwest-southeast-trending folds plunging southeasterly from the Llano Uplift to the west. The Glen Rose at this site represents cyclically alternating progradational cycles, resulting from repeated shoreline progradation into shallow, coastal embayments along the Cretaceous Interior Seaway. In this particular situation, it appears that herbivorous dinosaurs were using supratidal mudflats to access coastal marshes, which eventually prograded out over the flats with sea-level rise; carnivorous dinosaurs apparently followed. Although supratidal micrites are not common in this part of the section, this interesting occurrence suggests that some combination of lowstand and the structurally high nature of the small platform may have been responsible for the trackway development, whereas subsequent, rapid flooding may have contributed to its preservation. Hence, subsurface structure and its probable influence on Cretaceous coastal geomorphology may be important in explaining the origin and location of some trackway sites like this.