LARGE ENIGMATIC TRACE FOSSILS FROM THE GLEN ROSE FORMATION (LOWER CRETACEOUS), TEXAS

The Glen Rose Formation of Texas is famous for its dinosaur trace fossils, but little has been published on other trace fossils from this formation. The Heritage Museum of the Texas Hill Country tracksite in Comal County, Texas, discovered in 1982, preserves three parallel, linear features, in addition to several long tridactyl dinosaur trackways.

The trackbearing horizon is a peloidal micritic fossiliferous limestone. The fossils include foraminifera, ostracodes, and dasycladacean algae. These limestones are interbedded with layers of clay. The depositional environment is interpreted as low energy marginal marine-shoreline, which was exposed at least occasionally.

The dinosaur trackways have been tentatively identified as theropodan. Most of the trackways are oriented northeast-southwest. This might indicate the presence of a geographic barrier such as a shoreline.

Two of the linear features are close together, c. 0.6 meters apart. The third is about 4 meters northeast of the other two. Each is composed of a series of elongate imprints, about 20cm wide, oriented perpendicular to the trackway axis. They are preserved in concave epirelief and show a raised area on one side, interpreted as a back push-up mound which thus provides directional information on the trackway. The imprints are slightly concave anteriorly. All three are oriented northwest-southeast, roughly perpendicular to the main trend of the dinosaur trackways.

Several organisms have been suggested as the producers of these enigmatic trails: arthropods (crustaceans), gastropods or turtles. The sheer size of these features is an argument against invertebrates. Although huge gastropods seem to have been present they do not usually produce straight or parallel trails. If a shoreline did exert directional control on the dinosaur trackways it did not act in the same way on the producers of these features, and modern crabs and turtles come up on shore regularly. Turtles are known from the Glen Rose Formation of a size to produce such trails. These trails do not preserve the central tail drags often seen in modern turtle trails but this is not unexpected in light of the poor preservation of the associated dinosaur tracks. These would be the first turtle trails reported from the Glen Rose Formation.