PROBABLE STEGOSAUR TRACKS FROM THE JURASSIC MORRISON FORMATION, CURECANTI NATIONAL RECREATION AREA, COLORADO

Stegosaurs are prominently featured in Jurassic reconstructions, but attributable tracks from these animals are rarely described. A majority of these examples that are attributable to a stegosaurian thyreophoran are Deltapodus from European and Chinese sites. This study documents thirty-eight pes prints and three potential manus prints assignable to Deltapodus from the Kimmeridgian-Tithonian Brushy Basin Member of Morrison Formation in Curecanti National Recreation Area (CNRA) of southwest Colorado.

The tracks occur as slumped natural casts of very fine to fine grained sandstone from the base of thin sandstone beds, interpreted to be crevasse splay deposits in a 20m² area. The natural casts preserve in a wide variety of taphonomic states from ovate outlines to that of the characteristic triangular shape from which the ichnogenus Deltapodus name is defined. Well defined tracks show pes tracks with mesaxonic, tridactyl morphology and potential manus prints have an entaxonic, crescent form with a pollex trace preserved. Some pes prints show 1 to 3 mm striations, which are interpreted to be slide marks generated by the indenters’ skin tubercles against the muddy substrate during emplacement.

The CNRA specimens are attributable to Deltapodus brodricki based on length/width ratio data, which is one of the major differentiators of the three Deltapodus ichnospecies. Additionally, the manus prints share a similar shape with known D. brodricki. The pes prints are 29.9-50.2 cm long (41.2 cm average) and 18.3-33.1 cm wide (26.9 cm average), with length/width ratios of 1.31-1.74 cm (1.54 cm average) that is most consistent with D. brodericki. Deltapodus curriei differ in that they have smaller length/width ratio values. Deltapodus iberica has similar length/width ratios but does not appear to have a prominent pollex. There are few specimens of stegosaurian attributable tracks such as Deltapodus globally. The CNRA specimens provide more examples which may contribute to eventually defining allometric relationships in these ichnogenera, as well as helping to understand the taphonomic variations of D. brodericki as an ichnospecies. Additionally, this provides data that stegosaurs locally inhabited interdistributary floodplain areas during periods of humid climates during the Late Jurassic.