Morphological variations in dinosaur tridactyl tracks from the Candeleros Formation (Upper Cretaceous) of northwestern Patagonia, Argentina

Several dinosaur tridactyl tracks showing inter- and intra-trackways morphological variations from the Cenomanian Candeleros Formation of the Neuquén Basin, Argentina, have been documented. Footprints occur on three different stratigraphic levels and are impressed on medium-grained sandstone of fluvial origin. Level 1 includes deep tridactyl tracks with metatarsal impressions (average length of 31 cm) preserved as true tracks and undertracks that could have been generated due to simply sinking of the pes into a soft substrate and also probably by a continuous walk in a squatting position. Level 2 shows the largest tridactyl tracks (average length of 58 cm) recognized in this work, situated a few meters close to an already studied sauropod trackway. The different preservation between these bipedal and quadrupedal tracks could be explained due to different substrate properties and dissimilarities of the pedal shape between trackmakers. Level 3 comprises small to large tridactyl tracks (length from 6 to 34 cm) including intra-trackway morphological variation, that is interpreted as a result of changes in the substrate properties, mainly moisture content, along the tracking surface. Taking into account that the footprints studied here are poorly preserved, they were not assigned to any particular ichnotaxon. The large tridactyl tracks have been attributed to carcharodontosaurid theropod trackmakers while medium and small tracks to undetermined bipedal dinosaur. One pair of these large theropod tracks was closely documented on the same tracking surface of the aforementioned studied sauropod trackway, attributed to rebbachisaurids. This suggests that both carcharodontosaurid and rebbachisaurid trackmakers coexisted in the same habitats as the osteological record noted. This tracksite represents an example of the morphological variability of dinosaur tridactyl tracks controlled mainly by the substrate properties, and how an approach aimed at understanding footprint formation and preservation is necessary before beginning ichnotaxonomical studies, among others.