Traces of Floating Archosaurs: An Interpretation of the Enigmatic Trace Fossils from the Triassic of the Tabular Cover of Southern Spain

Abstract

abstractTwenty-six enigmatic footprints composed of three elongated parallel depressions (two lateral ones and a shorter central one located forward with respect to the others) are described from Middle Triassic red beds of the Tabular Cover in southern Spain. The purpose of this work is the interpretation of these enigmatic tracks. The footprints range from 28 to 48 cm in length and 23 to 44 cm in width. Limestones on top of the footprint-bearing sandstone contain invertebrate traces such as Planolites, Thalassinoides, and Rhizocorallium, and bivalves (Pleuromya, Trigonodus, and Unionites), typical of marine and brackish environments. They indicate that these footprints developed in a coastal or nearly coastal environment. Whereas the enigmatic trace fossils correspond to tridactyl footprints, the digital marks are parallel and do not show any divarication as in typical walking footprints of archosauria. These traces were left when the trackmaker was swimming in a waterbody deep enough for floating only occasionally touching the ground with the digit tips. The tracks formed as a result of the backward sweeping of the tips when the digits came in contact with the bottom while the animal was buoyed by water. They differ from typical swim traces of other archosaurs (such as theropods) that show three longitudinal scratches because in studied tracks the axial digit (III) of the bipedal tridactyl archosaur sinks in the sediment and gives a short and deep scratch. The other digit scratches (from digits II and IV) would correspond to the upper surface of the toes. The studied tracks correspond to Characichnos ichnofacies (swimming tracks composed by parallel scratch marks) and they were made by a bipedal tridactil archosaur or some functionally tridactyl, chirothere tracemaker.