The Fossil Trackway Pteraichnux Not Pterosaurian, but Crocodilian

The fossil trackway Pteraichnus saltwashensis Stokes 1957, from the Momson Formation of Arizona, originally attributed to a pterodactyloid pterosaur, is reassessed. We conclude that the assignment was incorrect because: 1 , Pteraichnus has five toes on the manus (all pterosaurs have four); and 2, pterosaurs did not walk quadrupedally. However, trackways similar in detail to the poorly preserved Pteraichnus can be simulated experimentally by a small caiman, and we suggest that Pteraichnus could have been made by a crocodilian. Experimental work on trackways, coupled with considerations of limb kinematics and substrate conditions, will permit the most robust inferences about paleoichnologic trackmakers, and will thus maximize the utility of fossil footprint data. INTRODUCTION whereas pterosaurs were digitigrade and IN 1957 Stokes described a trackway (Pterwould not have left a heel impression as seen aichnus saltwashensis) from the Morrison in Pteraichnus and the caiman tracks. FurFormation (Upper Jurassic) of Apache Counthermore, the articulation of the pterosaurian ty, Arizona, which he assigned to a "pteroforelimb (Padian, 1980) indicates that even dactyl" (=Pterodactyloidea sensu stricto) beif pterosaurs could have walked quadrupeCause of the narrow V-shaped heel, the four dally, which is unlikely, their trackways would subequal toes of the pes, and the unusual mahave differed considerably from Pteraichnus. nus print, which seemed to preserve an Second, when the Pteraichnus track is conimpression of the hypertrophied wing-finger sidered in the light of kinematics of the step (digit IV) as well as two of the three small cycle and interaction of the foot with the subdigits 1-111. Stokes added: "The apparent restrate, it corresponds in all appreciable reduction of digits in both manus and pes is spects to a similarly made trackway of a crocdistinctive and is the chief reason for placing odilian-a fact which we demonstrate the animal in the Pterodactyloidea." For experimentally with a living caiman. Our twenty years these tracks have served as the purpose in this paper is to show how these principal fossil evidence in support of the idea results might fit into a conceptual framework that when pterosaurs landed on the ground, of animal-sediment interactions, and to prothey must have walked quadrupedally (e.g., pose criteria for paleoichnologic analysis. Wellnhofer, 1978). Stokes' taxonomic inference on the basis PTERAICHNUS: DATA AND MEASUREMENTS of these tracks was ingenious, but we think Stokes' (1957) reconstruction of the Pterit must be called into question on at least two aichnus trackway, shown in Figure 1B, congrounds. First, detailed studies of anatomy sisted of a manus print of variable length, and functional morphology show that the averaging around 3% inches (8.3 cm), and a Pteraichnus tracks could not have been made pes approximately three inches long (7.5 cm). by a pterosaur. There are not four digits on Stokes described the manus print as a deep the manus print of Pteraichnus, as Stokes beimpression formed by the wing knuckle, with lieved, but five, although all five are not alshallower impressions of two of the three ways clearly preserved. This automatically smaller digits splayed laterally (not medially, removes pterosaurs, which have only four although they are the medial digits). A longer digits, from eligibility as possible trackmakposterior process of this track was taken for ers of Pteraichnus. Crocodiles, like pterodacthe impression of the wing-finger. tyloids, have a four-toed pes with a V-shaped There is some confusion in Stokes' mea"heel." However, crocodiles are plantigrade, surements of the trackway, which should be Copyright