The abundance and diversity of penetrative tracks: a critical re-evaluation of theropod ichnotaxa

Tetrapod ichnotaxonomy aims to classify tracks based on features that reflect trackmaker anatomy. Consequently, distinct ichnotaxa are assumed to represent distinct (though often unidentified) biological taxa. However, track shape is not only determined by trackmaker anatomy, but also by the properties of the substrate, the movements of the foot, the level of exposure, and any post-formational alteration. Because of these multiple sources of variation, determining if, and to what degree, a particular feature conveys anatomical information remains a major challenge. A major source of confusion has been penetrative tracks, which form when the foot sinks deeply into soft sediment, causing sediment to flow around the foot to seal its path. Penetrative tracks of long-toed, tridactyl trackmakers often show conspicuous features that appear to reflect foot anatomy but do in fact reflect the penetrative nature of the tracks. We studied dozens of tracksites in the Middle Jurassic of El Mers, Morocco and the Early Cretaceous Cameros Basin, Spain, showing that penetrative tracks are much more diverse and common than previously thought. We discuss formational mechanisms that explain the variation of several features commonly used to define ichnotaxa. We conclude that the type ichnospecies of Saurexallopus, Magnoavipes, Theroplantigrada, Ordexallopus, and Archaeornithipus are probably based on penetrative tracks and therefore nomina dubia.