A walk in the maze: Variation in Late Jurassic tridactyl dinosaur tracks - A case study from the Late Jurassic of the Swiss Jura Mountains (NW Switzerland)

Abstract

Background. Minute to medium-sized (FL less than 30 cm) tridactyl dinosaur tracks are the most abundant in the Late Jurassic tracksites of Highway A16 (Reuchenette Formation, Kimmeridgian) in the Jura Mountains (NW Switzerland). During excavations, two morphotypes, one gracile and one robust, were identified in the field. Furthermore, two large-sized theropod ichnospecies (Megalosauripus transjuranicus and Jurabrontes curtedulensis) and an ornithopod-like morphotype (Morphotype II) have recently been described at these sites. Methods. The quality of preservation (preservation grade), the depth of the footprint, the shape variation and the footprint proportions (FL/FW ratio and mesaxony) along the trackways have been analysed using 3D models and false-colour depth maps in order to determine the exact number of morphotypes present in the tracksites. Results. The study of the footprints (n = 93) collected during the excavations has made it possible to identify and characterize the two morphotypes distinguished in the field. The gracile morphotype is mainly characterized by a high footprint length/width ratio, high mesaxony, low divarication angles and clear, sharp claw marks and phalangeal pads (2-3-4). By contrast, the robust morphotype is characterized by a lower footprint length/width ratio, weaker mesaxony, slightly higher divarication angles and clear, sharp claw marks (when preserved), whereas the phalangeal pads are not clearly preserved although they might be present. Discussion. The analysis does not allow the two morphotypes to be associated within a morphological continuum. Thus, they cannot be a