TAILS OF SAURISCHIAN DINOSAURS IN THE EARLY JURASSIC OF THE NEWARK SUPERGROUP (EASTERN NORTH AMERICA)

Footprints are by far the most common tetrapod remains in the Newark Supergroup. In the Early Jurassic portion of the section, dinosaur tracks far outnumber footprints made by other tetrapods. Tail impressions are rare, mostly associated with the ornithischian ichnotaxon Anomoepus and more rarely with saurischian trackways.

Several theropod trackways have tail drags, and the pes prints in these trackways typically show digit I to be impressed, indicating that the animal was walking more upright (during normal locomotion this digit does not reach the ground). It is probable that if these trackmakers were walking more "normally" with their weight further forward, the footprints themselves would be indistinguishable from other grallatorids (Grallator, Anchisauripus, Eubrontes). In both Gigandipus and Hyphepus (Turners Falls Fm) the tail drag is a very narrow impression (usually < 1 cm) and slightly sinuous (more so in Hyphepus). The surrounding sediment is drawn into the tail impression, in linear striae, producing a 'herringbone'-like effect (the apex towards the anterior). These striae are probably due to mud being dragged along by the tail. A single tail impression is known for the prosauropod print Otozoum (Portland Fm): it is V-shaped in cross-section with convex sides and bears the 'herringbone striae'. The pes prints are unusually deeply impressed posteriorly, suggesting that as in the theropod examples, the animal was walking very 'upright'. A footprint locality in the earliest Jurassic Passaic Fm (New Jersey) has produced numerous grallatorids (theropod), Rhynchosauroides (lacertilian) and Batrachopus (crocodylomorph), as well as two isolated tail drags. These are preserved as natural casts, and in cross-section are U-shaped with a thin (<1cm) vertical 'keel' running down the center. Irregular 'ornamentation', rather than 'herringbone striae', is present.

The Passaic examples suggest that in Gigandipus and Hyphepus we are only seeing the ventral keel of the tail, which also explains the narrow width of the tail impression relative to the footprint size. In modern reptiles the ventral surface of the tail is flat and produces a much wider impression relative to the foot size. Thus this 'keel' may be a synapomorphy at the level of the Saurischia.