Paper No. 97-9
Presentation Time: 3:45 PM
TURNING BACK FROM A DINOSAUR TRACK: A MAMMALIFORM TRACKWAY FROM THE EARLY JURASSIC PORTLAND FORMATION, CONNECTICUT, USA
Early Jurassic rocks from the Hartford basin of the northeastern U.S. are well known for their dinosaur tracks, but non-dinosaurian tetrapod tracks are either rare or largely unrecognized. Here, we report the discovery of a small vertebrate trackway in a fluvial arkosic sandstone slab from the type locality of the Portland Formation in Portland, Connecticut. The slab, currently on display at Dinosaur State Park (Rocky Hill, Connecticut), is best known for its exquisitely expressed sauropodomorph (Otozoum) trackway with skin imprints. We studied the new trackway through a combination of direct observations, high-resolution digital photos, and photogrammetric models, the latter of which enhanced relief of surface features. The small vertebrate tracks, which we interpret as those of a mammaliform, are near the sixth and last Otozuom on the slab. The mammaliform trackway consists minimally of 25 indistinct tracks in one of two size classes. The 16 larger ones are interpreted as pes imprints, whereas the other nine are interpreted as those of the manus. The trackway is about 36 cm long and between 32-40 mm wide. Stride lengths vary considerably in accordance with gait changes. The trackway begins as a straight-forward, full-bound quadrupedal pattern, with paired and parallel pes and manus imprints, and strides of 63-68 mm. This pattern then shifts with an abrupt left turn with significantly shorter (43-46 mm) strides, followed by a more compact bipedal bounding pattern represented by pes imprints. This transition between different gaits coincides with the trackmaker’s closest approach to the Otozoum track, which we interpret as an avoidance reaction to a sediment rim around the dinosaur track. This trackway thus reflects a small mammaliform from the Early Jurassic changing its mind while in motion, a decision influenced by its local dinosaur-impacted landscape.