A NEW TYPE OF VERTEBRATE TRACE FOSSIL FROM THE LOWER JURASSIC PORTLAND FORMATION OF SOUTH HADLEY, MASSACHUSETTS

A Eubrontes trackway from the Lower Jurassic Portland Formation (Connecticut Valley) of South Hadley, MA shows a drag line associated with a trackway consisting of four Eubrontes impressions. This particular trackway once lent support to incorrect theories of bipedal dinosaurs as sluggish, tail-dragging creatures. Our results indicate that the linear drag structure is intimately related to the footprints and was formed after each footstep was taken. The width of the linear impression varies along the length of the trackway, but along its length it seems to be narrower than one would expect for the tail drag mark of an animal that made Eubrontes tracks. We propose here that this represents a tail drag mark of a prey (or scavenged) organism that was carried in the jaws of the predatory (possibly dilophosaurid) trackmaker. The tail of the carcass extended between the legs of the primary trackmaker. Evidence in support of this concept includes the following. The stride length is relatively short considering the size of the Eubrontes tracks, suggesting it was carrying something. The hallox impressions of the theropod tracks are visible, indicating that the animal was balanced toward its heels, further evidence that it was transporting a carcass. Crucially, the drag impression itself changes width along the length of the trackway, indicating that it represents the impression made by a slightly rotating tail of a dead prey tetrapod in transport, rather than the tail of the carnivorous therapod. Thus the drag impression was indeed formed by a tail, but not the tail of the Eubrontes trackmaker. The identity of the carried carcass species is unknown, but the body size and necessarily long tail is a good match to the coelophysoid dinosaur Podokesaurus holyokensis Talbot, 1911, known from skeletal remains in the Portland Formation of South Hadley, MA. The tail drag structure thus constitutes a new type of repichnid ichnofossil formed by the tail of a deceased tetrapod as it is dragged or carried by a larger predator or scavenger.