DISTINGUISHING THE TRIDACTYL DINOSAURIAN ICHNOGENERA ATREIPUS AND GRALLATOR: WHERE ARE THE LATEST TRIASSIC ORNITHISCHIA IN THE NEWARK SUPERGROUP?

The ichnogenera Atreipus and Grallator are superficially very similar: tridactyl pes prints with low divarication; however Atreipus is considered the print of an ornithischian while Grallator is a theropod track. In the Newark Supergroup (Late Triassic-Early Jurassic, eastern North America) both taxa first appear in the Carnian; Grallator survives the end-Triassic extinction event, but the last records of Atreipus date from the early part of the Rhaetian. Early Jurassic ornithischians are represented by Anomoepus, a tridactyl track easily distinguishable from Grallator. There is no footprint record of ornithischians from the later portion of the Rhaetian, yet their Early Jurassic presence in eastern North America suggests their presence up to the Triassic-Jurassic boundary. We speculate that the similarity in form between Grallator and Atreipus has led to the misidentification of Atreipus as Grallator, and that ornithischians did not become locally extinct before the end-Triassic mass extinction. The two taxa are distinguished in quadrupedal trackways, in which the manus is clearly ornithischian; Grallator manus prints are not known. However in bipedal Atreipus the pes is extremely similar to that of Grallator. To test this hypothesis we compare Atreipus and Grallator both qualitatively and quantitatively (using osteometric ratios), and redescribe both taxa in part, to ascertain if the two taxa can be reliably distinguished when manus prints are absent. Typically the phalangeal pads in Atreipus coalesce; there is a metatarsal-phalangeal pad on digit II; and the pes is more robust than that of Grallator.