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Atreipus

Atreipus is another ichnofossil from the Newark Super Group - clearly a glorious fossil location for dinosaur tracks! Tracks have been found from the Gettysburg Shale in Adams County Pennsylvania, the passaic Formation of Milford, New Jersey, and the Wolfville Formation of Kings County, Nova Scotia. It has three digits with poorly distinguished pads, however they can be found in some specimens.

The oldest Atreipus fossils ha been found in the Carnian age of the Triassic, and the youngest from the Norian age of the Triassic, straddling a time from of about 9.6 million or 12.4 million years between them. Its footprint features, however, are unknown in any skeletons, and thus trying to assign this footprint to any type of animal has been a struggle - it has been found to be completely incompatible with any known theropods, though it clearly was a very derived archosaur footprint. Hypotheses abound for an early Ornithischian or even a basal dinosaur, or even later Ornithischians like Tenontosaurus (well, something like it, since it came from the Cretaceous).

It was somewhat similar to Anomoepus though Anomoepus is much more in line with Ornithischians than Atreipus is. What can be concluded is that Atreipus was a dinosaur, or somthing very close to it, but it could either be a more derived ornithischian than Heterodontosaurus or a dinosaur more basal than Saurischians, such as a dinosaur belonging to neither Ornithischia or Saurischia. It had some beginning quadrupedal adaptations and no really close skeletal analogues are known.

Sources (Images and text):

https://en.wikipedia.org/wiki/Atreipus

Olsen, P. ., and Baird, D. 1996. The ichnogenus Atreipus and its significance for Triassic Biostratigraphy: in K. Padian (ed.), The Beginning of the Age of Dinosaurs, Faunal Change Across the Triassic-Jurassic Boundary, Cambridge University Press, New York, p. 61-87.

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