Banisterobates boisseaui

Fraser and Olsen 1996

Evidence: A single set of three-toed footprints, with small, inward facing hands preserved by two of the three tracks.

Holotype: VMNH 202

Carnian

Dry Fork Formation

Virginia; USA

Biology: 1 meter long – 1 kilogram

It is practically impossible to guess the size of an animal known from a tiny set of diminutive tracks (less than 2 millimeters in length). The describers even note that they might be confused with the tracks of a horseshoe crab (Fraser and Olsen 1996). The hands are much smaller than the hind feet, however, so it was probably partially bipedal. Assuming a heterodontosaur origin for the tracks, it would imply that even juveniles of the family were at least partially quadrupedal, in spite of the smaller forearms. It would have been an interesting, almost unnatural looking posture, but other tracks presumably belonging to heterodontosaurs have handprints.

Another interesting implication is their presence in a Carnian formation in North America. It is a very deep stratum for any dinosaur, let alone a primarily Jurassic family (Heterodontosauridae).

Evolution:

At first glance, the tracks seemed to me to belong to a theropod, but some small ornithopods have similar tracks (presumably). The handprints are reminiscent of Atreipus, which I once believed was a heterodontosaur, though they seem to be angled inward much more medially. Another possibility is a non-dinosaur dinosauromorph, but it is definitely within that clade, because no other archosaurs have feet that would match the exceptionally bird-like tracks. As Fraser and Olsen (1996) noted, it doesn’t really match any of the better-known dinosauromorphs (lagosuchids), leaving Ornithopoda as the most probable track-maker in this instance. In my own opinion, I would consider it a juvenile heterodontosaur, fabrosaur, or lesothosaur. Unfortunately, there aren’t any bones of these animals present anywhere else in North America, so it might belong in its own family of ornithopods (possibly including some of the Atreipus tracks, since some of those do not appear very theropod-like, as is usually presumed). If this was an ornithopods, it must have adopted a rather hunched, arch-backed posture to produce the tracks. However, we must assume that the tracks were made as the animal moved the way it would if it was a healthy individual, so a long-bodied ornithopods is unlikely. I lean toward the Heterodontosauridae because that family seems to be the best candidate for quadrupedal movement, without looking too unnatural. As a heterodontosaur, it was among the most unique, even as a juvenile, and probably deserves its own subfamily or better.

References:

D’Orazi Porchetti, S., U. Nicosia, P. Mietto, F. M. Petti, and M. Avanzini. 2008.

“Atreipus-like Footprints and their Co-occurrence with Evazoum from the Upper Carnian (Tuvalian) of Trentino-Alto Adige.” Studi Trentini li Scienze Naturali, Acta Geologica 83: 277-287.

Fraser, N. C., and P. E. Olsen. 1996. “A New Dinosauromorph Ichnogenus from the

Triassic of Virginia.” Jeffersoniana 7: 1-17.

Olsen, P. E., and D. Baird. 1986. “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: NY, Ney York. 61-87.