A large bipedal digitigrade Triassic pterosaur: evidence from footprints

Figure 1. Click to enlarge. (Along the bottom) Unnumbered and unnamed three-toed Late Triassic track (Conrad et al. 1987) here matched to the pes of a tiny, short-legged Triassic Eudimorphodon scaled up to match the tracks. Also shown to scale and to match tracks is a long-legged pterosaur, Austriadactylus, with relatively small feet, but the toes don’t match quite so well. Note: pedal digit 1 is short and so does not impress in this configuration with elevated proximal phalanges. This may represent a running track as only the distal phalanges impress.

Conrad et al. (1987)
illustrated an unnumbered and unnamed bipedal, digitigrade track with a stride length of one meter and the distance from the midline 3x the track width.

The only pedal match
I can come up with is to a tiny, possible juvenile, pterosaur Eudimorphodon cromptonellus, which had a short digit 1, a long digit 4 and an elevated proximal set of phalanges. It’s a good match only if the pterosaur was quite a bit larger (Fig. 2). Digit 5 was held hyper-flexed, off the substrate (Fig. 1). Digit 1 did not impress.

No archosauriform
and no bipedal archosauriform of the Late Triassic had sub-equal digits 2-4. E. cromptonellus has been described as diminutive and a possible juvenile or hatchling. If a hatchling, the 8x larger adult would still be half the size of the hypothetical trackmaker (Fig. 2).

Figure 2. Click to enlarge. If Eudimorphodon cromptonellus is a hatchling, then an 8x larger adult is only half the size of the hypothetical trackmaker.

A first
This would represent the first instance of a eudimorphodontid pterosaur in western North America. The large size is interesting because one anurognathid pterosaur known from western North America, Dimorphodon? weintraubi, is also much larger than European and Asian anurognathids. The other anurognatid, Mesadactylus, is more typical in size. An Asian anurognathid, currently represented by the IVPP embryo, is also 8x larger than European and Asian anurognathids. We learned earlier that size increase and decrease is a hallmark of pterosaur evolution.

Bipedal and digitigrade
These tracks, if indeed pterosaurian, confirm the origin hypothesis of Peters (2000a, b) as derived from bipedal and digitigrade fenestrasaur ancestors with elevated proximal phalanges and confirms the same for basal pterosaurs. The slight lateral angle of the digits and the width of the track confirms the lepidosaur/tritosaur ancestry of this ichnotaxon, provided with a splayed lepidosaurian femur.

Sorry
I didn’t consider or recognize this specimen in Peters (2011), the catalog of pterosaur pedes for trackmaker identification. This would have added to other digitigrade pterosaur tracks, most matched to anurognathids.

References
Conrad K, Lockley MG and Prince NK 1987. Triassic and Jurassic vertebrate-dominated trace fossil assemblages of the Cimarron Valley Region: Implications for paleoecology and biostratigraphy. New Mexico Geological Society Guidebook. 38th Field Conference, Northeastern New Mexico 1987. 127-138.
Jenkins FA Jr, Shubin NH, Gatesy SM and Padian K 1999. A primitive pterosaur of Late Triassic age from Greenland. Journal of the Society of Vertebrate Paleontology 19(3): 56A.
Jenkins FA Jr, Shubin NH, Gatesy SM and Padian K 1999. A diminutive pterosaur (Pterosauria: Eudimorphodontidae) from the Greenlandic Triassic. Bulletin of the Museum of Comparative Zoology, Harvard University 155(9): 487-506.
Peters, D 2000a. Description and Interpretation of Interphalangeal Lines in Tetrapods. Ichnos, 7: 11-41.
Peters, D 2000b. A redescription of four prolacertiform genera and implications for pterosaur phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106: 293-336.
Peters D 2011. A Catalog of Pterosaur Pedes for Trackmaker Identification
Ichnos 18(2):114-141. http://dx.doi.org/10.1080/10420940.2011.573605

wiki/Eudimorphodon