Brachycheirotherium – probably not produced by Typothorax

A recent paper on complete Typothorax specimens (Heckert et al. 2010, Fig. 1) also raised the possibility that Brachycheirotherium tracks (Fig. 1) could be made by aetosaurs like Typothorax.

Figure 1. Typothorax from Heckert et al. 2010 along with reconstructed pes, and Stagonolepis manus and pes. Brachycheirotherium track is too narrow for this tank-like taxon and digit 4 is too long.

Figure 1. Typothorax from Heckert et al. 2010 along with reconstructed pes, and Stagonolepis manus and pes. Brachycheirotherium track is too narrow for this tank-like taxon and digit 4 is too long in Brachycheirotherium compared to Typothorax. The Brachycheirotherium pedes line up on digit 2. The manus track is wider, but not as wide as reconstructed in Typothorax.

Close but no cigar.
The Typothorax pes is not the best match for Brachycheirotherium tracks. Such a narrow-gauge (pedal 2 is on the midline) would seem unlikely for such a wide tank-like reptile. The Stagonolepis pes is a better match by virtue of a relatively longer pedal digit 4. Ticinosuchus (Fig. 2) is also better by virtue of its more slender body, capable of producing such a narrow-gauge trackway in which pedal digits 2 were aligned along the midline during the step cycle, one of the traits of the Brachycheirotherium track (Fig. 1).

Figure 2. Ticinosuchus overall, hand, foot and skull.

Figure 2. Ticinosuchus overall, hand, foot and skull. Ticinosuchus nests as the ancestor taxon to aetosaurs.

References
Heckert AB, Lucas SG, Rinehart LF, Celesky MD, Spielmann JA and Hunt AP 2010. Articulated skeletons of the aetosaur Typothorax coccinarum Cope (Archosauria: Stagonolepididae) from the Upper Triassic Bull Canyon Formation (Revueltian: early-mid Norian), eastern New Mexico, USA. Journal of Vertebrate Paleontology 30 (3): 619–642.

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