Late Triassic Aetosaurs as the Trackmaker of the Tetrapod Footprint Ichnotaxon Brachychirotherium

Abstract

Brachychirotherium is the common ichnogenus of Late Triassic chirothere footprints well known from western Europe, North America, Argentina and South Africa. Although it has long been agreed by most workers that the trackmaker of Brachychirotherium was a derived crurotarsan archosaur, the trackmaker has been identified as either a rauisuchian or an aetosaur, and some workers attribute it to a primitive crocodylomorph (sphenosuchian). New knowledge of the osteology of the manus and pes of a large aetosaur, Typothorax coccinarum, indicates a close correspondence between the manus and pes structure of aetosaurs and the morphology of Brachychirotherium. Furthermore, functional analysis of complete skeletons indicates aetosaurs plausibly placed their feet in the narrow gauge, nearly the overstepped walk characteristic of Brachychirotherium. Brachychirotherium and aetosaurs have matched distributions, that is, they were Pangea-wide during the Late Triassic. The manus and pes morphology of rauisuchians and early crocodylomorphs (sphenosuchians) deviate from Brachychirotherium footprint morphology in key features, thus excluding their identification as trackmakers. Aetosaurs made Brachychirotherium footprints.

Keywords:

• Aetosaur
• Brachychirotherium
• Triassic
• Typothorax
• Footprint
• Chirothere

ACKNOWLEDGMENTS

Paul Sealey and Scott Sucher discovered the skeletons of Typothorax coccinarum that are now in the NMMNH collection. Matt Celeskey drew Figure 3. Bill Amaral facilitated access to the MCZ collection. Travel to the MCZ by Heckert was supported by an Appalachian State University Research Council Grant. Hendrik Klein helped with literature and images of Brachychirotherium. An anonymous reviewer and Murray Gingras provided helpful comments on a previous version of this manuscript.