Jaw biomechanics in the South American aetosaur Neoaetosauroides engaeus

Desojo, J.B., and S.F. Vizcaino. 2009. Jaw biomechanics in the South American aetosaur Neoaetosauroides engaeus. Paläontologische Zeitschrift online first. doi:10.1007/s12542-009-0032-6.

Abstract-The function of the jaw apparatus and the possible dietary habits of the aetosaur Neoaetosauroides engaeus from the Triassic of South America were analyzed in comparison with Northern Hemisphere aetosaurs Desmatosuchus haplocerus and Stagonolepis robertsoni and the living short-snouted crocodile Alligator mississippiensis. The adductor and depressor jaw musculature of these was reconstructed on the basis of dental and skeletal comparisons with living closest relatives’ extant phylogenetic bracket (EPB), followed by the analysis of the moment arms of these muscles to infer feeding habits. The aetosaurian skull design indicates that the total leverage of the inferred jaw musculature provides force rather than speed. However, within aetosaurs, the high ratios of muscle moment arms to bite moments indicate stronger bites in the northern Hemisphere forms, and faster ones in Neoaetosauroides. These differences indicate more developed crushing, chopping, and slicing capacities, especially at the back of the tooth series for D. haplocerus and S. robertsoni; whereas it opens a window to consider different abilities in which speed is involved for N. engaeus. There are differences among aetosaurs in dental characteristics, position of the supratemporal fenestra, location of the jaw joint relative to the tooth row, and shape of the lower jaw. Neoaetosauroides does not show evidence of dental serrations and wear facets, probably consistent with a relatively soft and non abrasive diet, for example soft leaves and/or larvae and insects without hard structures. It might be possible that Neoaetosauroides represents a tendency towards insectivorous feeding habits, exploiting a food source that was widespread in continental environments throughout the Triassic.

More evidence is presented in this paper that aetosaurs may have been insectivorous or omnivorous, rather than solely herbivorous, an idea that was first raised by Bryan Small back in 2002 for Desmatosuchus spurensis. Note that I consider Episcoposaurus (=Desmatosuchus) haplocerus to be a nomen dubium, thus this material is actually referable to the original type species of Desmatosuchus, D. spurensis (Case, 1920; Parker, 2008).

REFERENCES

Case, E. C , 1920. Preliminary description of a new suborder of phytosaurian reptiles with a description of a new species of Phytosaurus. Journal of Geology 28:524-535.

Parker, W.G. 2008. Description of new material of the aetosaur Desmatosuchus spurensis (Archosauria: Suchia) from the Chinle Formation of Arizona and a revision of the genus Desmatosuchus. PaleoBios 28: 1-40.

Small, B.J. 2002. Cranial anatomy of Desmatosuchus haplocerus (Reptilia: Archosauria: Stagonolepididae). Zoological Journal of the Linnean Society 136: 97-111.