Archosaurs and Their Kin: The Ruling Reptiles

Abstract

Archosauria includes birds, crocodylians, and a number of fossil groups such as dinosaurs and pterosaurs. They first appeared in the Early Triassic and since then have dominated terrestrial ecosystems. This chapter is a compilation of the available information on the inner ear morphology of archosaur crown groups and stem groups and an exploration of different aspects of the evolution of their otic anatomy. It is still not clear whether tympanic hearing was present in the basalmost members of stem archosaur clades. However, more derived taxa show a number of modifications that certainly improved their hearing sense, such as a larger metotic foramen and a more elongate cochlea. Impedance-matching hearing appeared many times independently in archosaurs, although it is currently problematic to know at which point this happened. In theropods, impedance-matching hearing appeared before the origin of birds and was retained in the crown-group. Pneumatization must play an important role in directional hearing and is likely to have influenced skull pneumatization in crocodylians. Exquisite sound production capacities were present not only in hadrosaurids but also in ankylosaurids. Elongation of the semicircular canals seems to be linked to the acquisition of a more upright posture and a more active lifestyle in archosaurs. Many crown groups show further elongation of the canals, with birds representing an extreme condition.