Evidence for high-frequency hearing in a Permian stem reptile.

Abstract

High-frequency hearing evolved in most amniote lineages in the Permian and Triassic. However, for reptiles, the evolutionary history of hearing is more elusive. Some have suggested that tympanic hearing evolved independently in each major lineage of reptile, largely based on gross observations of the stapes and outer ear region of fossils. On that basis, it was postulated that many Permian stem reptiles could not hear high frequencies. However, this hypothesis lacks quantitative support. To our knowledge, biomechanical analyses have not previously been conducted to determine the capacity for hearing in stem reptiles. Here, we conduct functional and morphometric analyses of the reptilian stapes including fossil data. Using harmonic response analyses, we demonstrate that the stapes of the stem reptile Youngina capensis could be displaced proximodistally at values similar to tympanic squamates. Additionally, Y. capensis is predicted to possess a tympanum based on a flexible discriminant analysis, while early-diverging stem reptiles are classified as low-frequency bone conductors. We show that hearing was facilitated by the formation of the annular ligament, improving stapedial movement, in addition to the evolution of a slender stapes and tympanum. Our results suggest that relatively high-frequency hearing was present in the common ancestor of reptiles, originating by the late Permian.