Patterns and processes in the early evolution of the tetrapod ear

Abstract

This article reviews some of the latest information on the evolution of the tetrapod ear region as seen in the fossil record. It looks at the changes that can be documented across the fish-tetrapod transition, the patterns that they show and what can be inferred of the processes that brought some of them about. These processes include an increased role for neural crest, and heterochronic processes such as pedomorphosis. The earliest tetrapods show a common pattern of a short stout stapes with a large stapedial foramen, that primitively contacted the palatal bones and probably supported the braincase. Modifications to this pattern can be seen in tandem with changes to the occiput and are bound up with changes to jaw and breathing mechanisms. By the Late Carboniferous, tetrapods had diversified into a range of groups showing a wide variety of otic morphologies, some of which were probably tympanic, while others were not, and some which are very different from those found in extant tetrapods. In amniotes, the evolution of a tympanic ear appears to correlate with consolidation and integration of the occiput to the skull roof. Competing phylogenies suggest different numbers of iterations for the origin of a tympanic ear, but a minimum of four separate occasions is implied.