A perspective from the Mesozoic: Evolutionary changes of the mammalian skull and their influence on feeding efficiency and high-frequency hearing.

Abstract

The complex evolutionary history behind modern mammalian chewing performance and hearing function is a result of several changes in the entire skeletomuscular system of the skull and lower jaw. Lately, exciting multifunctional 3D analytical methods and kinematic simulations of feeding functions in both modern and fossil mammals and their cynodont relatives approach this topic, giving fresh insights into the history of mammalian masticatory behaviors and their evolutionary trends. One crucial transformation in this context is the segregation of postdentary bones (becoming the mammalian middle ear) from the lower jaw, which is posited to have led to the important functional decoupling of the hearing and feeding systems. Evolution of the middle ear is regarded as the key transition that enhanced both mammalian chewing performance and hearing capacity. Three major functional parts undergo substantial evolutionary changes in this process that are anatomically linked to each other: the lower jaw and dentition, middle ear, and inner ear. Sound, transmitted via vibrations of the bony middle ear elements to the inner ear, is converted into movements of the endolymph fluid that shift hair cells of the organ of Corti, triggering neural stimuli perceived as hearing. Structural changes in one part of the system influence the function of the other two. In this review, I highlight recent advances in research focusing on the enhancement of both chewing performance and hearing ability in mammalian history to feature the mechanisms that led to the decoupling of the hearing system (i.e., middle and inner ear) from the feeding system.