The origin of the mammalian middle ear (MME) was an extreme transformation in the evolution of mammals from reptile-like ancestors. Initially devoted to feeding, the angular, surangular, articular, and quadrate bones of the ancestral reptilian jaw joint were “re-tooled” and miniaturized to form the tympanic ring, hammer, and anvil, which are entirely dedicated to conducting sound towards the inner ear. MME detachment – which perhaps independently arose in monotremes and other mammals – included gradual evolution of a secondary jaw joint via a dual-joint intermediate, starting approx. 250 million years ago. Strikingly, this process recapitulates during development, particularly in marsupials and monotremes, whose MME only detaches after the first postnatal weeks.
Funded by Hermon Slade Grant (HSF12/8) and a Discovery Grant (DP140102656), Prof. Stephen Wroe (UNSW) and I are applying an interdisciplinary, developmentally focused approach to characterize and quantify MME evolution with the help of UQI/UQ Centenary Scholarship funded PhD student Héctor Ramirez-Chaves. This project exploits Australia’s unique position of being home to the vast majority of marsupial species and the only two extant monotreme species to provide the first large scale, developmentally integrated investigation of how the mammalian middle ear evolved.
The middle ear of mammals consists of miniature versions of the bones that form the lower jaw joint in reptiles like Agamas (top left; scan source: www.digimorph.org). Fossils of early mammalian relatives as old as 245 million years, like Thrinaxodon liorhinus (top right; scan source: www.digimorph.org) show that this jaw joint became increasingly specialized for hearing, but still retained its chewing function. A similar transitional arrangement is still visible in the early development of marsupials like newborn rat kangaroos (bottom left) before the bones assume their exclusive hearing function in grown animals (an adult rat kangaroo is depicted at bottom right; rat kangaroo specimens were provided by the South Australian Museum). Green, Dentary; Pink, Angular/Ectotympanic; Turqoise, Articular/Hammer; Purple, Quadrate/Anvil.