The Ancestral Middle Ear

Exploring ancestral phenotypes and evolutionary development of the mammalian middle ear based on Early Cretaceous Jehol mammals

“…Attachment of the ectotympanic bone to the otic region and incorporation of the malleus-incus complex in the ossicular chain are two key events in the evolution of the mammalian middle ear; the former holds the tympanic membrane and the latter forms a functionally semi-independent unit that resulted in an increased bandwidth of hearing, particularly of high-frequency sounds [ 1 ]. Because the incus (quadrate) is recessed at the periotic in the cranium, it served as the anchor point for the gradual evolutionary shift of the malleus (articular), the gonial (prearticular) and the ectotympanic (angular) away from the dentary bone and its relocation at the base of the cranium [ 1 ].…”

“…Attachment of the ectotympanic bone to the otic region and incorporation of the malleus-incus complex in the ossicular chain are two key events in the evolution of the mammalian middle ear; the former holds the tympanic membrane and the latter forms a functionally semi-independent unit that resulted in an increased bandwidth of hearing, particularly of high-frequency sounds [ 1 ]. Because the incus (quadrate) is recessed at the periotic in the cranium, it served as the anchor point for the gradual evolutionary shift of the malleus (articular), the gonial (prearticular) and the ectotympanic (angular) away from the dentary bone and its relocation at the base of the cranium [ 1 ]. Thus, in addition to understanding the homology of the auditory bones, as reviewed by several authors [ 2 , 3 ], a focused subject in recent paleontological and developmental studies is how these jaw bones were detached from the dentary, which primarily concerns the role played by the Meckel's cartilage and the developmental genetic mechanisms regulating these processes [ 4–12 ].…”

“…In contrast, the ancestral phenotypes of the mammalian middle ear remain little known due to rareness of fossils. The middle ear of Didelphis was considered as the ancestral ear type for therians [ 1 , 13 ], but the saddle-shaped incudomalleolar joint [ 1 , 14–17 ] is already specialized for mammals as a whole. Similarly, the abutting contact of the malleus and incus in monotremes [ 18 , 19 ] is also peculiar [ 1 ], even though a similar pattern was claimed to be present in the Mesozoic multituberculate Jeholbaatar [ 20 ] and the eutriconodontan Yanoconodon [ 7 ].…”

“…The middle ear of Didelphis was considered as the ancestral ear type for therians [ 1 , 13 ], but the saddle-shaped incudomalleolar joint [ 1 , 14–17 ] is already specialized for mammals as a whole. Similarly, the abutting contact of the malleus and incus in monotremes [ 18 , 19 ] is also peculiar [ 1 ], even though a similar pattern was claimed to be present in the Mesozoic multituberculate Jeholbaatar [ 20 ] and the eutriconodontan Yanoconodon [ 7 ]. A morphological gap exists between the primary quadroarticular synovial jaw joint of non-mammalian cynodonts [ 21 ] and the middle ear of extant mammals.…”

Monotreme middle ear is not primitive for Mammalia

“…Wang et al 1 recently reported a specimen of Vilevolodon diplomylos (IMMNH-PV01699) 2 that adds to the increasing knowledge about the auditory apparatus of ‘haramiyidans’, an extinct Mesozoic group of mammaliaforms. The authors hypothesized that a middle ear with a monotreme-like incus and malleus and incudomallear articulation was primitive for mammals, which challenges the convention that the monotreme middle ear is specialized 3 or autapomorphic 4 in mammals. We raise concerns about terminology and identification of the incus presented by Wang et al and show that their analysis does not support their hypothesis; instead, it supports the one by Mao et al 5,6 .…”