Inner Ear Morphology of Basal‐Most Mammaliaform Morganucodon

Abstract

Extant therians are unique in their ability to hear at higher and at greater ranges of frequencies than most other vertebrates. This ability has been associated with changes in inner ear morphology, including elongation of the cochlear canal following loss of the lagena maculae, coiling of the cochlear canal, and stabilization of the hearing membrane through bony laminae. Presence and absence of these features is variable across Mesozoic mammaliaforms. To gain insights into the evolution of the inner ear and hearing in mammaliaforms, we document inner ear morphology in the basal‐most mammaliaform Morganucodon in unprecedented detail. We μCT scanned 30 petrosals of Morganucodon from several Jurassic fissure fillings in the UK. Surface reconstructions of the inner ears demonstrate that the cochlear canal gently varies in curvature and length (1.64–1.80 mm, measured from the posterior aspect of the cochlear foramen to the apex). All specimens show a distinct expansion of the apex of the cochlear canal suggesting the presence of a lagenar macula. Interestingly, the size of the apical expansion varies within our sample. None of the specimens preserve complete ossified laminae, but a shallow groove is visible on the ventrolateral surface of the cochlear canal endocasts, extending from the base of the canal (between the perilymphatic foramen and the fenestra vestibule) anteriorly towards the apex. We identify this groove as the attachment for the basilar membrane (termed here “base of secondary lamina”), as seen in extant monotremes. If the base of the secondary lamina is indeed reflective of basilar membrane length in Morganucodon then this could represents the first accurate reconstruction of the length of the basilar membrane in a fossil mammaliaform that retains a lagena. The base of the secondary lamina ends shortly before the apex of the canal and ranges between 1.17 and 1.45 mm. It moderately correlates (R2=0.58) with total cochlear canal length in our sample. Most intriguingly, the bony wall of the promontorium contains a network of canals that surrounds the cochlea, previously described as the circumpromontorium plexus. The plexus enters the cochlear canal next to the base of the secondary lamina and extends along the laminas full length. This strongly suggests that the blood supply to the cochlea might have been substantially different in Morganucodon compared to extant therians where blood vessel enter the cochlea through the cochlear foramen along with the cochlear nerve.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.