Meeting Abstract
The inner ear has undergone marked transformations in Mesozoic mammaliaforms, culminating in extant therians with greater ranges of hearing frequencies than most vertebrates. This ability has been associated with the elongation and coiling of the cochlear canal following the loss of the lagenar macula, ossification of the primary and secondary laminae, and ossification of the cribriform plate. The acquisition of these features in the few documented taxa appears mosaic, but could this reflect intraspecific or interspecific variation? To test whether cochlear canal length and curvature, presence of a lagenar macula, and ossification of laminae was variable in early mammaliaforms we micro-CT scanned 37 isolated petrosals of the basal mammaliaform Morganucodon from two Early Jurassic Glamorgan fissure fills. The endocasts reveal that the cochlear canal is short and fissure-specific with the average lengths ranging from 1.82 mm to 2.06 mm. In all specimens, the apex is gently curved and expanded, suggesting the presence of a lagenar macula. None of the specimens preserve ossified laminae, but a shallow groove is visible on the reconstructed endocasts, extending from the base of the canal, between the perilymphatic foramen and the fenestra vestibuli, to the apex. A similar groove for the base of the secondary osseous lamina is variably present in extant monotremes. Our qualitative data suggests the presence of a lagena and variation in cochlear length in Morganucodon. Interestingly, this variation is linked to different fissures and might represent variation in different populations or possibly in different species within Morganucodon. Quantitative assessment of cochlear shape using 3D geometric morphometrics is ongoing.
