Bhart-Anjan S. Bhullar
The Origin of the Avian Head as told by Transformations in the Fossil Record and Embryonic Development
Published Online:13 Apr 2018 Abstract Number:84.1
The head of birds is immediately and profoundly distinguishable from those of other major vertebrate clades, owing to a bewildering number of derived features including an expanded brain and eyes, a shortening of the maxillary face, and an expansion of the premaxillary region at the tip of the snout into the characteristic avian beak. Considered separately, multitudes of individual transformations are required to bridge the gap between an ancestral reptilian cranium and a bird skull. However, in our work, we have found that large-scale cranial evolution operates in such a way that multiple transformations have a single underlying developmental cause, resulting in character nonindependence and a simpler and more elegant evolutionary story than has previously been assumed. Foremost, we find that the avialan skull is fundamentally a juvenilized or paedomorphic version of the ancestral dinosaurian skull, and that many if not most of the iconic features of birds can be explained using a simple growth or scaling mechanism. In addition, the beak and skull roof â the entire modern avian feeding system, which features a great deal of mobility or kinesis â is patterned by one or a few early developmental influences. Finally, the highly modified avian skull roof is primarily influenced by the transformed brain, and indeed evidence suggests that a tight linkage between these structures is ancestral for Osteichthyes. We also found indications that the expanding brain restricted the size of the jaw-closing musculature. Remarkably, the fossil record supports many of these hypotheses, showing clear signals of juvenilization and correlated shifts in brain and skull, and also showing simultaneous transformation of the rostral beak and the palate as suggested by our developmental work. We find particular support for our hypotheses using new material of the most important fossil taxa demonstrating the 'modernization' of the avian skull, the Cretaceous toothed stem-birds Ichthyornis and Hesperornis. However, this material simultaneously demonstrates that the avian adductor chamber was, in at least some taxa, remarkably plesiomorphic even after the expansion of the brain, falsifying our hypotheses of a strict causal relationship between brain expansion and adductor diminution. Thus, despite the importance of developmental data in demonstrating the ways in which character transformations are interrelated during major evolutionary transitions, the fossil record is always the final arbiter of the merit of any large-scale evolutionary hypothesis.
Ramza Shahid, Pamela G. Gill, and Simone Hoffmann
Inner Ear Morphology of Basal-Most Mammaliaform Morganucodon
Published Online:20 Apr 2018 Abstract Number:780.9
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 ÎCT 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 (1.64â1.80 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.