Meeting Abstract
110.4 Sunday, Jan. 6 The evolution and development of the archosaurian head and the origin of the bird skull BHULLAR, B.-A.S*; MARUGAN-LOBON, J.; RACIMO, F.; BEVER, G.S.; ROWE, T.B.; NORELL, M.A.; ABZHANOV, A.; Harvard; Univ. Auton. de Madrid; Max Planck Inst. Evol. Anthro.; NYCOM; Univ. Tex, Aus.; Amer. Mus. Nat. Hist.; Harvard bbhullar@fas.harvard.edu
The bird skull is a highly specialized structure that has diverged considerably from the ancestral cranial plan of the archosaurian “ruling reptiles,” whose modern representatives are birds and crocodilians. We investigated the developmental mechanisms underlying this divergence on several scales. On a broad scale, we propose that the heterochronic mechanism of progenetic paedomorphosis explains many seemingly disparate transitions from a more ancestral archosaurian skull to an avian skull. The bird lineage was juvenilized in several steps relative to ancestral forms, an insight obtained using a geometric morphometric analysis that included both phylogenetic and ontogenetic breadth. Early avialans in particular clustered with the juveniles and embryos of other archosaurs, with more crownward taxa moving farther and farther backward along a trajectory corresponding to ontogenetic progression. Definitionally, these results describe paedomorphosis, but the particular mechanism of progenesis was strongly supported by corresponding decreases in body size and in time to sexual and somatic maturity. Furthermore, some modularity exists in the heterochronic transformations here identified, notably in that the premaxilla, which forms the distinctive bird bill, grows peramorphically against a global paedomorphic background in which the orbits and brain become relatively larger and the maxillary region smaller. Given a broad evolutionary developmental mechanism for major changes in skull form toward birds, we have subsequently undertaken investigations of the specific molecular mechanisms behind the transformations elucidated by our phylogenetic/ontogenetic morphometric work, with some success in testing hypotheses of gene function using manipulation of model organisms.
