Meeting Abstract

S10-9  Thursday, Jan. 7 14:00  Bird jaws and the origin of Aves BHULLAR, B.-A.S.; Yale University bhart-anjan.bhullar@yale.edu http://people.earth.yale.edu/profile/bhart-anjan-bhullar/about

The avian body plan is profoundly modified compared to its distant reptilian antecedents. In the trunk, a series of transformations associated with flight are an obvious driving force behind this modification. However, the bird head is also dramatically different from that of other reptiles. Here I provide an overview of the steps leading from the early divergences of reptilia to the base of Aves, with a particular focus on the beak. There is now considerable evidence that the basic plan of the avian head is that of a juvenilized or paedomorphic theropod dinosaur. A dramatic reduction of the facial region ordinarily would lead to a very short rostrum, save that the avian premaxilla became hugely enlarged in compensation, to form the upper beak. At least a part of this transformation was enabled by the simple addition of a median zone of gene expression and proliferation early in the embryonic patterning of the face. Previously we published on the expression of two genes, Fgf8 and Lef1, indicating signaling at two different embryonic time points; additional expression data support the general pattern. Moreover, the fossil record indicates that, as the premaxillae enlarged, the maxillae became concomittantly smaller. I will present some preliminary evidence that this occurred through a reduction of anterior proliferation and thus ossification in the maxillary primordia. Finally, I will show that new data from fossil taxa support a dramatic and sudden transformation of both the upper rostral and the palatal skeleton near the crown clade aves, perhaps suggesting a discrete developmental shift. I will then review the limited data on the possible function of the beak at its origin, and will cautiously speculate about its role as a prehensile organ replacing the hands.