Meeting Abstract

27.3  Jan. 5  How Dinosaurs Build Beaks: Homology Between Avian Rhamphotheca and Diapsid Facial Scales HIERONYMUS, TL*; WITMER, LM; Ohio University; Ohio University th108702@ohiou.edu

Studying the evolution of facial integument in non-avian dinosaurs requires an initial framework of proposed transformational homologs for various diapsid skin features, including facial scales and avian rhamphothecae. The most thoroughly developed homology concepts that link bird beaks and lizard scales were proposed before the advent of molecular development studies and modern phylogenetic systematics, and as such are ripe for re-examination in light of a large body of new evidence. In this study, specimens from 170 extant and extinct diapsid taxa were surveyed for the relative topology of skin features, dermatocranial elements, and osteological correlates (OCs) of trigeminal (CN V) innervation. An unexpected overlap between the OC of the maxillary nerve and the frontonasal mass (FNM) -derived premaxilla in extant birds suggests a possible caudal migration of FNM-derived ectomesenchymal cells that may play a role in pattern formation in the avian beak. The lateral plate of the compound rhamphotheca found in basal birds is more closely associated with the caudally projecting maxillary process of the premaxilla than with the dermatome of the underlying maxillary nerve, as would be expected if FNM ectomesenchyme exerts control over epidermal beak development in the maxillary rostrum. The inclusion of new nerve and bone data lends credence to an existing hypothesis of transformational homology between the rostral premaxillary and mandibular scales of squamates and the avian beak that was initially based solely on epidermal similarity. These results are congruent with the six parallel occurrences of premaxillary beaks in maniraptoran theropods, and may have heuristic value for investigating beak evolution in more distantly related taxa.