Wnt gene expression during appendage development of the sepiolid squid Euprymna scolopes

their downstream targets in the lamprey head. In jawed vertebrates, Edn signaling patterns the pharyngeal skeleton and positions the jaw joint through the action of target genes such as Hand, Bapx, Chordin, Gdf5, Gsc, Dlx, and Barx. We find gnathostome-like expression of Edns and most Edn target genes in the lamprey head, indicating that Ednmediated pharyngeal skeleton patterning predates the evolution of the jaw. This suggests that jaw evolution was driven by relatively minor alterations to an ancient Edn-based pharyngeal pattern, rather than by the de novo evolution of a gnathostome-specific pharyngeal patterning mechanism.