Because vertebrate jaw evolution involved modification of the anteriormost pharyngeal arch, it is important to understand the skeletal patterning of the lamprey pharyngeal arch. In this study, we visualized mucocartilage, which constitutes most of the skeletal elements in the anterior pharyngeal arches of the lamprey, and traced the development of these skeletal elements. We found that the basic framework of the mucocartilage skeletal elements is established in stage-30 larvae (about 1-month-old at 16°C) and that the expression pattern of the SoxE homolog, LjSoxE3, prefigures the development of the skeletal elements in the craniofacial region. This enabled us to trace the developmental pattern of the anterior pharyngeal arch skeletal elements. We obtained evidence that endothelin signaling is involved in development of the ventral element of the first pharyngeal arch. These results suggest that endothelin signaling was already involved in the specification for the ventral skeleton and that the gnathostome jaw innovation must have been achieved by modifying downstream regulatory systems. J. Exp. Zool. (Mol. Dev. Evol.) 316:339–346, 2011. © 2011 Wiley-Liss, Inc.