Vertebrate Cartilage Evolution Figure 7. The diversity of skeletal tissues in the larval lamprey head. Based on morphology and gene expression, we speculate that lamprey larvae may possess as many as 11 distinct types of skeletal tissue. This includes 3 kinds of cellular cartilage, 7 kinds of mucocartilage, and a mucocartilage-like skeletal tissue unique to the dorsal fin. The diagram shows the first 6 pharyngeal arches. The upper lip mesenchyme is derived from the premandibular cranial neural crest (pm). For reference, cellular morphology and gene expression in gnathostome cellular cartilage and joint tissue are shown in the shaded rows. doi:10.1371/journal.pone.0022474.g007 that the lamprey head skeleton approximates the basal vertebrate condition, our results have two significant evolutionary implica- tions. First, they suggest that the evolution of the gnathostome head skeleton involved a marked reduction in skeletal tissue diversity. This implies that the basal vertebrate state is the possession of multiple cartilage-like tissue types, while gnathos- tomes display a derived dependence on a single kind of cellular cartilage. Second, our results suggest that this loss of skeletal tissue PLoS ONE | www.plosone.org 8 July 2011 | Volume 6 | Issue 7 | e22474