Fig. 1

Skeletal anatomy of lamprey ammocoetes. (A) Traditional assessment of lamprey skeletal tissues. A firm distinction is present between hyaline-like cartilages (red) and mucocartilages (blue). (B) Modified assessment of lamprey skeletal tissues which corroborates previous findings as well as results from this study. The sclerotome and oral papillae have not been as thoroughly studied as other skeletal tissues, so their classification is beyond the scope of this study. For these results, we posit that the absence of type II collagen in the branchial arches in Petromyzon marinus is a derived rather than ancestral trait for lamprey. The distinction between hyaline and mucocartilage is first and foremost determined by histological features, and subtypes are determined by differences in gene expression which are further elaborated in Figure S1. Keywords: ba: branchial arches; ebb: epibranchial bar; ec: endostilic cartilage; evb: external velar bar, hbb; hypobranchial bar; hy: hyoid; ivb: internal velar bar; lb: lateral bar; ll: lower lip; mf: medial flap; nc: nasal capsule; nt: notochord; oc: otic capsule; oh: oral hood; op: oral papilla; pr: parachordal process; sc: sclerotome; so: subotic mucocartilage; tr: trabecular process; ul: upper lip; vlp: ventrolateral plate; vmlb: ventromedial longitudinal bar; vp: ventral pharynx