Fig. 1. The branchiostegal ray-opercle series of hyoid dermal bones. Left-side views, anterior to the left and dorsal to the top. (A) The series (blue) decorates the hyoid arch, here shown isolated from the rest of the head skeleton, of Anchoa hepsetus, a member of the herring family of teleost fish [Clupeidae, from Nelson (Nelson, 1970)], and zebrafish Danio rerio, a minnow (Cyprinidae), and considered to be more highly derived with respect to their hyobranchial structures than herrings. The opercle (op) is the dorsalmost bone, branchiostegal rays (bsr) are located ventrally. The branchiostegal rays are generally described as being present as an anterior-posterior series, as makes sense from the adult morphology in Anchoa. Primary patterning in zebrafish appears DV, the anterior-posterior orientation seems secondarily derived, reflecting an anterior rotation of the ceratohyal (c) (see Kimmel et al., 2001b) to which the rays attach. Dorsal to the branchiostegal series, and just ventral to the opercle is the subopercle (s), also considered a member of the branchiostegal-opercle meristic series (Hubbs, 1920). There is a prominent size gradient within the series, including all three types of bones. Other bones included in the drawing are the hyomandibula (h), interopercle (i), metapterygium (m), preopercle (p), and quadrate (q). (B) The series in these modern fish may have evolved within the Osteichthyes from a more uniform set of branchiostegal ray-like elements, as represented by the condition in the late-Devonian fossil acanthodian Homalocantus. Acanthodians are bony fish sometimes considered to be the osteichthyan sister group (Miles, 1973; Maisey, 1986; Schultze, 1993), their series of hyoid bony spicules might be homologous to the osteichthyan branchiostegal-opercle series (see Janvier, 1996). The DV size gradient was present in the most basal osteichthyans, here represented by the late-Devonian paleoniscid fossil Moythomasia, thought to be a representative of the lineage of ray-finned fish (Actinopterygia) that gives rise to teleosts (reviewed by Carroll, 1988). The dorsal-most bone of the series (dark blue) was considered by Hubbs and others to be homologous to the teleost opercle. Drawings adapted from Janvier (Janvier, 1996).