Fig. 6

Evolution of the vertebrate neurocranium to explain the establishment of composite neurocranium, as a series of evolutionary grades from a simple primitive ancestor (a) to modern gnathostomes (d). Cephalic neural crest-derived skeletal elements are colored pink except for the mandibular arch- and posterior process ectomesenchyme colored darker pink and purple, respectively. Paraxial mesodermal elements are colored light blue. For every evolutionary stage, diagrammatic composition of the neurocranium is presented on the lower right. a Hypothetical ancestral condition. The forebrain is only weakly developed, and the paraxial mesoderm is coextensive with the notochord that extends almost to the rostral tip of the body axis. Around the neural tube, the neural crest-derived ectomesenchyme is only seen around the nasal-hypophyseal placodes (nhp) as anterior and posterior processes (ap, pp). The neurocranium of this animal (only the part that covers the brain, excluding the nasal capsule) is formed only of the mesodermal tissue, consisting of the premandibular mesoderm-derived element and the parachordal. The neural crest is mostly forming oro-visceral skeleton. The rostral ectomesenchyme in the premandibular domain (pp) may have been utilized to form a part of the oral apparatus. b Idealized monorhinous jawless state, corresponding to modern cyclostomes and possibly stem gnathostomes as well. The premandibular ectomesenchyme is utilized as a part of the oral apparatus, like the upper lip of the lamprey larva (ul). c Early jawed stage as seen in a basal placoderm like Romundina [21]. The mandibular arch is dorsoventrally divided to differentiate into the upper jaw (mx) and the lower jaw (mn). d Idealised embryonic scheme of crown gnathostomes based on the elasmobranch morphology. The premandibular ectomesenchyme (corresponding to the posterior process in the ancestor) is now forming a rostral part of the neurocranium to support the expanded forebrain, and the entire neurocranium consists of a prechordal region derived from the neural crest (trabecula) and a chordal region derived from the mesoderm (orbital cartilage and parachordals). The importance of the prechordal cranium increases with the expansion of the forebrain. Note, in (c) and (d), that the posterior part of the chordal cranium has incorporated an occipital component, derived from a few rostral somites. ap, anterior process; br1, branchial arch 1; e, eye; hb, hindbrain; hy, hyoid arch; lamptr, lamprey trabecula; llp, lower lip; ma, mandibular arch; mb, midbrain; mn, mandibular process; mo, mouth; mx, maxillary process; nas, nasal capsule; nhp, nasohypophyseal plate; oc, orbital cartilage; occ, occipital cartilage; ot, otic vesicle; pc, parachordal cartilage; pm, premandibular mesoderm; pmp, premedian plate; pp., posterior process; tel., telencephaon; tr, trabecula; trc, transverse commissure in the lamprey (hypophyseal commissure in the hagfish); ul, upper lip