Fig. 5

Cranial anatomy of stem gnathostomes. a–b The galeaspid Shuyu, composite reconstruction based on reference [20]. a Combined ventral (left) and dorsal (right) image showing the arteries of the pharynx on the left, and the cranial cavity and associated structures on the right. b Lateral view showing cranial cavity, nasohypophyseal duct and inner ear. The double-headed black arrow indicates the length of the mandibular-premandibular region, measured from the anterior margin of the spiracular gill pouch to the tip of the snout. c–d The osteostracan Norselaspis, composite reconstruction based on reference [19]. In (c), note the anterior position of the pharynx compared to Shuyu, and the expansion of the saccular region of the inner ear into multiple long, branched canals (ot.ext) with a presumed sensory function. e-g The acanthothoracid (primitive placoderm) Romundina, redrawn from reference [22]. In this and the next taxon, only the anterior half of the neurocranium is shown. f Neurocranium in lateral view. g Cranial cavity in lateral view. In (g), note that the projecting precerebral part of the neurocranium is ventral to the cranial cavity. The nasal (or rostronasal) capsule is separated from the neurocranium by a fissure. The double-headed red arrow indicates the part of the neurocranium that floors the cranial cavity between the hypophysis and nasal sacs, and can thus be considered a trabecular region sensu stricto. Note that the inferred length of the mandibular-premandibular region (black arrow) is based on the position of palatoquadrate articulations and the exit of the hyomandibular trunk of n VII, and thus relates to the lateral margin of the neurocranium; in the midline the parachordal domain probably extends further forward. h-j The arthrodire (derived placoderm) Kujdanowiaspis, composite reconstruction based on reference [15]. The precerebral part of the neurocranium has been lost. As in Romundina, a fissure separates the rostronasal capsule from the neurocranium proper. e.hyo.art, efferent hyoidean artery; e.mand.art, efferent mandibular artery (generally termed efferent pseudobranchial artery in early gnathostomes); hyo.gill.po, hyoidean gill pouch; hyp, hypophysis; ju, dorsal jugular vein; nas, nasal sac; nas.cap, (rostro) nasal capsule; ot.ext., extension of the inner ear; pin, pineal foramen; II, optic nerve; V, trigeminal nerve; VII, hyomandibular branch of facial nerve