Fig. 1

a Interpretation of the head in dorsal view of the Early Cambrian stem vertebrate Haikouichthys from the Chengjiang fauna of Yunnan Province, China, showing the presumed cartilaginous braincase (stippled) at the anterior end of the notochord. The braincase evidently included the three major sense organs of vertebrates: paired nasal openings (nas.op), paired eyes, and probably an acoustico-lateral line system suggested by paired otic capsules (otic caps.). b Dorsal view of an incomplete headshield of the arandaspid agnathan Sacabambaspis from the Ordovician of Bolivia, showing the unique paired pineal openings (pi), sensory grooves (sg) of the lateral line system, and an anterior orbitonasal opening (ono). This housed both eye capsules, with the nasal openings separated by an internasal bone (inb). cSacabambaspis (Ordovician, Bolivia); reconstruction of the whole fish in left dorsolateral view, showing the position of the eyes, paired pineal openings (pi), sensory groove system (sg), and branchial openings (br.op). a modified from Janvier (2003, Fig. 2c); b, c modified from Gagnier (1995, Figs. 3 and 4)