Illustrations of skull of Acanthocleithron chapini (AMNH 6575, paratype). Top: Dorsal view of skull with pectoral girdle, jaws and suspensorium. Middle: Lateral view of skull with pectoral girdle, jaws and suspensorium. Bottom: Ventral view of skull with pectoral girdle, jaws and suspensorium. Abbreviations: ANG – angulo-articular, ANP – anterior nuchal plate, BO – basioccipital, C5 – 5th centrum, C6 – 6th centrum, CC – complex centrum, CHY – ceratohyal, CLE – cleithrum, COR – coracoid, DN – dentary, DPR – dorsal-fin proximal radial, DSP – dorsal spine, DSPT – dorsal spinelet, EHY – epihyal, ESA – elastic spring apparatus, fac. HYOM – facet for the hyomandibular, FR – frontal, FR f. – frontal fontanelle, HHY – hypohyal, HYOM – hyomandibular, IO1-4 – infraorbitals 1-4, IOP – interopercle, LE – lateral ethmoid, MAX – maxilla, ME – mesethmoid, MNP – medial nuchal plate, MPT – metapterygoid, NA – nasal, NP – nuchal plates, OP – opercle, PA – palatine, PMX – premaxilla, PNP – posterior nuchal plate, pr. 4 – parapophysis of 4th centrum, pr. 5 – parapophysis of 5th centrum, PSP – pectoral spine, PTO – pterotic, PTSCL – posttemporo-supracleithrum, QU – quadrate, R6 – rib of 6th centrum, SO – parieto-supraoccipital, SP – sphenotic, TR – tripus, UHY – urohyal. Illustrations by Thomas R. Vigliotta