Fig. 7

Radiograph of an alcohol preserved acronurus stage of an unidentified species of the genus Acanthurus. USNM 279390, 24.6 mm SL, Hawaii. Scale bar is 5 mm. The scalpel-like spine on the caudal peduncle is already moderately well developed. Note that there are two basal pterygiophores in the first interhaemal space; all other taxa of acanthurids have at least two basal pterygiophores (and up to seven) in this space, except G. friedmani, which has only one. Also note that two basal pterygiophores are present in the tenth interneural space, which is the majority condition among acanthurids; a minority of taxa, including Glarithurus friedmani, have only one basal pterygiophore in this space; a few taxa have three basal pterygiophores there