Redescription of the type and only specimen of Aigialosaurus dalmaticus (BSP 1902II501), an “aigialo-saurian” squamate from the Cenomanian–Turonian of Hvar, Croatia, and, at the time of collection, the Italian island of Lesina, indicates that previous reviews of the specimen include erroneous anatomical interpretations. The type specimen presents a new and unique pelvic anatomy: the ilium of A. dalmaticus is an elongate element with well-developed anterior and posterior iliac processes. A scenario is presented for mosasaurid pelvic evolution, whereby the anteriorly elongate and dorsally directed ilium of mosasauroids is not derived from the forward rotation of the posterior iliac process, but rather the reduction of that process and the elaboration of the anterior iliac process. Recent phylogenetic analysis of Mosasauroidea finds A. dalmaticus to be the sistergroup to Opetiosaurus bucchichi and all other mosasaurids and “aigialosaurs.” We examine the character state assignments for A. dalmaticus in that study and refute those assignments owing to the absence of preservation of all four characters states found to separate A. dalmaticus from O. bucchichi.

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