Amphistium paradoxum (from, 'on both sides', 'sail', and 'extraordinary'), the only species classified under the genus Amphistium and the family Amphistiidae, is a fossil fish which has been identified as a Paleogene relative of the flatfish, and as a transitional fossil. In a typical modern flatfish, the head is asymmetric with both eyes on one side of the head. In Amphistium, the transition from the typical symmetric head of a vertebrate is incomplete, with one eye placed near the top of the head. Amphistium is among the many fossil fish species known from the Monte Bolca Lagerstätte of Ypresian Italy. Heteronectes is a related, and very similar fossil from a slightly earlier strata of France. Heteronectes is sometimes also placed in the family Amphistiidae, though other studies find it to be more basal than any other flatfish taxon.