''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 The Paleogene Period ( ; also spelled Palaeogene or Palæogene) is a geologic period and system that spans 43 million years from the end of the Cretaceous Period Ma (million years ago) to the beginning of the Neogene Period Ma. It is the fir ...

relative of the

flatfish A flatfish is a member of the Ray-finned fish, ray-finned demersal fish Order (biology), suborder Pleuronectoidei, also called the Heterosomata. In many species, both eyes lie on one side of the head, one or the other migrating through or around ...

, and as a

transitional fossil A transitional fossil is any fossilized remains of a life form that exhibits traits common to both an ancestral group and its derived descendant group. This is especially important where the descendant group is sharply differentiated by gross ...

. 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 A Fossil-Lagerstätte (, from ''Lager'' 'storage, lair' '' Stätte'' 'place'; plural ''Lagerstätten'') is a sedimentary deposit that preserves an exceptionally high amount of palaeontological information. ''Konzentrat-Lagerstätten'' preserv ...

'' of

Ypresian In the geologic timescale the Ypresian is the oldest age (geology), age or lowest stage (stratigraphy), stratigraphic stage of the Eocene. It spans the time between , is preceded by the Thanetian Age (part of the Paleocene) and is followed by th ...

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.

References

Transitional fossils † Prehistoric percomorph genera Monotypic prehistoric ray-finned fish genera Eocene fish of Europe Ypresian genera Fossils of Italy Fossil taxa described in 1835 Taxa named by Louis Agassiz {{Pleuronectiformes-stub