Amphistium is a fossil fish which has been identified as an early relative of the Flatfish

Flatfish

The flatfish are an order of ray-finned fish, also called the Heterosomata, sometimes classified as a suborder of Perciformes. In many species, both eyes lie on one side of the head, one or the other migrating through and around the head during development...

, and as a Transitional fossil

Transitional fossil

A transitional fossil is any fossilized remains of a lifeform that exhibits characteristics of two distinct taxonomic groups. A transitional fossil is the fossil of an organism near the branching point where major individual lineages diverge...

. 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

Monte Bolca

Monte Bolca is a lagerstätte near Verona, Italy that was one of the first fossil sites with high quality preservation known to Europeans, and is still an important source of fossils from the Eocene....

 lagerstatte

Lagerstätte

A Lagerstätte is a sedimentary deposit that exhibits extraordinary fossil richness or completeness.Palaeontologists distinguish two kinds....

 of Lutetian

Lutetian

The Lutetian is, in the geologic timescale, a stage or age in the Eocene. It spans the time between and . The Lutetian is preceded by the Ypresian and is followed by the Bartonian. Together with the Bartonian it is sometimes referred to as the Middle Eocene subepoch...

 Italy. Heteronectes

Heteronectes

Heteronectes is a fossil fish which has been identified as an early 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...

is a related, and very similar fossil from slightly earlier strata of France.