Amphistium and Heteronectes

Amphistium and Heteronectes were two genera of fish representing the transition toward modern flatfishes, which in their adult form have two eyes on the same side of the head. Flatfish when hatched are symmetrical, but during their growth to maturity one of the eyes migrates to the other side of the head.

Typical modern flatfish wait for prey by lying down on top of the substrate, on one side, and have their two eyes on the same side of their head (rather than having them symmetrically located as is typical for vertebrates, which would have one eye uselessly facing downward in a similar position). The fossils of Amphistium and Heteronectes show a transitional state, with one eye in an intermediate position, moved near the top of the head. This is intermediate between the typical location for eyes in vertebrates symmetrically placed in the head, and the typical location for eyes in flatfishes on the same side of the head. Alternative explanations for this are excluded. For example, there are several fossil specimens of Amphistium known from the Monte Bolca lagerstatten, so it is not just an isolated malformed fish.

The challenge to evolution
The evolution of this asymmetry in the positioning of the eyes is a long-outstanding problem for gradual evolution. St. George Jackson Mivart wrote in 1871 (that is, twelve years after the publication of On the Origin of Species):

"Such sudden changes, however, are not those favoured by the Darwinian theory, and indeed the accidental occurrence of such a spontaneous transformation is hardly conceivable. ... It seems, even, that such an incipient transformation must rather have been injurious."

The discovery of these fossils demonstrates that, no matter how inconceivable the transformation was, it did happen; and that the incipient transformation was not so injurious as to preclude the existence of fish in such a state. One of the fossils is of a fish that was successful enough as a predator to have eaten another fish. This discovery shows that the argument from implausibility, as applied by Mivart, is unreliable.