Amphistium

Abstract: The asymmetry of flatfish is an exceptional morphological specialization that arises in development: starting from a symmetrical larva/juvenile, the skull is remodelled so that one eye migrates over the top of the skull to sit next to its fellow on one (or other) side of the animal. The evolutionary origins of this arrangement have been unclear. Matt Friedman re-examined Eocene (47-million-year-old) fossils of the fish Amphistium and describes a new genus that represents the most primitive flatfish known. In these fish, the migrating eye never gets further than the dorsal midline, even in fully adult fishes. This is a graphic example of a transitional form spotted in the fossil record, confirming that the evolution of the specialized flatfish bodyplan was a gradual process. A re-examination of the Eocene fish Amphistium and a description of a new genus prove that they are the most primitive members of the flatfish family. In these fish, the migrating eye never gets farther than the dorsal midline, even in fully adult fishes, providing perhaps the most graphic and dramatic examples known of a transitional form spotted in the fossil record. All adult flatfishes (Pleuronectiformes), including the gastronomically familiar plaice, sole, turbot and halibut, have highly asymmetrical skulls, with both eyes placed on one side of the head. This arrangement, one of the most extraordinary anatomical specializations among vertebrates, arises through migration of one eye during late larval development. Although the transformation of symmetrical larvae into asymmetrical juveniles is well documented1,2,3,4,5,6,7, the evolutionary origins of flatfish asymmetry are uncertain1,2 because there are no transitional forms linking flatfishes with their symmetrical relatives8,9. The supposed inviability of such intermediates gave pleuronectiforms a prominent role in evolutionary debates10,11,12,13,14,15,16, leading to attacks on natural selection11 and arguments for saltatory change14,15. Here I show that Amphistium and the new genus Heteronectes, both extinct spiny-finned fishes from the Eocene epoch of Europe, are the most primitive pleuronectiforms known. The orbital region of the skull in both taxa is strongly asymmetrical, as in living flatfishes, but these genera retain many primitive characters unknown in extant forms. Most remarkably, orbital migration was incomplete in Amphistium and Heteronectes, with eyes remaining on opposite sides of the head in post-metamorphic individuals. This condition is intermediate between that in living pleuronectiforms and the arrangement found in other fishes. Amphistium and Heteronectes indicate that the evolution of the profound cranial asymmetry of extant flatfishes was gradual in nature.