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Updated: Jan 9, 2026

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The evolutionary origin of flatfish asymmetry.

Matt Friedman1

  • 1Committee on Evolutionary Biology, University of Chicago, 1025 East 57th Street, Chicago, Illinois 60637, USA. mattf@uchicago.edu

Nature
|July 11, 2008
PubMed
Summary

Fossil fish discoveries reveal gradual evolution of flatfish asymmetry. Primitive flatfish ancestors, Amphistium and Heteronectes, show intermediate eye migration, supporting gradual evolutionary change.

Area of Science:

  • Paleontology
  • Evolutionary Biology
  • Ichthyology

Background:

  • Flatfishes (Pleuronectiformes) exhibit extreme skull asymmetry with both eyes on one side, a unique vertebrate trait.
  • This asymmetry develops via eye migration during larval stages, but the evolutionary path remains unclear due to a lack of transitional fossils.
  • Previous debates questioned natural selection's role in flatfish evolution, suggesting saltatory (abrupt) changes.

Purpose of the Study:

  • To investigate the evolutionary origins of flatfish cranial asymmetry.
  • To identify and describe the most primitive known pleuronectiform fishes.
  • To provide evidence for the gradual nature of flatfish eye migration and skull asymmetry.

Main Methods:

  • Paleontological analysis of fossil spiny-finned fishes from the Eocene epoch.

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  • Comparative anatomical study of skull morphology in extinct and extant flatfishes.
  • Examination of orbital region and eye placement in fossil specimens.

    • Main Results:

    • Discovery and description of Amphistium and Heteronectes, the most primitive pleuronectiforms known.
    • These fossil taxa display strong cranial asymmetry, similar to modern flatfishes.
    • Crucially, orbital migration was incomplete in Amphistium and Heteronectes, with eyes on opposite sides post-metamorphosis, representing an intermediate stage.

    Conclusions:

    • Amphistium and Heteronectes provide crucial transitional evidence for flatfish evolution.
    • The profound cranial asymmetry of flatfishes evolved gradually, not through saltatory change.
    • These findings resolve long-standing debates about the evolution of pleuronectiform asymmetry.