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What evidence supports the idea that flatfishes evolved from symmetrical fish ancestors?

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Body plan and sexual dimorphism in the common sole (Solea solea) and dusky flounder (Syacium papillosum). — pleuronectiform Body plan and sexual dimorphism in the common sole (*Solea solea*) and dusky flounder (*Syacium papillosum*).

The evidence supporting the evolution of pleuronectiforms (flatfishes) from symmetrical ancestors is primarily found in their developmental biology and anatomical structure. While adult flatfishes are uniquely asymmetrical—with both eyes positioned on one side of their compressed bodies—larval flatfishes are born with an eye on each side of the head, mirroring the bilateral symmetry found in most other fishes and vertebrates.1

During a critical phase of development known as metamorphosis, one eye migrates across the head to the opposite side, a process that triggers significant osteological changes in the skull to accommodate the shift. This developmental pattern suggests that the order evolved from a generalized, symmetrical percoid ancestor, such as a sea bass, that likely adopted a lifestyle of habitually resting on its side on the ocean floor.1