Friedman 2008 described an asymmetric Eocene fish,
Heteronectes, which he described as “the most primitive pleuronectiforms known” due to the incomplete migration of the lower orbit. Heteronectes will be added to the LRT soon.
are planktonic, swimming closer to the surface. They require sunlight to swim upright, so at night hatchlings swim erratically.
According to a BMC blog by Christopher Foote
flatfish experts, “Friedman and Schreiber think that the flatfish’s anatomical makeover followed a change in its behavior. When threatened, some modern fish are known to lie flat on their side on the seafloor and briefly bury themselves in the sand. Others tip over to play possum, only to leap up and snatch unsuspecting prey. Perhaps the flatfish’s predecessor was a bilateral open water fish particularly adept at this kind of stealth.”
Friedman M 2008. The evolutionary origin of flatfish asymmetry. Nature 454:209–212.
Friedman M 2012. Osteology of †Heteronectes chaneti (Acanthomorpha, Pleuronectiformes), an Eocene stem flatfish, with a discussion of flatfish sister-group relationships. Journal of Vertebrate Paleontology (32) 4: 735-756; doi: 10.1080/02724634.2012.661352
Harrington RC, et al. (6 co-authors) 2016. Phylogenomic analysis of carangimorph fishes reveals flatfish asymmetry arose in a blink of the evolutionary eye. BMC Evolutionary Biology 16 (224).