A revised origin of flatfish in the LRT

Harrington et al. 2016
used genomics to study flatfish (flounder, turbot and sole). Their analysis of extant taxa nested the thread fin, Polydactylus sexfilis (Fig. 1) as a proximal ancestor and Psettodus erumei as an extant basal flatfish.

Adding Polydactylus
to the large reptile tree (LRT, 1570 taxa, not yet revised) recovered it with Thunnus, the tuna (Fig. 3), far from Hippoglossus the flatfish.

So…
I took another look at the drawing data I had for Hippoglossus (Fig. 2) and re-scored it appropriately. Then Hippoglossus nested with Thunnus and Polydactylus was the first outgroup taxon.

As you might imagine,
a flounder was a difficult fish to add to the LRT when I knew so little about fish skulls. Now I know a little better.

Like adult tuna,
adult flatfish are able predators. Unlike tuna, camouflaged adult flatfish wait on their right side nearly buried in the sandy sea floor. Their exposed eyes provide a 360º view. When prey swims by, flatfish attack with a rapid push of their buried pectoral fin and a quick jet of expelled gill water.

Symmetrical hatchlings
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.”

References
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).

wiki/Spiny_turbot
wiki/Flatfish

News articles:
sci-news.com/paleontology/article00431.html
bmcevolbiol.biomedcentral.com/articles/10.1186/s12862-016-0786-x
pbs.org/wgbh/nova/article/flatfish-evolution/