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A revised origin of flatfish in the LRT

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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.

Figure 1. The thread fin, Polydactylus, enters the LRT close to flatfish + tuna. ” data-medium-file=”″ data-large-file=”″ class=”size-full wp-image-38916″ src=”″ alt=”Figure 1. The thread fin, Polydactylus, enters the LRT close to flatfish + tuna. ” width=”584″ height=”748″ srcset=” 584w, 117w, 234w, 588w” sizes=”(max-width: 584px) 100vw, 584px” />

Figure 1. The thread fin, Polydactylus, enters the LRT close to flatfish + tuna.

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.

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.

Figure 2. Revised skull colors for Hippoglossus the flounder now nests it with Thunnus the tuna. Note the lack of circumorbital bones, like more derived taxa. 

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.

Figure 3. Revised colors applied to this skull of Thunnus, the tuna. ” data-medium-file=”″ data-large-file=”″ class=”size-full wp-image-38919″ src=”″ alt=”Figure 3. Revised colors applied to this skull of Thunnus, the tuna. ” width=”584″ height=”665″ srcset=” 584w, 132w, 263w, 588w” sizes=”(max-width: 584px) 100vw, 584px” />

Figure 3. Revised colors applied to this skull of Thunnus, the tuna.

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.”

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).


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