Flatfish origin no longer ‘uncertain’ ca. 2008

We took an earlier look
at flatfish origins here. Today, more taxa and a look at earlier hypotheses proposing a dual origin for flatfish.

Friedman 2008 wrote, “The evolutionary origins of flatfish asymmetry are uncertain because there are no transitional forms linking flatfishes with their symmetrical relatives. Here I show that Amphistium (Fig. 4) and the new genus Heteronectes (Fig. 3), both extinct spiny-finned fishes from the Eocene epoch of Europe, are the most primitive pleuronectiforms known. The orbital region of the skull in both taxa is strongly asymmetrical, as in living flatfishes, but these genera retain many primitive characters unknown in extant forms. Most remarkably, orbital migration was incomplete in Amphistium and Heteronectes, with eyes remaining on opposite sides of the head in post-metamorphic individuals.”

Friedman 2008 was a major discovery supported by the LRT in 2019.
But it did not go as deep as the LRT (Fig. 9, see below).

If you’re interested in seeing how flatfish operate in vivo,
here’s a video from YouTube:

Here
in the large reptile tree (LRT, 1579 taxa, subset Fig. 9) we can trace the origin of flatfish immediately to the tuna (Thunnus, Fig. 1) and thread fin (Polydactylus, Fig. 2) and then further back to jawless Silurian fish.

Thunnus thyrnnus (Linneaus 1758; 4.6m long; Fig. 1) is the extant Atlantic tuna. Traditionally it is considered a member of the perch family. Here it nests following the Late Carboniferous Coccocephalichthys wildi. The jugal is retained. The squamosal is absent. The maxilla appears to retain one tiny tooth. Note the lacrimal contacts the ventral jugal, creating an orbit not confluent with a lateral temporal fenestra. The tip of the premaxilla descends and carries tiny teeth.

Polydactylus oxtonemus (Girard 1858; up to 23cm, some species up to 2m; Fig. 2) is the extant Atlantic threadfin. Traditionally considered a Perciforme (perch family) here Polydactylus is shallows-dwelling transitional taxon from free-swmming Thunnus (tuna) to flattened Heteronectes and the later flatfish. Note the five thread-likerays/feelers anterior to the pectoral fin arising from the gular bone. These drag along the sea floor similar to sea robins, like Prionotus.

Heteronectes chaneti (Friedman 2008, 2012) is an Eocene fish with an assymetric face originally considered basal to extant flatfish. Note the retention of circumorbital bones. It is transitional between Polydactylus (Fig. 2) and Amphistium (Fig. 4).

Amphistium paradoxum (Agassiz 1835) is an Eocene fish with an asymmetric face, transitional to extant flatfish. It also has the flatfish shape, so it probably swam broadside down.

Flatfish
are found over sand or mud flats and beaches, and among mangroves. They feed mostly on crustaceans, as well as chaetognaths, polychaetes, fishes, and some plant material. The common presence of small juveniles throughout the year suggests a prolonged spawning season. The reproduction of a few related species has been studied and they appear to be protandrous, sex changing from male to female with growth (Motomura 2004). I mean, with all the other changes… why not?

For awhile fish workers were thinking
“Flatfish can be divided into two groups: the three species of spiny turbot that make up the family Psettodidae, and the much larger suborder Pleuronectoidei.  Unsurprisingly, fish biologists long assumed that both groups of flatfish evolved from a single common ancestor; it is hard to imagine such a bizarre adaptation having evolved multiple times.”

“Recently however, this common-sense assumption has come under attack. Several studies have found support for the distinct flatfish adaptation having evolved on two separate occasions. Is the flatfish body-plan not as unique as it appears?” (Fig. 7).

Harrington et al. 2016 report,
“Here, we recovered significant support for flatfish monophyly and relationships among carangimorphs through analysis of over 1,000 UCE loci.”

The LRT employs fossil taxa
(Fig. 9) and, like Harrington 2014 (Fig. 8) does not find a dual origin for flatfish. So, finally a genomic study matches a phenomic study!

References
Bloch ME and Schneider JG 1801.Systema Ichthyologiae Iconibus cx Ilustratum. Post obitum auctoris opus inchoatum absolvit, correxit, interpolavit Jo. Gottlob Schneider, Saxo. Berolini. Sumtibus Auctoris Impressum et Bibliopolio Sanderiano Commissum. i-lx + 1-584, Pls. 1-110.
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
Girard CF 1858. Notes upon various new genera and new species of fishes, in the museum of the Smithsonian Institution, and collected in connection with the United States and Mexican boundary survey: Major William Emory, Commissioner. Proceedings of the Academy of Natural Sciences of Philadelphia. 10: 167-171.
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).
Linnaeus C 1758. Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata.
McCoy F 1855. A synopsis of the classification of the British Palaeozoic rocks, with a systematic description of the British Palaeozoic fossils. Fasciculus 3, Mollusca and Palaeozoic fishes. British Palaeozoic Fossils, Part II. Palaeontology 407-666.