Caught in the Middle: Transitional Flatfishes

One of the quirkiest products of evolution is the flatfish, of which there are several different species in the Pleuronectiformes order. They inhabit the sea-floor, and display some rather clumsly-looking adaptations. The ancestors of the flatfish, like other species of bony fish, were not flattened horizontally. But over time, they evolved into flat creatures that glide over the sea floor. The evolutionary history of these creatures isthought to be recapitulated in their development, which can be seen in the following videos:

As you can see, one of key developments is the migration of one eye from the bottom-facing side to the top. You can view the result of this curious adaptation in the picture below of the witch flounder (Glyptocephalus cynoglossus), which I snapped at an aquarium in Quebec City. The flatfish presents a textbook example of an evolutionary adaptation, although for a long time fossils that captured a transitional state between a symmetrical and asymmetrical skull were lacking. But this has changed with a new paper in this week's Nature, which describes Eocene fossils that support a gradual evolution of the flatfish.

Matt Friedman, the author of the paper, made his discovery while searching through neglected specimens in the basements of museums. One of the specimens is of a new species, which Friedman named Heteronectes chaneti, and it displays asymmetrical skull bones. The frontal bone on one side is smaller than on the other, and the orbit is displaced. However, unlike in most modern flatfish species, the orbit does not cross the dorsal midline to the other side. It represents an intermediate condition. Friedman also studied specimens of the Amphistium genus, which were once thought to have symmetrical skulls. However, Friedman used a computed tomography (CT) scan to study these skulls with greater detail, and was able to determine that they too exhibit an asymmetrical cranial morphology. He was also able to determine that these specimens are of adults by noting that they had completely mineralized skulls, which would not be the case if these were juveniles. Furthermore, all the Amphistium specimens are within a consistent size range, making it unlikely that they belong to a juvenile stage. The anatomical characteristics were also compared with the living Psettodes, a primitive flatfish, in order to confirm that the fossil specimens correspond to the adult stage. Friedman was also able to place Amphistium in the same lineage as modern flatfish species, by noting other shared characteristics in addition to the displaced orbits. Heteronectes and Amphistium also retain certain features of the ancestral perciformes. Some flatfish species tend to flatten towards one side, whereas the more primitive ones like Psettodes are observed to flatten in either direction. The Amphistium specimens display a similar pattern, with some flattening to the left and others to the right.

Friedman's findings provide strong evidence for a gradual evolution of Pleuronectiformes by bringing key fossil evidence to the table. They also highlight the fact that many exciting discoveries are made not only out in the field, but also in dusty basements of museums, where forgotten specimens are waiting to be examined with modern techniques and a fresh perspective. You can read more about this paper at The Loom, Not Exactly Rocket Science, and GrrlScientist's blog.

(Heteronectes chaneti)

Friedman, M. (2008). The evolutionary origin of flatfish asymmetry. Nature, 454(7201), 209-212. DOI: 10.1038/nature07108