Examining the anatomy of Amphistium and Heteronectes with a diverse range of techniques (including CAT scanning and chemical preparation, which dissolves the rock surrounding the fossil skeleton), Friedman discovered that both represent primitive flatfishes with a somewhat asymmetrical skull. Nevertheless, in both cases the eyes remain on opposite sides of the head in adults. In other words, the fossils show incomplete asymmetry, displaying an intermediate condition between what is found in ordinary symmetrical fishes and extraordinary asymmetrical flatfishes.
This discovery rejects the notion that flatfishes must have arisen suddenly as "hopeful monsters" and documents two steps in the gradual assembly of one of the most bizarre body plans found among vertebrates. The position of Amphistium and Heteronectes within the fish evolutionary tree is confirmed by many aspects of their anatomy. Features unrelated to asymmetry link these fossils with flatfishes, but the specimens also show characters more primitive than those found in any living form.
Adding a further twist to the story, the right eye migrated in some specimens of Amphistium, while the left eye moved in other specimens. This is unlike most living species of flatfishes, where individuals are always right- or left-eyed. It also indicates that that mixed 'handedness' is primitive for flatfishes.
"There is a broad implication for the tempo and mode of evolution here," Friedman concluded. "Scientists had long assumed flatfishes must have arisen suddenly because they could not imagine the adaptive significance of intermediates, but this work delivers clear evidence that such intermediates did exist, and therefore, that flatfish asymmetry arose gradually."
The fossils described in Nature give clues to the lifestyle of th
|Contact: Greg Borzo|