From “Mystery of the Flatfish Head Solved” (ScienceDaily, June 25, 2012), we learn:
A new fossil discovery described in the latest issue of the Journal of Vertebrate Paleontology by Oxford University researcher Dr Matt Friedman finally solves the mystery. Friedman’s fossil fish, named Heteronectes (meaning ‘different swimmer’), was found in 50 million year old marine rocks from northern Italy. This study provides the first detailed description of a primitive flatfish, revealing that the migrated eye had not yet crossed to the opposite side of the skull in early members of this group. Heteronectes, with its flattened form, shows the perfect intermediate stage between most fish with eyes on each side of the head and specialized flatfishes where both eyes are on the same side.
Interesting. In living flatfish, the migration takes place during the fish’s lifetime. Here’s how it works:
North Pacific Halibut, a member of the Flounder Family of fish, are unique because they have a biological characteristic that only the Flounder Family has. When they are first hatched from the egg they swim upright and have one eye on each side of their head like all other species of fish. At about five weeks of age and one inch in length, one eye “migrates” over the top of the head so that both eyes are on the same side of the head. At this time the juvenile Halibut “lays over” on its’ side with both eyes on the upward or top side. As the fish grows the under side becomes white, the top side becomes a mottled darker variation of colors resembling the sea bottom, and their body flattens into an oval shape: thus the nickname “Flatfish”.
The fossil’s a great catch, but it’s unclear how it solves the “mystery.”
We know that the eyes migrate; they do so in every individual flatfish. The question of how an apparently complex developmental process is superintended over millions of years, with many accompanying adaptations required, and extinction the cost of failure – and how and why the process started – is still out there.
See the migration in developmental images:
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