CHICAGOA newly identified fossil and the reinterpretation of previously known fossils, all from Europe and about 50 million years old, fill in a "missing link" in the evolution of flatfishes and explain one of nature's most extraordinary phenomena.
All living flatfishes, which include halibut, flounder and sole, have a bizarre structural adaptation: both eyes are on one side of their head. What is even more remarkable is that every flatfish is born symmetrical, with one eye on each side of its skull. However, as it develops from a larva to a juvenile, it undergoes a metamorphosis where one eye moves (or "migrates") gradually up and over the top of the head, coming to rest in its adult position on the opposite side of the skull.
This unique specialization provides a clear survival advantage: it allows flatfishes to use both of their eyes to look up when they are lying on the seafloor. But scientists have had no idea how the forces of evolution gave rise to this curious structural adaptation because no fishesliving or fossilwith intermediate characteristics of this adaptation have ever been identified.
What might have led to this adaptation? What evolutionary advantage might intermediates have possessed? Many famous evolutionary thinkers, including Jean-Baptiste Lamarck, Charles Darwin, Alfred Russel Wallace, and Richard Goldschmidt, have had their say on this issue, but the new discoveries to be published in the journal Nature July 10, 2008, settle the matter.
"This problem of the evolution of asymmetrical flatfishes was particularly puzzling to biologists because it was very hard to explain what evolutionary forces might have led to this transition," said Matt Friedman, a research associate at The Field Museum, a graduate student at the University of Chicago and author of the study. "How can you arrive at the pattern seen in living flatfishes via gradual evolution? There seems to be no adaptive reason to start down the gradual
|Contact: Greg Borzo|