Flatfish are unusual vertebrates, in that adults are highly asymmetric. Their young are normal enough, but as they get older, they adopt the behavioural traits of mature organisms and undergo significant developmental changes. In particular, one eye migrates around the skull so that both are located on the upper side of the fish. There has been no evidence for this developmental process in the fossil record - until now. According to the Editor of Nature, "a graphic example of a transitional form [has been] spotted in the fossil record, confirming that the evolution of the specialized flatfish bodyplan was a gradual process."
"The drawings show the most ancient fossil (left) with symmetrical eye sockets. Amphistium and Heteronectes, the reexamined fossil fish, show asymmetrical skulls, but maintain eyes on opposite sides of the head. The other two fish drawings show the asymmetrical eye locations and skull of the most current flatfish species." (Source Science News)
The finds are deemed significant because at least one stage of partially offset eyes has been documented. This is considered to challenge advocates of punctuated equilibrium (who argue for sudden jumps) and also creationists (who are supposed to defend the creation of species in their present form).
Matt Friedman, the author of the research paper, says that the fossils are important because "they help to settle a long-standing evolutionary debate and shed light on the mode and tempo of evolutionary change". Apparently, finding one intermediate stage dated at 47 million years is sufficient to say that the tempo was "gradual" and that it occurred over "over thousands to millions of years". We have no earlier fossils without asymmetry, and fully asymmetric flatfish appear in the fossil record at the same time. At very least, we can say that these data do not justify the word "gradual"!
What about the description of the fossils as "transitional"? Morphologically, there is no questioning their intermediate status. The interesting questions relate to understanding this observation. The morphological changes are developmental: they occur in every flatfish living today. These changes do not involve any genetic change - the genome is the same before and after the development of asymmetry. The important difference is that the new fossils are mature, not young. But surely the first explanatory options to be explored relate to developmental mechanisms. Are there environmental factors leading to the retarded development of these fish? Or are there epigenetic influences which mean that normal development was impaired, and these animals represent stunted growth? Curiously, there is no exploration of these options in the research paper or in the science media.
There are further questions about the proposed genetic interpretation of the fossils as transitional. What was driving change? Janvier recognises the importance of asking this question: "What could have been the selective advantage of this unusual anatomy?" He refers to several possible scenarios that have been proposed. However, a friend commented: "The fact the migration of the eye takes place in living creatures should have alerted people to the fact that there is no (significant) selective disadvantage to having an asymmetrical skull."
Arguments over selection forces by Darwinists will no doubt continue, but the quest is, at best, for a plausible 'just-so' story. The real problem is that empirical work with natural selection does not confirm that it is a strong force capable of doing what Darwinists want it to do. Furthermore, it begs the question whether eye migration is a cause or an effect. In 2006, Schreiber concluded:
"Behavior and skull asymmetries precede metamorphosis, and the development of lateralized behaviors was independent of eye position in larvae treated with thyroid hormone and in symmetrical variants. Therefore, lateralized behavior is not an adaptive response to eye translocation, but rather must result from changing vestibular responses to thyroid hormone."
The most noticeable feature of the research concerns the evolutionary hype that has emerged from the journal Nature and from the science media. We are not witnessing an impartial evaluation of the data, but a construction of an argument to "lay to rest" criticisms of Darwinism and evolutionary transformation. For more on this, see Luskin's comments on the National Geographic coverage of the story.
The evolutionary origin of flatfish asymmetry
Nature 454, 209-212 (10 July 2008) | doi:10.1038/nature07108
All adult flatfishes (Pleuronectiformes), including the gastronomically familiar plaice, sole, turbot and halibut, have highly asymmetrical skulls, with both eyes placed on one side of the head. This arrangement, one of the most extraordinary anatomical specializations among vertebrates, arises through migration of one eye during late larval development. Although the transformation of symmetrical larvae into asymmetrical juveniles is well documented, the evolutionary origins of flatfish asymmetry are uncertain because there are no transitional forms linking flatfishes with their symmetrical relatives. The supposed inviability of such intermediates gave pleuronectiforms a prominent role in evolutionary debates, leading to attacks on natural selection and arguments for saltatory change. Here I show that Amphistium and the new genus Heteronectes, 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. This condition is intermediate between that in living pleuronectiforms and the arrangement found in other fishes. Amphistium and Heteronectes indicate that the evolution of the profound cranial asymmetry of extant flatfishes was gradual in nature.
Cressey, D. The eyes have it, firstname.lastname@example.org, 9 July 2008 | doi:10.1038/news.2008.946
Janvier, P., Squint of the fossil flatfish, Nature 454, 169-170 (10 July 2008) | doi:10.1038/454169a
Luskin, C. National Geographic Finds Opportunity to Conflate Intelligent Design with Creationism while Misreporting Fish Fossil (Evolution News & Views, July 10, 2008)
Schreiber, A.M. Asymmetric craniofacial remodeling and lateralized behavior in larval flatfish, Journal of Experimental Biology 209, 610-621, 2006.
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