June 25, 2008 | David F. Coppedge

Not Another Tetrapod Missing Link

Fossils don’t contain light bulbs, but almost every time a new one is found, scientists claim it sheds light on evolution.  The BBC News kept that tradition going with this line, “Scientists say a fossil of a four-legged fish sheds new light on the process of evolution.”  What, exactly, was found?  Whatever Ventastega curonica was, it would be hard to claim it helped shed any light on evolution, because according to the article, it was an “evolutionary dead end.”
    A close look at the article reveals other evolutionary conundrums.  Ventastega was placed after Tiktaalik, but was more primitive.  The BBC News article also said, “Scientists once believed that these early amphibious animals descended in a linear fashion, but this discovery instead confirms these creatures diversified into different branches along the way.”  The animal is made up of a curious mosaic of features.  It would have looked something like an alligator, they said, but allegedly had fishy features like a tail fin and gills.  Interesting, also, is the fact that this fish-o-gator from Latvia was found in sand.  Somehow these sediments, said to be 365 million years old, had not solidified into rock.
    Since popular news reports tend to exaggerate, a look at the original paper in Nature1 might shed light on the wattage of this fossil.  The Editor’s Summary in the June 26 issue said it “resembles a simple intermediate between Tiktaalik and Acanthostega, with the skull shape of an early tetrapod, but proportions more closely resembling a fish.”  Sounds promising so far.  The next sentence, however, undermined the missing-link story in a one-two punch: “But the picture is more complicated than that, due to the unexpected morphological diversity of early tetrapods, and the fact that their initial diversification was earlier than had been thought.
    On to the original paper.  Ahlberg et al opened by claiming that the long-mysterious fish-to-tetrapod gap has been beginning to close, and that their fossil narrows it further.  But then they said that the paucity of complete fossils makes it hard to fill in the gap.  Even after the highly-publicized find of Tiktaalik, “Acanthostega and Ichthyostega are still the only Devonian tetrapods known from near-complete skeletons,” they said, adding: “We know less about the fish�tetrapod transition than the taxic diversity suggests.
    The fossils are not new discoveries.  They had been collected between 1970 and 2001.  In addition, the fossil did not declare itself a transitional form.  This deduction was done with software.  The team plugged various traits they deemed significant into tree-building algorithms.  Though they got consistent results with different permutations, the interpretations were not straightforward.  The fossil contained both “primitive” and “derived” (evolved) features.  The paper suggested that the authors were puzzled about where to fit the pieces from Ventastega and other specimens.
    They ended by saying it was “tempting to interpret Ventastega as a straightforward evolutionary intermediate” (i.e., missing link).  “However, this simple picture should be approached with a degree of caution.”  Why?  Because it contains trait combinations that are substantially different from alleged earlier fossils.  “At a minimum this demonstrates the presence of considerable morphological diversification among the earliest tetrapods,” they said, ending on a positive note that this fossil and Tiktaalik fit expectations of what a transitional form “at a particular point in the phylogeny” should look like.
    All the paper’s caution was cast to the wind by the popular press.  Science Daily, with artwork to prove it, trumpeted, “New Fossils Of Extremely Primitive 4-Legged Creatures Close The Gap Between Fish And Land Animals.”  National Geographic News admitted that the diversity of the Devonian tetrapods was surprising, but nevertheless labeled them as “Fishy Ancestors of Humans”.  Only on page 2 was some caution sprinkled in: “So researchers have a rough idea of the major evolutionary changes that took place but still have their work cut out for them when it comes to filling in the gaps.”
    Shaun Doyle critiqued the claims being made about this fossil in an article on Creation Ministries International.

1.  Ahlberg, Clack et al, “Ventastega curonica and the origin of tetrapod morphology,” Nature 453, 1199-1204 (26 June 2008) | doi:10.1038/nature06991.

Probably this was just Jennifer Clack’s latest attempt to one-up Neil Shubin’s fish-a-pod after the press gushed on him without shame or restraint (04/06/2006) and made him a celebrity (01/16/2008).  Clack had been the darling of PBS till this rival muscled in.  Remember her sermonette on 04/06/2006 that one skeleton is unlikely to unlock the key to understanding of evolution, and that the concept of missing links, though having a powerful grasp on the imagination, contains unfounded notions of evolutionary progress?
    Does anyone really believe a fish-o-gator (or whatever it was) from Latvia decided to swim over to Canada where the evolving conditions were better?  We could make up a better case for an evolutionary sequence with living fish and amphibians than these ideologues can with fragmentary fossils that their worldview demands be placed into ancient epochs without observers.  When the Darwin story collapses, sociologists will use these phylogenetic rivalries to “shed light” on how scientists can deceive themselves into seeing what they want to see.

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Categories: Fossils, Marine Biology

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