Casey Luskin, a lawyer for the Discovery Institute, hasn’t yet discovered that eponymous means having the same name as. When “Garth Brooks” puts out a debut album called “Garth Brooks,” it’s referred to as his eponymous album. And when Neil Shubin, the author of a scientific paper describing his discovery, refers to the bones in Tiktaalik’s reinforced fin as being homologous to the eponymous wrist bones in tetrapods, he means the bones with the same names. And Casey complains that the author hasn’t specified which bones.
Previously, he quote-mined:
Shubin has another paper in Nature specifically on Tiktaalik‘s fin, entitled, “The pectoral fin of Tiktaalik roseae and the origin of the tetrapod limb.” In this paper, there’s much more discussion of the “wrist,” as the first sentence of the abstract contains a confession of retroactive ignorance that states, “Wrists, ankles and digits distinguish tetrapod limbs from fins, but direct evidence on the origin of these features has been unavailable.”
That’s right, Casey. There has been plenty of circumstantial evidence. Shubin evaluated it and went looking for direct evidence in rocks of the right age and ecological circumstances; and he found it. What’s to carp at?
He makes much of the fact that Ahlberg and Clack in 2006 were careful to point out that these are still fin bones and not digits. Very good, Casey! They aren’t claiming that legs appeared “fully formed” from fins. It’s a transitional form–functional in its swampy environment, but not on dry land. In fact, he argues as if Shubin hadn’t already called Tiktaalik’s limb a fin in the title of his paper. He tries to turn scientific accuracy into a weakness: as rhetoric, that stinks like a dead fish.
Finally, Casey goes for the stale authority. He quotes another paper from fifteen years ago, stating that modern lungfishes and others have sturdier fins than ancient fishes in “the forms that we know.” The lesser point is that no one says that the modern lungfishes etc. are the same as ancient fossils. They have been evolving too and are not relevant. The main point is that Tiktaalik was not known 15 years ago. It’s a newly discovered form that is more developed, lived in a different environment, and had sturdier limb-fins. I’d call this argument a pair of red herrings.
Shubin et al. go on to acknowledge, “Limb skeletons differ from those of fins mainly by the presence of bones that comprise mobile wrists, ankles and digits.” It would thus indeed be very impressive to find a fish with a wrist, ankle, or digits in its fin. Does Tiktaalik have these bones?
No, and Shubin doesn’t say that it does. He says that there are bones in arrangements and places that could develop into wrists and digits, and that those bones allow extension and flexion is a similar way to tetrapod joints. Casey claims that modern fish that prop themselves up out of water have similar mechanisms for extension and flexion. The ones I’ve seen use straight fins.
Casey, flogging his straw man again, calls for a diagram comparing Tiktaalik’s limb to a tetrapod limb. Then he shows Shubin’s diagram, in which Tiktaalik is placed neatly between the extinct fish Panderichthyis and the extinct tetrapod Acanthostega, with its “eight little piggies” made famous by Stephen Jay Gould. Apparently Casey doesn’t understand standard zoology diagrams of homologous bones, either.
I guess Casey hasn’t studied either discography or zoology.
homologies in fish and tetrapod limbs