Dec 4, 2008 — Has all the colorful artwork of the fish-a-pod been for naught? Three European biologists that Neil Shubin’ famous Tiktaalik fossil, supposedly of a fish evolving into a four-footed land dweller (see 05/03/2006, 11/13/2008) and achieving near iconic status in the media (01/16/2008) is not a missing link after all.
The situation is much more complex, argued Boisvert et al in a letter to Nature.1 Those who enjoy the full jargon can follow their reasoning:
The pectoral fin skeletons of Panderichthys and Tiktaalik share certain unusual features such as a blade-like radius and a longitudinal ridge-and-groove on the flexor surface of the ulna. These can tentatively be interpreted as attributes of the ‘elpistostegid’ segment of the tetrapod stem lineage and thus ancestral for the tetrapod forelimb. Given that recent phylogenies consistently place Panderichthys below Tiktaalik in the tetrapod stem group, it is surprising to discover that its pectoral fin skeleton is more limb-like than that of its supposedly more derived relative. In Tiktaalik4, like in ‘osteolepiforms’ and rhizodonts (more primitive fish members of the stem group), the ulna and ulnare are of similar size. The axis of the fin comprises two more elements distal to the ulnare, and the distal radials are arranged pinnately around this axis. In contrast, in Panderichthys and tetrapods, the ulna is much longer than the ulnare, the ulnare is the last axial element, and the distal radials/digits are arranged in a transverse fan shape11 (Fig. 3). It is difficult to say whether this character distribution implies that Tiktaalik is autapomorphic,2 that Panderichthys and tetrapods are convergent, or that Panderichthys is closer to tetrapods than Tiktaalik. At any rate, it demonstrates that the fish-tetrapod transition was accompanied by significant character incongruence in functionally important structures.Being translated, that last sentence says that if these fossils represent an evolutionary line from fish to tetrapod, the features are all mixed up and out of sequence – including the “functionally important structures.”
1. Boisvert, Mark-Kurik and Ahlberg, “The pectoral fin of Panderichthys and the origin of digits,” Nature 456, 636-638 (4 December 2008) | doi:10.1038/nature07339.
2. Autapomorphic means a derived trait unique to any given taxon, i.e. shared by the ingroup taxa, but excluded from its outgroup taxa. Convergent refers to traits that appear similar but are not phylogenetically related.
If this tale were told by Paul Harvey, “the rest of the story” would undo the first of the story. It wouldn’t bother the evolutionists, though, because that’s what Darwinism is all about, anyway: telling a good story (see 12/22/2003 commentary).
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