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New paper on fish fingers



Catherine A. Boisvert, Elga Mark-Kurik & Per E. Ahlberg, "The pectoral fin
of Panderichthys and the origin of digits," Nature 456 (4 Dec 2008), pp.
636-638.

A decade or so back, Panderichthys was the popular choice of "fishapod" (by
which I mean, roughly, non-tetrapod tetrapodomorph) closest to Tetrapoda,
though its forelimb morphology was, um, unfortunate.  More recently
Tiktaalik was discovered and found closer to Tetrapod condition, among other
things in having "distal radials" that were plausibly homologous with
digits.  New paper redescribes Panderichthys's pectoral 'pendage on basis of
CT scans of a new specimen.  Turns out the earlier reconstruction missed
(forgivably: the bones were covered in scaly integument in the specimen it
was based on) an interbone gap: Panderichthys had distal radials, which the
earlier rec. had combined with the Ulnare as a single big bone.  The new
reconstruction looks more tetrapod-like than Tiktaalik: P. has a single arc
of distal radials where we have fingers, whereas T. has further axial
structure distal to the Ulnare, with terminal stuff branching off at more
than one level.

Comment: in addition to looking more like a Tetrapod hand, the new
reconstruction is a lot less weird-looking than the old one (whose oversize
Ulnare gave it a rather lobster-claw look).

Paper ends with the phylogenetic remark that the branching order between P.
and T. and Tetrapods is in question again, and "At any rate, it demonstrates
that the fish-tetrapod transition was accompanied by significant character
incongruence in functionally important structures."

Comment: maybe we shouldn't be too surprised at that.  There seems to be an
analogous situation at the bottom of the mammal tree: Trithelodonts are more
mammalesque in some features, Tritylodonts in others, and then there's
Brazilodon... (Key word: "mosaic evolution".)  My impression is that
dinosaur workers are finding something similar at the bottom of birds.
Maybe it is to be expected that new clades with major morphological
novelties will arise from "bushes" in which there has been extensive
"experimentation."

--

Allen Hazen
Philosophy Department
University of Melbourne