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Forthcoming theropod paper of note on the manus
To appear in the January 2003 issue of Trends in
Ecology & Evolution:
Frietson Galis, Martin Kundrat, Barry Sinervo. An old
controversy solved: bird embryos have five fingers.
The numbering of theropod hands -- the best paper,
thus far, on some aspects of the question, remains
that by G.P. Wagner and Jacques Gauthier -- is
problematic, given the poor fossil record of basal
taxa. It is possible that avian dinoosaurs exhibit
embryogenetic reduction of manus digit I earlier than
some have thought vs. the idea of (as Frietson et al.
write in their mss.) "a repatterning of the initial
embryonal Anlage followed by the development of the
digits that are present later on".
Much further work is needed, indeed. But, unless I am
misreading the literature, it is presumed theropod
hands consist of I, II, III. The recent embryological
research may revise this, as avian embryos show five
digits, with I and V lost. Could it be that theropod
hands are not I-III, but II-IV? Feduccia & Nowicki,
earlier this year, beat the mantra drums of avians
being anything but Dinosauria on the basis of
embryology -- but they are out-of-tune.
The subject of the manus is, actually, quite
interesting. The Wagner/Gauthier Frame Shift
Hypothesis proposes that, from theropod I-III, there
transpired the shift to II-IV among immediate clades
ancestral to extant avian theropods. Both Russel
Lande, and Frietson Galis, among others, have
demonstrated that changes in embryogenesis are rather
constrained, and there are examples of this among
diverse taxa. Frietson in the forthcoming paper gives
examples from extant avians having variously
positioned pedal digits, examples of homeotic
transitions of digit identity: among woodpeckers
(digit IV) and trogons (digit II) function in identity
as digit I. Among pelicans and swifts, digit I
partially functions in identity as II, III, or IV.
It is, as she notes, unclear yet the adaptive
advantage of homeotic shift from I-III into II-IV
among basal avian theropods. Among some basal avian
theropods, "the transition from four to three fingers
in bird ancestors occurred by the reduction of the
most posterior digit" as in Ornitholestes.
As she and her colleagues propose, there is a period
of transitions among ceratosaurs and tetanuran
ancestors of avians of independent digit reduction in
the hands. Thus, she believes that Theropoda have hand
digits II-IV (I and V lost).
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