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Re: The Back-up New Papers
Er... I haven't read the paper yet, and development is not my forte,
but theropod outgroups like Massospondylus clearly show a furcula
homolog that is unfused clavicles (Yates and Vasconcelos, 2005),
while the basal archosaurian interclavicle (e.g. Erpetosuchus) is an
unpaired xiphoid element. Also, unlike modern crocodilians, basal
crurotarsans have both clavicles and an interclavicle. Clavicles are
THE first elements to ossify in the pectoral girdle in squamates,
have a more similar position to the furcula, also develop from paired
condensations, also never coexist with furculae, and also undergo
intramembraneous ossification. Is it just me, or does this
hypothesis ignore paleontological evidence as much as the recent
"theropods have manual digits II-III-IV" paper did?
Could be. Many of the listed similarities between the croc interclavicle
and the furcula are common to all dermal bones or to the entire dermal
shoulder girdle. For instance, it's normal for... bony vertebrates that
the dermal shoulder girdle (interclavicles, clavicles, cleithra*,
anocleithra**, supracleithra**, postcleithra**, extrascapulars**,
whatnots**) completes its ossification before the endochondral shoulder
girdle (scapulocoracoid) even starts, that it appears as early as the
earliest skull bones (which are also dermal), and that all that happens
at a generally early stage in development.
Moreover, an overlapping or identical set of authors (including
Vickaryous) published a great paper about the homology of the two
coracoids of amniotes a few years ago. In passing, that paper mentions
that the interclavicle of monotremes is a fusion product of two dermal
bones (the craniolateral "arms") and an endochondral bone (the median,
caudal part); monotremes have clavicles, too, so they have two pairs of
elongate dermal bones in that region.
Thanks to *Massospondylus* and the like, it's an inescapable conclusion
that the furcula consists of the fused clavicles. If it's also
homologous to the crocodilian "interclavicle", that means the latter is
not an interclavicle. I'm not familiar with crurotarsan pectoral
girdles, so I can't judge if that, in turn, is nonsense; if it is, the
crocodilian interclavicle can't be homologous to the furcula.
BTW, that the entoplastron of turtles is homologous to the interclavicle
is textbook wisdom, and rightly so. Check out the entoplastron of the
Middle Jurassic stem-turtle *Condorchelys* in the March issue of JVP
(fig. 6A, B of Sterli & de la Fuente, p. 358) -- it's shaped like an
ordinary amniote interclavicle.
* Absent in crown-group diapsids, as well as in... therapsids or something.
** Absent in crown-group tetrapods or larger clades.