[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

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.