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Re: Bird origins



To: dinosaur@usc.edu
Subject: Re: Bird origins

Francoise Forel wrote:

> We know that derived structures looking much like feather rachis were
> present in pterosaurs and *Sinosauropteryx*. There are two ways to
> interpret these strutures:

> 1) They are the result of convergence and are not shared by every
> member (or at least the least advanced members) of the group including the
> last common ancestor of pterosaurs and *Sinosauropteryx* and all of its
> descendants (Ornithodira).

Of course, the body covering of pterosaurs is far different from that of birds.
I think David Unwin (correct me here if I'm wrong) mentioned that at least not
all of the filament impressions can be interpreted as fur, and that some of
them are actually the actinofibrillae that support the wing.

> 2) They are shared by all Ornithodirans exept forms that
> secondarily lost or reduced them (ornithischians, sauropodomorphs, maybe
> some theropods). Pterosaurs had them, as well as *Lagerpeton*,
> *Scleromochlus*, *Marasuchus*, *Pseudolagosuchus*, herrerasaurids,
> *Eoraptor*, the most basal dinosaurs, coelophysids, etc...
> I believe in the second hypothesis. By the way, could the possible lack of
> rachis in sauropodomorphs and ornithischians be a diagnostic point for the
> Phytodinosauria?

Does this mean you place the primary division within dinosaurs between the
Theropods and Phytodinosaurs (as Bakker does) and not between Saurischians and
Ornithischians?

It's quite possible that, if "rachis" are a diagnostic feature of
Ornithodirans, then Sauropodomorphs and Ornithischians (Predentata to Bakker),
either lost them independently, or that juveniles had them at least sparsely
(like altritial bird chicks).

Raymond Ancog
Mines and Geosciences Bureau
Philippines
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