Re: Tiktaalik

[David Marjanovic <david.marjanovic@gmx.at>]

> Titanosaurs across the board display opisthocoelous, procoelous,
> amphicoelous and biconvex centra.  The most parsimonious explanation 
> behind
> this diversity in caudal morphology is that the front and back surfaces
> could switch between one and the other conditions (e.g., amphicoelous or
> opisthocoelous directly from procoelous), without always having to pass
> through intermediate morphologies (e.g., weakly procoelous, amphiplatyan
> conditions).

Hey, we're back to dinosaurs! I can't allow that... ;-)

In this case I don't have a better idea. However, gradual changes are also 
suspected to occur. There are opisthocoelous salamanders in whose vertebrae 
the ball contains a little socket in its center. Gradually enlarge that 
socket, and you get a procoelous vertebra. (Without an amphicoelous or 
amphiplatyan intermediate!) This is thought to be how the procoelous 
salamanders (the Cretaceous Noterpetontidae... IIRC) came into being.