'SAILBACKS' (was Re: Deinosuchus and Tyrannosaurus)

[Pieter.Depuydt@rug.ac.be]

> On Sun, 22 Jun 1997 Dinogeorge@aol.com wrote:
> 
> > 
> > Dorsal scutes are a primitive feature for practically all of Archosauria, 
> > and
> > crocs retain this feature presumably because it is useful to have the back
> > protected. It could well have arisen in early archosaurs for protection from
> > above from larger predators, such as therapsids. (Incidentally, therapsids
> > did not have dorsal scutes. Although I'm not terribly familiar with the
> > group, I can't recall any dorsally armored therapsids. This is probably why
> > they were able to evolve finbacks and such.)
> > 
> Nice speculation, however one group of armoured tetrapods did evolve 
> sail backed forms, the dissorophoid temnospondyls. You can see a 
> gradation of forms from things like Cacops with just a vetebral row of 
> dorsal osteoderms to Aspidosaurus with tall peaked osteoderms fused to 
> the neural spines to Platyhistrix with very elongated neural 
> spine/osteoderm structures that rival Dimetrodon in proportion. As an 
> aside though the tips of these spines probably protuded free, the 
> distribution of ornament suggests that a web of skin extended at least 
> some of the way up the "sail".

Well if my memory does not play tricks on me, also non-dinosaurian 
archosaurs already developed various sail-backed forms by Middle and 
Late Triassic times, for example the odd Lotosaurus (a rauisuchian 
according to some) and the enigmatic Spinosuchus, with laterally 
flattened elongated neural spines. 
There is also a record of a creature called Ctenosauriscus, probably 
archosaurian (or archosauriform?), with very elongated, spiny, rod-like
neural spines, very reminiscent of those from the early Permian sail-backed
sphenacodontid 'pelycosaurs'. A similar spine has been found in the Middle 
Triassic of Devon, UK.

PS: Therapsids did not evolve 'sailbacks' either, but non-therapsid 
synapsids (edaphosaurs and ' sphenacodonts') did (yes I know, 
nitpicking)

Pieter Depuydt