Re: 'SAILBACKS' (was Re: Deinosuchus and Tyrannosaurus)

[Pieter.Depuydt@rug.ac.be]

June 24, George Olshevsky wrote:

> You are quite correct. I should note that none of these genera was dorsally
> armored; they had lost the primitive archosaurian dorsal armor secondarily.
> _Spinosuchus_ is a very interesting case (no pun intended, but if you see the
> pun, award yourself ten points), since the vertebral anatomy strongly
> resembles that of the much smaller animals _Megalancosaurus_,
> _Drepanosaurus_, and _Dolabrosaurus_ and suggests it should be classified in
> the same family.

This is an interesting suggestion.  Long and Murry doubt the 
archosaurian affinities of Spinosuchus (Case 1921, ten points)(but 
the honour goes to von Huene who designated the series of vertebrae 
as the holotype of Spinosuchus; Case referred the vertebrae to 
Coelophysis I think), and consider it as Neodiapsida incertae sedis. 
The neural spines of S. look indeed superficially similar to those of 
Megalancosaurus (I have no detailed figure of S.). However the 
affinities of the 'drepanosaurs' are not quite clear either AFAIK. 
Renesto considers M. as a possible Archosauromorph 
(he tentatively places M. as an outgroup to Prolacertiformes and Archosauria). 
Renesto indeed suggests arboreal habits for Megalancosaurus, with the
elongation of the neural spines probably related to the development of a 
strong transversospinalis musculature. He also points to the great 
similarity of the scapula of Megalancosaurus to that of Longisquama 
(a convergence related to arboreality?).

 The spines of _Spinosuchus_ aren't laterally flattened but
> are rather spikelike, with very prominent craniocaudal dorsal expansions,
> unlike the spines of _Lotosaurus_ and _Ctenosauriscus_, which are flattened
> and lack craniocaudal dorsal expansions. The sail of _Lotosaurus_ is much
> lower than that of _Ctenosauriscus_. Perhaps these sails were connected with
> thermal aspects of changing metabolic regimes; who knows? But the elongate
> spines of _Megalancosaurus_ and its relatives seem to have more to do with
> controlling body flexibility and operating a prehensile tail in an arboreal
> environment than with thermoregulation.

Note: The spine from the Otter Sandstone (Devon) is very long, 
strap like, with changing diameters ('nodes') along its course. 
Although Milner and Benton tentatively assign it to a ctenosauriscid archosaur,
it seems to be a fourth type of Triassic spine...?

Pieter Depuydt.