Scleromochlus (was Re: Origin of Birds-arboreal biped)

[Pieter.Depuydt@rug.ac.be]

> On Mon, 8 Dec 1997, G & J Bloomfield wrote:
> 
> (various snips)
> >      The upper Triassic archosaur Scleromochlus taylori had been 
> > suggested as one such beast by Von Huene  in 1914.*  He apparently was 
> > exploring the possibility of it being a pterosaur ancestor, but it seems 
> > to me that it would "work better" as a bird/dinosaur type as is being 
> > discussed.  I've rearranged the published skeleton(from Wellnhofer's 
> > pterosaur book) and attempted to restore it as an arboreal, 
> > "protofeathered," obligate biped, with its small forelimbs free to help 
> > it balance itself as it jumped and clambered about in branches.  The 
> > recent findings of possible primitive feathers are seeming to make this 
> > more and more plausible.  It would seem to fill the bill in many ways, 
> > and some form like this does seem to me to be the most likely candidate 
> > for a "proto-bird."  

> > I realize that this particular animal was not 
> > well-preserved, but is there anything known for certain about it that 
> > would rule this out?  It does seem to be a perfectly good, tiny archosaur 
> > in the right time slot.

Jack wrote:
> _Scleromochlus_ is preserved only as an impression.  Dr. James Clark has
> told me that about all we have to argue with on this little creature are
> limb proportions.  

Indeed, Scleromochlus taylori is described from natural molds in 
sandstone and is very imperfectly known. Apart from von Huene's work, 
it was redescribed by Benton and Clark in their review of the Elgin 
vertebrates published in Paleontology in ?1989. They hypothetized 
Scleromochlus as a "dune jumper", an agile creature adapted to 
locomotion on loose sand slopes, parallelling present day desert 
lizards (agamids?), based on the taphonomy and paleoenvironment of 
the Lossiemouth sandstone: i.e. an oasis or wadi like environment 
surrounded by sand dunes.
In the SVP memoir 2, Sereno published a new reconstruction of 
Scleromochlus, rather different from the classical "gerbil or 
rabbit"- like one we all know. This new reconstruction shows a very 
lightly build, bipedal animal, very similar in body proportions to 
Marasuchus (formerly 'Lagosuchus').
But again, S. is very imperfectly known, its place within Ornithodira 
(whether closer to Pterosauria or Dinosauria) is not established, and 
it might even turn out not to be an ornithodiran at all.

Pieter Depuydt.