Re: Where have all the ornithischians gone?

[Tim Williams <twilliams_alpha@hotmail.com>]

Peter Buchholz wrote:

(Nice to hear from you, Pete!)

> If I recall correctly (all literature is in boxes at the moment, so bear 
> with me), _Technosaurus_ was based entirely on skull elements, some of 
> which may be prosauropod, and some of which apear to be (to me anyway) 
> actually ornithischian.

The type material for _Technosaurus smalli_  includes a right dentary from a 
putative ornithischian, and a premaxilla and partial mandible from a 
putative sauropodomorph.  The dentary (TTUP P 9021) was nominated by Sereno 
as the lectoype for _Technosaurus_.  The type material also includes an 
element that Chatterjee interpreted as an astragalus, but Sereno could not 
identify as such.  So for all intents and purposes, I think you're right: 
_Technosaurus_ is known only from cranial elements, possibly pertaining to 
both an ornithischian and a sauropodomorph.

David Peters wrote:

> This is the kind of material that needs to be looked at with the eye
> that MAYBE  the pubis is not retroverted in this taxon, (Is that why it
> appears to be mixed up with sauropodomorph material? I don't know and
> haven't seen).

Even if the pubis of a basal ornithischian was not retroverted, this would 
not necessarily exclude it from the Ornithischia.  So I don't see the need a 
clade named "Paraornithischia".  A retroverted pubis is derived for 
dinosaurs, and we might expect the first ornithischians to retain the 
propubic pelvis primitive for dinosaurs.

As for putting _Lotosaurus_ and _Silesaurus_ in the Ornithischia (or 
"Paraornithischia"), I don't see a compelling case for this at all.  Perhaps 
there are vague resemblences seen when eyeballing the specimens - and maybe 
it does help to squint.  :-)  But we'll need to analyze the individual 
characters present in the specimens in order to assess the phylogenetic 
affiities of _Silesaurus_ and _Lotosaurus_.  In the past, we've been led 
dangerously astray by eyeballing (e.g., _Longisquama_ or _Cosesaurus_ as 
pro-avians; _Acrocanthosaurus_ as a spinosaurid, etc).

Back to the the scarcity of ornithischians in the Late Triassic and Early 
Jurassic... this problem is not unique to the Ornithischia, but also occurs 
for other dino groups.  For example, tetanurans are exceedingly rare during 
the LT/EJ.  We have _Cryolophosaurus_ (carnosaur), _Zupaysaurus_ (basal 
tetanuran, though possibly ceratosaurian), maybe _"Megalosaurus" cambrensis_ 
(ceratosaur?), and a few other bits and pieces.  But the dominant theropods 
at this time were ceratosaurs, as well as things like _Eoraptor_ and 
herrerasaurs that may not even be theropods.  It seems that the Tetanurae 
did not 'bloom' until the mid-Jurassic, and the same may be true for the 
ornithischians.

Cheers

Tim