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Re: Where have all the ornithischians gone?



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