Re: More on Moser and prosauropod sacrals

[m.moser@lrz.uni-muenchen.de]

Jaime Headden wrote

>   My only comment on this issue will be observation on one specimen of
> *Silesaurus,* ZPAL Ab III/404, in which the sacrum is a
> unified object of four vertebrae, comprising dorsosacrals 1 and 2, plus
> the two primordial archosaurian sacrals, and no caudosacral,
> based on the large size and contact of sacral ribs and ilia in the last
> two elements. Despite this, the dirst dorsosacral is fused
> to the second, but is almost complete anterior to the anterior extent of
> the ilium, and does not contact it. It is, essentially, a
> fused "lumbar," being a dorsal without facets for the ribs, and thus
> *Silesaurus* may be described as having a synsacrum. The reason
> I mention this is the exclusionary data for defining possible dorsosacrals
> (or caudosacrals) based on how many vertebrae are defined
> by extent of the preacetabular alae or contact with the ilium. As this is
> a primitive near-dinosaur (or possible dinosaur), it seems
> particularly relevant in considerations of possible sacral compositions in
> experimentations in other basal dinosaurs, including
> "prosauropods" as to sacral recruitment.

The well illustrated sacrum of Silesaurus IMHO confirms my position
regarding Massospondylus: There are only two sacrals present (S1, S2, with
key features that are present in S1 and S2 of Plateosaurus, Anchisaurus,
Vulcanodon etc.). In Silesaurus the two last dorsals are fused to the
sacrum. As these dorsals have no contact to the ilium, they are better
termed sacrodorsals than dorsosacrals, as the term sacral is reserved for
vertebrae that have a bony contact with the ilium. In both Silesaurus and
Massospondylus (ok, only the specimen figured by Cooper) the transversal
processes of the last two dorsals, although stoutly built, are too short
to reach the ilium. In Silesaurus as opposed to full grown Massospondylus
(Cooper 1981 fig. 39A) the postacetabular process of the ilium is not
developed enough to support a caudosacral. I conceive that the last
mentioned specimen of Massospondylus has also an elongated preacetabular
process and I'm very interested in seeing evidence of a true dorsosacral
as promised by Adam Yates, and where in relation to SR1 the dorsosacral
rib DSR1 is attached to the ilium.

Kind regards

MarKus Moser