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Re: More on Moser and prosauropod sacrals
Adam Yates wrote
> I admire Marcus's optimism, but for the moment I'm not
> persuaded by his arguments and maintain that a dorsosacral
> is the basal condition for Prosauropoda + Sauropoda. The
> presence of a caudosacral is an unusual synapomorphy of
> Plateosaurus (and as far as I can tell Plateosaurus alone).
A caudosacral is present in Plateosaurus (some "Sellosaurus"), Efraasia
(incl. other "Sellosaurus"), Gresslyosaurus (fused with S2), and Ruehleia:
CS1 in all specimens I have seen so far, AND in Vulcanodon (only 3 sacrals
present, the earlier ASSUMPTION of presence of a dorsosacral was based on
the ASSUMPTION that the ilium when complete shows a sauropod like expanded
processus praeacetabularis, which is not preserved at all). Therefore the
basal sacral condition is with a caudosacral.
> A dorsosacral is most definitely present in Massospondylus (I
> am in the fortunate position of having a number of Masso.
> specimens at my disposal). Most [ALL] are undescribed or
> unfigured but Cecilio Vasconcelos is working hard to
> rectify this problem (at least postcranially).
I wait for that paper. If a dorsosacral is present indeed, then this must
be one similar to the Melanorosaurus style sacrum described by Heerden &
Galton, i. e. not homologous with the sauropod style, in which DSR1 is
attached to the ilium cranially of SR1, while in Melanorosaurus DSR1 is
attached to the ilium dorsally of SR1. The length of Massospondylus ilia
does not allow for a sauropod style sacrum.
> Marcus himself
> has shown that the last sacral vertebra of Masso. is the
> second primordial but argues upon the basis of Cooper's
> illustrations (of a sacrum not in contact with the ilium) that
> there were only two sacral vertebrae (ie. the two primordial
> sacrals and no dorsosacral).
Please note that I argue for a sacrum of TWO vertebrae in THAT specimen
(the only sacrum illustration published so far), which is apparently not
full grown and therefore the expanded transversal process of the first
caudal does not reach the ilium yet. The transversal process of the last
dorsal ("s1" of Cooper) is too short to reach the ilium either. It is
altogether impossible however that the "incipient fourth sacral" ("s1a",
Cooper) had reached any part of the ilium - no way for a four
sacrals-sacrum. So using only vertebral characters it seems to be
ambiguous whether the next ontogenetical step is the inclusion of a
caudosacral or a dorsosacral. But a look at the ilium (not only of this
specimen) reveals that only the postacetabular process is elongated enough
for supporting a sacral rib, but not the preacetabular process. Thus only
a prosauropod style sacrum with a caudosacral is to be expected.
> Much better specimens at the
> BPI and SAM show very clearly that this is not the case. So
> I guess the argument will be how much of the Prosauropoda
> has a Plateosaurus-like arrangement and how much has the
> Massospondylus-like arrangement. Based on my
> observations of Coloradisaurus, Riojasaurus (there are
> better specimens than the one illustrated by Bonaparte),
> Anchisaurus and Efraasia I'd say most had dorsosacrals.
> Most of these sacra are extremely similar to that of
> Massospondylus (Anchisaurus is the odd one out with an
> unusual sacrum and consequently I'm rather less certain
> about its homologies).
Efraasia (pers. obs.) and Anchisaurus (cf. Galton 1976 fig. 26B) have
clearly S1, S2, and CS1. I would very much like to see Coloradisaurus &
Riojasaurus, and all the bunch of Chinese prosauropods and would-be
sauropods of which there have been no/too bad illustrations yet published.
> Anyway my heartiest congratulations to Marcus on a very
> thorough and valuable monograph. It contains a trove of
> information and lots of food for thought.
Many thanks for your discussion and kind words.
I hope my work will stimulate more scientists to crawl beneath there
mounted skeletons - to have a look at those interesting sacrals.