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Re: Spinosaurus



In a message dated 96-11-26 22:58:51 EST,
T.Williams@cclru.randwick.unsw.edu.au (Tim Williams) writes:

<< It also fits in with the close phylogenetic relationship which many
 authors believe existed between _Becklespinax_ and _Acrocanthosaurus_
 (e.g. Greg Paul's _PDW_, where they are congeneric - as _A.
 altispinax_ and _A. atokensis_).   >>

I would keep them in different genera but agree that they belong in the same
family (Sinraptoridae, as far as I'm concerned, along with _Sinraptor_,
Piatnitzkysaurus_, _Metriacanthosaurus_, and _Yangchuanosaurus_, among
others; _B._ and _A._ were the most-derived-known members). The centra of
_B._ are clearly those of an old individual, with lots of pachyostosis of
some kind (visible in photos but I haven't yet seen the specimen in person),
yet it was only about 60% the size of _A._ (_A._ is nearly _Tyrannosaurus
rex_ size, but _B._ is more the size of an average _Allosaurus_.) Also, the
neural spines of _B._ are taller than those of _A._ relative to centrum
diameter. Another difference: the neural spines of _B._ contact each other
solidly at their tips, immobilizing the vertebrae, but those of _A._ do not
(as far as I know) anywhere in the vertebral column. For whatever it was
worth, _A._ had a more flexible back than _B._, with a more compact dorsal
ridge.

It's possible that some of these differences are related to old age in _B._,
so that young individuals would have more _Acrocanthosaurus_-like vertebrae,
but with only one _B._ specimen known, I take the differences at face value.