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Re: Amazing Tendaguru and the most prolific localities in the world
Mike Taylor wrote;
By the way, the name _Giraffatitan_ is not widely loved. Wilson and Sereno
(1998) among others prefer the original name, _Brachiosaurus brancai_. The
paper that erected the subgenus _Giraffatitan_, Paul 1988, did so on the
basis of differences between the anterior dorsals of the two
_Brachiosaurus_ species; however, the only known anterior dorsal of _B.
altithorax_ turned out to be from _Supersaurus_, a diplodocid (Curtice et
al. 1996), so it's hardly surprising that it
was different from those of _B. brancai_! Only the seven most posterior
dorsals are preserved in the type specimen FMNH 25107 of _B. altithorax_;
these are most likely D5-D11, but since the length of the _Brachiosaurus_
dorsal column is not known, we can't be sure.
Well, I for one like _Giraffatitan_ (although perhaps "love" is too strong a
word). Setting aside Greg Paul's evidence (I agree with Mike's
interpretation of the dorsals), I think one can still build a strong case
for separating the Morrison (_B. altithorax_) and Tendaguru (_B. brancai_)
brachiosaur species into separate genera, and thereby supporting the
validity of the genus _Giraffatitan_.
(1) Salgado et al. (1997) found that the characters that have traditionally
been used to unite _altithorax_ and_brancai_ to the exclusion of other
brachiosaurid species are actually primitive titanosauriform traits.
However, other studies seem to disagree (e.g., Wilson and Sereno, 1998;
Naish et al., 2004). The length of the humerus relative to the femur has
been regarded as diagnostic for _Brachiosaurus_, but I have heard that (a)
the humerus of _B. brancai_ is incomplete; (b) the mounted skeleton of _B.
brancai_ is a composite of several individuals, meaning there is no
guarantee that the humerus and femur belong to the same individual (making a
humerus/femur ratio worthless); and (c) a similar humerus/femur ratio (or
estimated ratio) is seen in _Atlasaurus_. I have yet to verify (a), (b) or
(c) myself (they could all be wrong), but they may be worth looking into.
(2) A brachiosaurid skull (USNM 5730) from the Morrison Formation, described
by Carpenter and Tidwell (1998), has been referred to _Brachiosaurus_.
(BTW, this is the same skull that was previously and erroneously thought to
belong to _Apatosaurus_.) USNM 5730 clearly belongs to a basal
titanosauriform, probably a brachiosaurid, as it resembles the skull of _B.
brancai_ more than that of any other known sauropod species (e.g., both
share the "stepped", subrectangular muzzle). However, in other respects,
USNM 5730 shows some plesiomorphic characters, especially in the snout
(shorter maxilla with more teeth), and perhaps in the dentition too (the
only known tooth of USNM 5730 is more like the teeth of_Camarasaurus_ than
_B. altithorax_). USNM 5730 is currently classified as _Brachiosaurus_ sp.,
and it might be generically distinct from the skull of _B. brancai_. Of
course, this depends on how finely calibrated your genus-meter is; but a
phylogenetic analysis may find that USNM 5730 and the _B. brancai_ skull do
not come up as monophyletic. It is also worth noting that we don't have
many skulls for basal titanosauriforms / brachiosaurids for comparison.
(3) I also prefer to keep the two species in separate genera due to
biogeographical considerations. To put _altithorax_ and _brancai_ in the
same genus tends to overstate the similarities between the Morrison and
Tendaguru dinofaunas, which I don't think were as similar as traditionally
thought. In the same vein, the idea that _Allosaurus_, _Ceratosaurus_ and
_Barosaurus_ were also shared by the Tendaguru and Morrison Formations is
probably due to a combination of inadequate material and incorrect generic
referrals: the theropod material is generically indeterminate, and the
Tendaguru "Barosaurus" is probably _Tornieria_. As for
_Dryosaurus_/_Dysalotosaurus_ - these may also be separate genera too.
Other _B. altithorax_ material is disappointingly scarce.
Very true - and although I think this material is referrable to
_Brachiosaurus_ (e.g., the "Ultrasaurus" scapula), I don't think we can be
sure that it belongs to the species _B. altithorax_. There may be more than
one _Brachiosaurus_ species in the Morrison, just as there are more than one
_Apatosaurus_, _Diplodocus_, _Camarasaurus_ and (maybe) _Barosaurus_ and
_Haplocanthosaurus_ species across the length and depth of the Morrison