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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 Formation.