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Re: Giraffatitan



Grefory Paul wrote:

Because there are significant differences between what is known of B.
altithorax and brancai I applied a new subgeneric title to the latter in case
additional fossils of the former verify the designation, or require a generic
separation. Wanted to do it myself since the African species deserved a cool and
appropriate name. Although the Morrison brachiosaur skull tends to favor
separation, the situation has not changed enough yet for full distinction to be
proposed. If and when additional Morrison remains require a generic separation then
it has to be Giraffatitan.

The skull in question (USNM 5730), designated "_Brachiosaurus_" sp. is certainly different from that of _B. brancai_; but whether it warrants generic separation from the Tendaguru brachiosaur (_brancai_) is a subjective decision. Further, we cannot be 100% sure that USNM 5730 belongs to _Brachiosaurus altithorax_ since, although the skull certainly comes from a brachiosaurid (or at least a basal titanosauriform), it was not associated with _B. altithorax_ skeletal elements AFAIK, and there may be more than one brachiosaur in the Morrison. Carpenter and Tidwell (1998) deliberately held off referring USNM 5730 to _altithorax_.


Less important than how different _brancai_ and _alithorax_ are is where they fall out in phylogenetic analysis. If the characters shared by _brancai_ and _altithorax_ are all basal titanosauriform characters, and the two species share no unique derived characters, then this is a strong case for splitting the species into separate genera. Relationships within the basal titanosauriform nexus are a little tangled and confused, and many characters used to diagnose the Brachiosauridae have been shown to be basal titanosauriform characters.

The "tall shoulders" of brachiosaurids (forelimbs much longer than the hindlimbs) have been regarded as the hallmark of this group, and this character alone should be enough to bring together _brancai_ and _altithorax_ (and perhaps some European taxa) to the exclusion of other titanosauriforms. However, it has been suggested that the striking forelimb/hindlimb disparity of brachiosaurs has been exaggerated, and inspired by the Humbolt mount of _B. brancai_ that was composed of elements from different-sized individuals:

http://dml.cmnh.org/1999Nov/msg00122.html

And, yes, _Giraffatitan_ is a cool name, IMHO.


Tim

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