sábado, 25 de setembro de 2010

Today sees the publication of the new Journal of Vertebrate Paleontology, and with it my paper on the two best-known brachiosaurs and why they’re not congeneric (Taylor 2009).  This of course is why I have been coyly referring to “Brachiosaurus” brancai in the last few months … I couldn’t bear to make the leap straight to saying Giraffatitan, a name that is going to take me a while to get used to.
But before we go lunging into the details, here is my skeletal reconstruction of Brachiosaurus proper, taken from the paper:
Skeletal reconstruction of Brachiosaurus altithorax, with Homo sapiens and Canis familiaris for scale, from Taylor (2009:fig. 7). White bones represent the elements of the holotype FMNH P 25107. Light grey bones represent material referred to B. altithorax: the Felch Quarry skull USNM 5730, the cervical vertebrae BYU 12866 (C?5) and BYU 12867 (C?10), the "Ultrasauros" scapulocoracoid BYU 9462, the Potter Creek left humerus USNM 21903, left radius and right metacarpal III BYU 4744, and the left metacarpal II OMNH 01138. Dark grey bones modified from Paul's (1988) reconstruction of Giraffatitan brancai. Scale bar equals 2 m.
Those of you familiar with Greg Paul’s classic reconstruction of Giraffatitan brancai will immediately recognise that Real Brachiosaurus is rather differently proportioned, especially in having a longer torso and tail.
This paper has been in the works for some time, and while it was in review and then in press at JVP, it led double life as Chapter 2 of my dissertation.  (For most of its gestation period, the paper’s title was just “Brachiosaurus brancai is not Brachiosaurus“, and the folder where I keep all the project files is still called “bb-is-not-b”).  In the end, I chickened out and went for a longer, more formal, title.
So why are the two species not congeneric?  Well, it’s a long story, and you can read about the detail in the paper, but the bottom line is that virtually every bone that is known from both species differs in significant respects between them.
Of course, I am not the first to suggest that the African brachiosaurid that we know and love isn’t exactly Brachiosaurus.  Credit for that goes to Greg Paul, who more than twenty years ago executed a then-new skeletal reconstruction of that species (the very same reconstruction that is now considered the classic), and in doing so noticed some differences between the American type species Brachiosaurus altithorax and the African referred species “Brachiosaurus” brancai (Paul 1988).  Paul hedged his bets, though: rather than erect a new genus for the African animal, he proposed a subgenus Brachiosaurus (Giraffatitan), so that the full name of the species would become Brachiosaurus (Giraffatitan) brancai; and that of the type species would become Brachiosaurus (Brachiosaurus) altithorax.  Unsurprisingly, this cumbersome nomenclatural scheme did not catch on, and I have not been able to locate a single subsequent reference to these subgenera in the literature.
Second caudal vertebrae of Brachiosaurus altithorax and Brachiosaurus brancai, equally scaled, from Taylor (2009:fig. 3). A, B, B. altithorax holotype FMNH P 25107; C-G, B. brancai referred specimen HMN Aa. A, C, posterior; B, D, F, right lateral; E, G, anterior. A-B modified from Riggs (1904:pl. LXXV); C-E modified from Janensch (1950a:pl. 2), F-G modified from Janensch (1929:fig. 15). Scale bar equals 50 cm.
That didn’t mean the idea was dead, though: three years later, George Olshevsky’s self-published mega-revision of dinosaur taxonomy proposed raising the name Giraffatitan to genus level (Olshevsky 1991).  Although this genus became popular on the Internet (it cropped up, for example, in Mike Keesey’s much-lamented Dinosauricon web-site), it was almost completely ignored in the technical literature, and even Greg Paul himself subsequently seems to have reverted to using the name Brachiosaurus brancai (e.g. Paul 1994:246).
Why was the new name overlooked?  Partly, I suspect, just because it’s so butt ugly — everyone knows and loves Brachiosaurus brancai, and the name itself has a definite poetry to it that Giraffatitan sorely lacks.  But mostly it’s because Paul didn’t really make a case for the separation that he proposed — wrongly stating, for example, that “the caudals, scapula, coracoid, humerus, ilium, and femur of B. altithorax and B. brancai are very similar” (Paul 1988:7).
That’s how things stood a few years back when I started to take a serious interest in Migeod’s Tendaguru brachiosaurid, which lives in the basement of the Natural History Museum in London.  It quickly started to seem to me that it wasn’t the same thing as what everyone means by Brachiosaurus, but to make sense of it all, I needed first to figure out what the Brachiosaurus actually does mean.  That meant visiting the type material of both species, in Chicago and Berlin, and really looking closely.
Well, I don’t want to go on all day — apart from anything, England play Croatia in a World Cup qualifier in just over an hour — so I’ll just show you some of the the differences between the dorsal vertebrae of the two species.  (You’ll have seen the caudals up above — I just threw them in to break up all that text).
Dorsal vertebrae of Brachiosaurus altithorax and Brachiosaurus brancai in posterior and lateral views, equally scaled, from Taylor (2009:fig. 1). A, B, E, F, I, J, M, N, B. altithorax holotype FMNH P 25107, modified from Riggs (1904:pl. LXXII); C, D, G, H, K, L, O, P, B. brancai lectotype HMN SII, modified from Janensch (1950a:figs. 53, 54, 56, 60-62, 64) except H, photograph by author. Neural arch and spine of K sheared to correct for distortion. A, D, E, H, I, L, M, P, posterior; B, F, G, J, N, right lateral; C, K, O, left lateral reflected. A, B, dorsal 6; C, D, dorsal 4; E-H, dorsal 8; I-L, dorsal 10; M, N, P, dorsal 12; O, dorsals 11 and 12. Corresponding vertebrae from each specimen are shown together except that dorsal 4 is not known from B. altithorax so dorsal 6, the most anterior known vertebra, is instead shown next to dorsal 4 of B. brancai. Scale bar equals 50 cm.
Lots and lots of differences here — I will quote from the Systematic Paleontology section on the type species: “Postspinal lamina absent from dorsal vertebrae (character 130); distal ends of transverse processes of dorsal vertebrae transition smoothly onto dorsal surfaces of transverse processes (character 142); spinodiapophyseal and spinopostzygapophyseal laminae on middle and posterior dorsal vertebrae contact each other (character 146); posterior dorsal centra subcircular in cross-section (character 151); posterior dorsal neural spines progressively expand mediolaterally through most of their length (“petal” or “paddle” shaped) (character 155); mid-dorsals about one third longer than posterior dorsals (see Paul, 1988:7); middorsals only about 20% taller than posterior dorsals (see Paul, 1988:8); dorsal centra long (Janensch, 1950a:72) so that dorsal column is over twice humerus length (Paul, 1988:8); transverse processes of dorsal vertebrae oriented horizontally (Paul, 1988:8); dorsal neural spines oriented close to vertical in lateral view; dorsal neural spines triangular in lateral view, diminishing smoothly in anteroposterior width from wide base upwards; deep inverted triangular ligament rugosities on anterior and posterior faces of neural spines” …. *gasp*
So anyway: the upshot of all this is that “Brachiosaurus” brancai differs from Brachiosaurus altithorax more than, say, Barosaurus does from Diplodocus; and so it must be placed in its own genus … and that genus has to be Giraffatitan, because of the ICZN’s principle of priority.  And THAT is why the very end of the paper — the last sentence of the Acknowledgements — reads:
Finally, I beg forgiveness from all brachiosaur lovers, that so beautiful an animal as “Brachiosaurus” brancai now has to be known by so inelegant a name as Giraffatitan.
Anyway, go and read the paper; full-resolution figures are freely available if you want to look more closely than the JVP’s PDF allows.


(And, yes, Randy, I know what your comment is going to say; go ahead and say it anyway, it’ll give me a chance to explain why your approach is wrong :-))

Fonte: http://svpow.wordpress.com/2009/09/09/brachiosaurus-brancai-is-not-brachiosaurus/

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