A Re-Evaluation of Brachiosaurus Altithorax (Riggs, 1903)
Total Page：16
File Type：pdf, Size：1020Kb
Journal of Vertebrate Paleontology 29(3):787–806, September 2009 # 2009 by the Society of Vertebrate Paleontology ARTICLE A RE-EVALUATION OF BRACHIOSAURUS ALTITHORAX RIGGS 1903 (DINOSAURIA, SAUROPODA) AND ITS GENERIC SEPARATION FROM GIRAFFATITAN BRANCAI (JANENSCH 1914) MICHAEL P. TAYLOR* Palaeobiology Research Group, School of Earth and Environmental Sciences, University of Portsmouth, Burnaby Road, Portsmouth PO1 3QL, United Kingdom, [email protected] ABSTRACT—Although the macronarian sauropod Brachiosaurus is one of the most iconic dinosaurs, its popular image is based almost entirely on the referred African species Brachiosaurus brancai rather than the North American type species Brachiosaurus altithorax. Reconsideration of Janensch’s referral of the African species to the American genus shows that it was based on only four synapomorphies and would not be considered a convincing argument today. Detailed study of the bones of both species show that they are distinguished by at least 26 characters of the dorsal and caudal vertebrae, coracoids, humeri, ilia, and femora, with the dorsal vertebrae being particularly different between the two species. These animals must therefore be considered generically separate, and the genus name Giraffatitan Paul 1988 must be used for “Brachiosaurus” brancai, in the combination Giraffatitan brancai. A phylogenetic analysis treating the two species as separate OTUs nevertheless recovers them as sister taxa in all most parsimonious trees, reaffirming a mono- phyletic Brachiosauridae, although only one additional step is required for Giraffatitan to clade among somphospondy- lians to the exclusion of Brachiosaurus. The American Brachiosaurus is shown to be somewhat different from Giraffatitan in overall bodily proportions: it had a longer and deeper trunk and probably a longer and taller tail, carried a greater proportion of its mass on the forelimbs, and may have had somewhat sprawled forelimbs. Even though it was overall a larger animal than the Giraffatitan lectotype, the Brachiosaurus holotype was probably immature, as its coracoids were not fused to its scapulae. INTRODUCTION Nomenclature for vertebral laminae follows that of Wilson (1999). The sauropod dinosaur Brachiosaurus Riggs, 1903 is one of Anatomical Abbreviations—ACDL, anterior centrodiapophy- the most iconic of all prehistoric animals, immediately recogniz- seal lamina; PCPL, posterior centroparapophyseal lamina; able by its great size, tall shoulders, long neck and helmet-like PODL, postzygadiapophyseal lamina; PPDL, paradiapophyseal skull. However, much of the distinctive morphology attributed lamina; PRPL, prezygaparapophyseal lamina; SPPL, spinopara- to Brachiosaurus is known only from the referred species, pophyseal lamina. B. brancai Janensch, 1914, and not from the type species Institutional Abbreviations—BMNH, Natural History Muse- B. altithorax Riggs, 1903. That B. brancai belongs to Brachio- um, London, United Kingdom; BYU, Brigham Young Universi- saurus was asserted but not convincingly demonstrated by ty, Provo, Utah; FMNH, Field Museum of Natural History, Janensch (1914), and contradicted but not disproved by Paul Chicago, Illinois; HMN, Humboldt Museum fu¨r Naturkunde, (1988). This study reviews the history of the two species, assesses Berlin, Germany; OMNH, Oklahoma Museum of Natural His- the similarities and differences between them, assesses their rela- tory, Norman, Oklahoma; USNM, National Museum of Natural tionships within a broader phylogenetic context, and discusses History, Smithsonian Institution, Washington D.C. the implications for the phylogenetic nomenclature of sauropods. Anatomical Nomenclature—The term Gracility Index (GI) is HISTORICAL BACKGROUND introduced to quantify the gracility of the humeri and other long bones discussed in this study, and is defined as the ratio between the Initial Finds proximodistal length of the bone and its minimum transverse width. The type species of the genus Brachiosaurus is Brachiosaurus Many different sets of directions have been used to describe altithorax, founded on a partial skeleton collected from the Grand sauropod coracoids, with the edge furthest from the scapular River valley of western Colorado by the Field Columbian Museum articulation having been variously described as median (e.g., paleontological expedition of 1900 under the leadership of Elmer Seeley, 1882), inferior (Riggs, 1904), anteromedial (Powell, S. Riggs (now accessioned as specimen FMNH P 25107). It com- 1992), distal (Curry Rogers, 2001) and anterior (Upchurch prises the last seven dorsal vertebrae, sacrum, the first two caudal et al., 2004), and the designation of the other directions varying vertebrae (one in very poor condition), left coracoid, right humer- similarly. I follow Upchurch et al. (2004) in describing the cora- us, ilium and femur, fragmentary left ilium, and dorsal ribs. The coid as though the scapulocoracoid were oriented horizontally: the type specimen does not contain any material from the skull, neck, scapular articular surface is designated posterior, so that the gle- anterior dorsal region, median or posterior parts of the tail, distal noid surface of the coracoid is considered to face posteroventrally. parts of the limbs or feet; nor has such material been confidently referred to the species (although see below). Brachiosaurus altithorax was first reported, unnamed, by Riggs (1901), and subsequently named and briefly described on *Current address: Department of Earth Sciences, University College the basis of some but not all elements, the dorsal vertebrae not London, Gower Street, London WC1E 6BT, United Kingdom yet having been prepared (Riggs, 1903). After preparation was 787 788 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 29, NO. 3, 2009 complete, the material was more fully described and figured in a condition, while most cervical vertebrae are lacking nearly all monograph on the new family Brachiosauridae (Riggs, 1904). processes and laminae. An initial assessment of the material Riggs (1903:299, 1904:230) assigned the coracoid to the right indicates that it probably represents a second distinct Tendaguru side, but it is from the left: the orientation of the scapular margin brachiosaur (Taylor, 2005). indicates that the coracoid as figured by Riggs (1903:fig. 3, 1904: Apart from the specimens recovered by the German expedi- pl. LXXV, fig. 4) and as displayed in the FMNH collection is tions, Migeod’s specimen is the only Tendaguru brachiosaur either a right coracoid in medial view or a left coracoid in lateral material to have been been reported. However, several later view; and the posteromedial-anterolateral orientation of the cor- finds of sauropod material in the U.S.A. have been referred, acoid foramen and lateral flaring of the bone to support the with varying degrees of certainty, to Brachiosaurus. glenoid articular surface (observed in photos provided by Phil Potter Creek Humerus—As recounted by Jensen (1985, 1987), Mannion) show that it is the latter. Eddie and Vivian Jones collected a large left humerus from the Brachiosaurus is also known from a second species, Brachio- Uncompahgre Upwarp of Colorado and donated it to the Smith- saurus brancai, excavated from Tendaguru in Tanzania by the sonian Institution where it is accessioned as USNM 21903. It was German expeditions of 1909-1912 (Maier, 2003). Janensch (1914) designated Brachiosaurus (Anonymous, 1959) although no rea- initially also named a second Tendaguru species, Brachiosaurus son for this assignment was published; it was subsequently de- fraasi Janensch, 1914, but subsequently synonymized this species scribed very briefly and inadequately by Jensen (1987:606-607). with B. brancai (Janensch, 1929:5, 1935-1936:153, 1950a:31). Un- Although its great length of 213 cm (pers. obs.) is compatible like the type species, B. brancai is known from many specimens of with a brachiosaurid identity, it is in some other respects differ- varying degrees of completeness, in total including almost all skel- ent from the humeri of both B. altithorax and B. brancai, al- etal elements. The original type specimen, “Skelett S” (Janensch, though some of these differences may be due to errors in the 1914:86) was subsequently found (e.g., Janensch, 1929:8) to consist significant restoration that this element has undergone. The of two individuals, which were designated SI (the smaller) and SII bone may well represent Brachiosaurus altithorax, but cannot (the larger and more complete). Janensch never explicitly desig- be confidently referred to this species, in part because its true nated these two specimens as a syntype series or nominated either proportions are concealed by restoration (Wedel and Taylor, in specimen as a lectotype; I therefore propose HMN SII as the prep.). It can therefore be discounted in terms of contributing to lectotype specimen of Brachiosaurus brancai. The skull, with its an understanding of the relationship between B. altithorax and distinctive nasal arch, and the very long neck, are known only B. brancai. from B. brancai, and it is primarily from this species that nearly Other Potter Creek Material—Further brachiosaurid material all previous skeletal reconstructions and life restorations have was recovered from the Potter Creek quarry in 1971 and 1975 been executed, beginning with that of Matthew (1915: fig. 24). (Jensen, 1987:592-593), including a mid-dorsal vertebra, incom- The sole exception is the partial reconstruction that Paul (1998: plete left ilium, left radius and right metacarpal.