neural spine bifurcation in sauropod dinosaurs of the - PalArch

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Wedel & Taylor, Neural Spine Bifurcation in Sauropods PalArch’s Journal of Vertebrate Palaeontology, 10(1) (2013) apophysis and the right rib is also broken. BYU 12613 might represent a small adult Diplodocus or Kaatedocus (Tschopp & Mateus, 2012) or a subadult in which the neural arch and cervical ribs fused relatively early in ontogeny; current evidence is insufficient to falsify either hypothesis. Nevertheless, it indicates that small individuals of known taxa can have fully fused vertebrae and deeply bifurcated neural spines. Size can be a particularly misleading indicator of ontogenetic age for isolated vertebrae, because vertebrae change so dramatically in size along the vertebral column in sauropods. In the famous mounted skeleton of Diplodocus carnegii CM 84/94, the largest post-axial cervical vertebra, C15, has a posterior centrum diameter more than three and a half times that of C3, the smallest (245 vs 69 mm; Hatcher, 1901: 38; figure 8). Although open neurocentral joints (properly synchondroses while developing and synostoses when fused) indicate juvenile or subadult status in extant crocodilians (Brochu, 1996; Ikejiri, 2012) and in fossil archosaurs (Irmis, 2007), including sauropods (e.g. Peterson & Gilmore, 1902; Myers & Fiorillo, 2009; Remes et al., 2009), it is not clear that the reverse is true. In other words, fused neurocentral synostoses do not necessarily indicate the attainment of either full size or skeletal maturity. Regarding full size, the MB.R.2180 (formerly HM SI) lectotype specimen of Giraffatitan brancai is only about 75% the size of the mounted paralectotype MB.R.2181 (formerly HM SII) and only Figure 8. Third and fifteenth cervical vertebrae of Diplodocus carnegii CM 84/94 in posterior view. The cotyle diameters of the vertebrae are 69 and 245 mm, respectively. Modified from Hatcher (1901: plate 6). 66% the size of the largest known individuals of Giraffatitan from Tendaguru, but has fully fused neurocentral synostoses throughout the presacral vertebrae. In comparing neurocentral fusion with respect to other markers of skeletal maturity, we note that Apatosaurus ajax YPM 1860, G. brancai MB.R.2181, and Diplodocus carnegii CM 84/94 all have fused neucentral synostoses in all of the vertebrae that can be assessed, but YPM 1860 has a substantially unfused sacrum, MB.R.2181 has an unfused scapulocoracoid joint, and CM 84/94 has unfused cervical ribs in the anterior cervical vertebrae. Fusion of neurocentral synostoses is probably a necessary but not sufficient criterion for inferring adulthood in sauropods. The sacrum and in particular the sacricostal yokes formed by the fused sacral ribs are the bony interfaces between the axial skeleton and the hindlimbs, so we might expect them to be biomechanically important and for their formation to be closely related to the attainment of adult size. But as criteria for inferring adulthood in sauropods, fusion of the sacral vertebrae and formation of the sacricostal yokes are also problematic. From a practical standpoint, sacral fusion can be difficult to assess, especially if during preparation matrix is left around the sacrum or between the sacral ribs. As a more biological consideration, the sacrum and sacricostal yokes are not always the last elements to fuse in ontogeny, and the timing of sacral fusion relative to other fusions varies among taxa. Apatosaurus ajax YPM 1860 has fused neural arches and cervical ribs but a very incompletely fused sacrum (Ostrom & McIntosh, 1966: plate 29), whereas Diplodocus carnegii CM 84/94 has fused scapulocoracoids, the five sacral centra coossified and a sacricostal yoke uniting the ribs of S2-S5 (S1-S4 of Hatcher, 1901, who described S1 as an eleventh dorsal), but the cervical ribs of the anterior cervical vertebrae are unfused (Hatcher, 1901: plate 3). Brachiosaurus altithorax FMNH P 25017 has a fully fused sacrum and sacricostal yokes, but the scapulocoracoid joint is still open (Riggs, 1904: plates 73 and 75). Other macro-scale indicators of skeletochronology in sauropods suffer the same problem of variation among taxa. In Diplodocus carnegii CM 84/94 cervical rib fusion apparently followed the incorporation of S5 into the sacrum and fusion of the scapulocoracoid joint. This contrasts © PalArch Foundation 8
Wedel & Taylor, Neural Spine Bifurcation in Sauropods PalArch’s Journal of Vertebrate Palaeontology, 10(1) (2013) with the pattern in Apatosaurus ajax YPM 1860, in which the cervical ribs are fused but S1 and S5 are not fused to the sacrum, and in Giraffatitan brancai MB.R.2181, in which the cervical ribs are all fused but the scapulocoracoid joint is open. MB.R.2181 also has plates of ossified interspinous ligaments between the neural spines of D11 and D12 (Janensch, 1950: figure 62), so even the ossification of these ligaments, which in other cases has been taken as a sign of advanced age (McIntosh, Miller, et al., 1996), may not always indicate adulthood. The inconsistent sequencing of all of these macro-scale skeletal changes in sauropod skeletons is summarized in table 1. The most reliable method for determining cessation of growth is the formation of an external fundamental system (EFS) in the outer cortex of a bone (Sander et al., 2004). However, it is possible that not all sauropods formed an EFS, or, if an EFS formed, it may have been quickly remodelled to the point of being unidentifiable (Klein et al., 2012). Even if an EFS is present, it can only be assessed by histological sectioning, which requires destructive sampling (even if only drilling cores), is timeconsuming, and has been done for few individual sauropods. The implication of the foregoing discussion is that the readily available ways of determining adulthood in sauropods are all inexact and frequently conflict with each other. For the purposes of this paper we will refer to the large mounted skeletons – Apatosaurus louisae CM 3018, Diplodocus carnegii CM 84/94, Giraffatitan brancai MB.R.2181, and others – and individuals of like size as ‘adults’ because they are latest ontogenetic stages that are well-represented by existing fossils, and as such they form the de facto comparative basis for our understanding of these taxa. It is possible that when complete skeletons become available for even larger individuals, such as the Apatosaurus represented by OMNH 1670, we will have to revise our idea of what constitutes adult morphology for certain taxa. We refer to the large mounted skeletons as adults without implying that they had finished growing or had developed external fundamental systems, or that smaller individuals were necessarily subadult. ‘Adult’ is used herein as a term of convenience, not a biological fact. Data from Descriptive Monographs Before examining the data from Woodruff & Fowler (2012), it will be useful to review previously published observations on neural spine bifurcation in the Morrison Formation sauropods (figure 9). We include Suuwassea emilieae in this compilation, although its status as an independent source of data depends on whether it is a valid taxon or a juvenile of a known diplodocid, as argued by Woodruff & Fowler (2012). We will revisit this point in detail in a later section; for now we are merely reviewing the data available before the publication of Woodruff & Fowler (2012), and drawing what inferences we can. S1S S4S S1C S5C S1R S5R CR SC LO A. ajax YPM 1860 no no no no yes no no A. excelsus YPM 1981 yes no yes no A. excelsus YPM 1980 no yes yes yes yes yes yes no no D. carnegii CM 84/94 no no yes yes no yes no yes no H. delfsi CM 879 yes yes no no no no no no no H. delfsi CM 572 yes no yes yes yes yes yes no B. altithorax FMNH P 25107 no no yes yes yes yes no no G. brancai MB.R.2181* yes yes yes yes no yes * The sacral vertebrae are not preserved in MB.R.2181, but are already fused in the smaller specimen ‘Aa’ (see Janensch, 1950: figures 74 and 75). Table 1. The timing of macroscopic changes in sauropod skeletons over ontogeny is not consistent among taxa. Although this may not be surprising from an evolutionary standpoint, it complicates attempts to determine the relative ontogenetic age of sauropods with non-histological methods. Some of these differences may reflect taxonomic rather than ontogenetic variation; the larger point is that most of these specimens are taken to represent the adult morphology of their respective taxa, but every one has at least one major joint unfused. ‘yes‘ indicates fusion, ‘no‘ indicates lack of fusion, and empty cells indicate that the relevant material is not preserved. Abbreviations: S1S, sacral 1 spine fused to other spines; S4S, sacral 4 spine fused to other spines; S1C, sacral 1 centrum fused to other sacral centra; S5C, sacral 5 centrum fused to other sacral centra; S1R, sacral 1 rib fused to sacricostal yoke; S5R, sacral 5 rib fused to sacricostal yoke; CR, all available cervical ribs fused; SC, scapula and coracoid fused; LO, ligaments ossified in at least some dorsal neural spines. © PalArch Foundation 9
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