Woodruff D Cary, Fowler Denver W
Museum of the Rockies, Montana State University, Bozeman, MT, USA.
J Morphol. 2012 Jul;273(7):754-64. doi: 10.1002/jmor.20021. Epub 2012 Mar 28.
Within Diplodocoidea (Dinosauria: Sauropoda), phylogenetic position of the three subclades Rebbachisauridae, Dicraeosauridae, and Diplodocidae is strongly influenced by a relatively small number of characters. Neural spine bifurcation, especially within the cervical vertebrae, is considered to be a derived character, with taxa that lack this feature regarded as relatively basal. Our analysis of dorsal and cervical vertebrae from small-sized diplodocoids (representing at least 18 individuals) reveals that neural spine bifurcation is less well developed or absent in smaller specimens. New preparation of the roughly 200-cm long diplodocid juvenile Sauriermuseum Aathal 0009 reveals simple nonbifurcated cervical neural spines, strongly reminiscent of more basal sauropods such as Omeisaurus. An identical pattern of ontogenetically linked bifurcation has also been observed in several specimens of the basal macronarian Camarasaurus, suggesting that this is characteristic of several clades of Sauropoda. We suggest that neural spine bifurcation performs a biomechanical function related to horizontal positioning of the neck that may become significant only at the onset of a larger body size, hence, its apparent absence or weaker development in smaller specimens. These results have significant implications for the taxonomy and phylogenetic position of taxa described from specimens of small body size. On the basis of shallow bifurcation of its cervical and dorsal neural spines, the small diplodocid Suuwassea is more parsimoniously interpreted as an immature specimen of an already recognized diplodocid taxon. Our findings emphasize the view that nonmature dinosaurs often exhibit morphologies more similar to their ancestral state and may therefore occupy a more basal position in phylogenetic analyses than would mature specimens of the same species. In light of this, we stress the need for phylogenetic reanalysis of sauropod clades where vital characters may be ontogenetically variable, particularly when data is derived from small individuals.
在梁龙超科（恐龙纲：蜥脚亚目）中，雷巴齐斯龙科、叉龙科和梁龙科这三个亚科的系统发育位置受到相对较少特征的强烈影响。神经棘分叉，尤其是在颈椎内，被认为是一个衍生特征，缺乏这一特征的类群被视为相对原始。我们对小型梁龙超科动物（至少代表18个个体）的背椎和颈椎进行分析后发现，较小的标本中神经棘分叉发育较差或不存在。对约2米长的梁龙科幼年个体索里耶博物馆阿阿萨尔0009的重新清理显示，其颈椎神经棘简单且未分叉，这与更原始的蜥脚类动物如峨嵋龙极为相似。在基础大鼻龙类的圆顶龙的几个标本中也观察到了与个体发育相关的相同分叉模式，这表明这是蜥脚亚目几个类群的特征。我们认为，神经棘分叉执行着与颈部水平定位相关的生物力学功能，而这一功能可能仅在体型变大时才变得重要，因此，在较小的标本中它明显缺失或发育较弱。这些结果对根据小体型标本描述的类群的分类和系统发育位置具有重要意义。基于其颈椎和背椎神经棘的浅分叉，小型梁龙科的苏瓦塞龙更简约地被解释为一个已被认可的梁龙科类群的未成熟标本。我们的发现强调了这样一种观点，即未成熟恐龙通常表现出与其祖先状态更相似的形态，因此在系统发育分析中可能比同一物种的成熟标本占据更原始的位置。鉴于此，我们强调有必要对蜥脚类类群进行系统发育重新分析，因为关键特征可能在个体发育过程中发生变化，特别是当数据来自小个体时。
