New information on a juvenile sauropod specimen from the Morrison Formation and the reassessment of its systematic position
New information on a juvenile sauropod specimen from the Morrison Formation and the reassessment of its systematic position
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DOI：
10.1111/j.1475-4983.2012.01139.x
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发表时间：
2012-05-01
期刊：
影响因子：
2.6
通讯作者：
Pabst, Ben
中科院分区：
文献类型：
--
作者：
Carballido, Jose L.;Marpmann, Jean S.;Pabst, Ben
人们对蜥脚类恐龙个体发育的形态变化知之甚少，因此很难确定非常年轻个体的系统亲缘关系。这里介绍了关于几乎完整的幼年蜥脚类恐龙 (SMA 0009) 的新信息，估计总长度约为 2 m。该标本被描述为梁龙科，因为存在该类群的一些假定的共源性。然而，最近的进一步制备揭示了梁龙科特征的缺失和长龙衍生特征的存在。为了测试该样本的亲和力，进行了系统发育分析。严格的共识树将标本恢复为基础泰坦龙类，与腕龙和长颈龙之间的关系尚未解决。然而，这里建议腕龙科的归属是基于该类群广泛接受的单系性（只有当 SMA 0009 被置于该类群内时才恢复）。尽管不能完全排除新分类单元的存在，但标本中衍生特征和拟形特征的组合表明它属于腕龙。需要执行十六个额外步骤才能将该标本归入梁龙科。将此标本归入该组的成本很高，因为 SMA 0009 中不存在一些梁龙科特征，例如颈椎中缺乏分开的前颈突突椎板、前腔前尾椎中心、前尾椎中的组成侧椎板、拉长的中尾椎、短颈肋和 最后，系统位置揭示了一些主要的个体发生转变。这些影响气动结构（例如，从颈椎中的简单胸腔到复杂胸腔的变化以及背椎侧向凹陷的发展），但也包括神经脊柱的未记录的转变（例如棘干椎板的发育、背椎神经棘的加宽）和某些部位的异速生长。 四肢骨。
Morphological changes in the ontogeny of sauropods are poorly known, making difficult to establish the systematic affinities of very young individuals. New information on an almost complete juvenile sauropod (SMA 0009) with an estimated total length of about 2 m is here presented. The specimen was described as a diplodocid owing to the presence of some putative synapomorphies of this group. However, recent further preparation revealed the absence of diplodocid characters and the presence of macronarian derived characters. To test the affinities of this specimen, a phylogenetic analysis was conducted. The strict consensus tree recovers the specimen as a basal titanosauriform, in an unresolved relation with Brachiosaurus and Giraffatitan. Nevertheless, a brachiosaurid assignment is here suggested in base of the widely accepted monophyly of this group (only recovered when SMA 0009 is placed within this group). Although the existence of a new taxon cannot be completely ruled out, the combination of derived and plesiomorphic characters in the specimen suggests its assignment to Brachiosaurus. Sixteen extra steps are needed to place this specimen within Diplodocidae. The high cost to place this specimen within this group is owing to the fact that several diplodocid characters are absent in SMA 0009, such as the absence of divided centroprezygapophyseal lamina in cervical vertebrae, procoelous anterior caudal centra, composed lateral lamina in anterior caudal vertebrae, elongated middle caudal vertebrae, short cervical ribs and caudolateral projection of distal condyle of metatarsal I. Finally, the systematic position reveals few major ontogenetic transformations. These affect the pneumatic structures (e.g. change from simple pleurocoels in the cervical vertebrae to complex pleurocoels and the development of lateral excavations in the dorsal vertebrae) but also include unrecorded transformations of the neural spine (e.g. the development of the spinodiapophyseal lamina, the widening of the neural spines in the dorsal vertebrae) and allometric growth in some limb bones.