Osteological correlates for quadrupedality in ornithischian dinosaurs
Osteological correlates for quadrupedality in ornithischian dinosaurs
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DOI：
10.4202/app.2012.0065
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发表时间：
2014-03-01
影响因子：
1.8
通讯作者：
Barrett, Paul M.
中科院分区：
文献类型：
--
作者：
Maidment, Susannah C. R.;Barrett, Paul M.
在四足动物的进化中，从两足祖先进化成四足动物是一个非常罕见的转变，但在食草恐龙的鸟臀亚目中已经发生过几次。剑龙目、甲龙目和角龙科都是四足动物，而基鸟臀目和基鸟脚目都是两足动物。然而，在禽龙目鸟脚亚目（包括鸭嘴龙）和非角龙目角龙目中的立场是有争议的，因为四足动物的骨骼学相关性还没有被确定。我们研究了一套特征，以前提出的骨骼相关的两足或四足恐龙。这些包括离散的解剖特征，我们评估为四足动物使用字符优化到一个复合的分支图，和比例的比例，我们评估为相关的重建节点祖先状态使用平方变化简约，然后优化。我们还研究了这些特征与身体大小的相关性。尺骨近端的前外侧突、蹄状的手爪、横向加宽的髂骨、第四粗隆缩小以及股骨长于胫骨，这些都与鸟臀目恐龙的四足性有密切关系。沿着角龙类的“干”谱系，四足动物的特征是以逐步的方式获得的，前肢特征的发展先于后肢的变化。与此相反，禽龙类鸟脚类显示出一个马赛克的字符状态，表明不同程度的兼性四足动物，可能是由于各种不同的原因。鸭嘴龙被发现拥有与四足动物有关的所有特征，并且可能主要是四足动物。一般来说，四足鸟臀目似乎并没有被他们的两足祖先限制在一个特定的字符获取顺序。
The evolution of quadrupedality from bipedal ancestors is an exceptionally rare transition in tetrapod evolution, but it has occurred several times within the herbivorous dinosaur clade Ornithischia. Stegosauria, Ankylosauria, and Ceratopsidae are all uncontroversially quadrupedal, while basal ornithischians and basal ornithopods are uncontroversially bipedal. However, stance in iguanodontian ornithopods, including the hadrosaurs, and in non-ceratopsid ceratopsians is debated because robust osteological correlates of quadrupedality have not been identified. We examine a suite of characteristics that have been previously proposed as osteological correlates for bipedality or quadrupedality in dinosaurs. These include both discrete anatomical features, which we assess as correlates for quadrupedality using character optimization onto a composite cladogram, and proportional ratios, which we assess as correlates by reconstructing nodal ancestral states using squared-change parsimony, followed by optimization. We also examine the correlation of these features with body size. An anterolateral process on the proximal ulna, hoof-shaped manual unguals, a transversely broadened ilium, a reduced fourth trochanter and a femur longer than the tibia are found to be robust correlates of quadrupedality in ornithischian dinosaurs. Along the ceratopsid "stem" lineage, quadrupedal characters were acquired in a stepwise fashion, with forelimb characters developing prior to changes in the hind limb. In contrast, iguanodontid ornithopods display a mosaic of character states, indicating varying degrees of facultative quadrupedality that probably arose for a variety of different reasons. Hadrosaurs are found to possess all character states associated with quadrupedality and were probably predominantly quadrupedal. In general, quadrupedal ornithischians do not appear to have been constrained by their bipedal ancestry to a particular order of character acquisition.