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1. chinaXiv:201711.01914 [pdf]

Morphological coevolution of the pygostyle and tail feathers in Early Cretaceous birds

WANG Wei; Jingmai K. O’CONNOR
Subjects: Biology >> Zoology

The transformation from a long reptilian tail to a shortened tail ending in a pygostyle and accompanied by aerodynamic fanning rectrices is one of the most remarkable adaptations of early avian evolution. However, no fossils directly capture this transition, and information regarding the structural morphology and the early evolution of the pygostyle in Mesozoic birds and their integuments is relatively limited. Here we provide a review of the pygostyle morphology of Early Cretaceous birds with comparison to the structure in living birds. This study emphasizes the convergent evolution of distally co-ossified caudal vertebrae in non-avian maniraptorans and early birds. There further exist distinct differences in pygostyle morphology between Sapeornithiformes, Confuciusornithiformes, Enantiornithes, and Ornithuromorpha. The morphology of the pygostyle and rectrices in early ornithuromorphs appear similar to that of extant birds, whereas the pygostyle in more primitive birds does not appear morphologically capable of supporting the rectricial bulbs and musculature necessary to control an aerodynamic fan-shaped tail. The rectricial bulbs and rectricial fan appear to have coevolved with the plough-shaped pygostyle early in the evolution of the Ornithuromorpha. This study also shows that the confuciusornithiform pygostyle was more similar to that of enantiornithines than previously recognized, consistent with the presence of nearly identical ornamental tail feathers in both groups.

submitted time 2017-11-07 From cooperative journals:《古脊椎动物学报》 Hits277Downloads150 Comment 0

2. chinaXiv:201708.00188 [pdf]

The morphology of Chiappeavis magnapremaxillo (Pengornithidae: Enantiornithes) and a comparison of aerodynamic function in Early Cretaceous avian tail fans

Jingmai K. O’CONNOR; ZHENG Xiao-Ting; HU Han; WANG Xiao-Li; ZHOU Zhong-He
Subjects: Geosciences >> Geology

We provide a complete description of the skeletal anatomy of the holotype of Chiappeavis magnapremaxillo, the first enantiornithine to preserve a rectricial fan, suggesting that possibly rectricial bulbs were present in basal members of this clade. Notably, Chiappeavis preserves a primitive palatal morphology in which the vomers reach the premaxillae similar to Archaeopteryx but unlike the condition in the Late Cretaceous enantiornithine Gobipteryx. If rectricial bulbs were present, pengornithid pygostyle morphology suggests they were minimally developed. We estimate the lift generated by the tail fan preserved in this specimen and compare it to the tail fans preserved in other Early Cretaceous birds. Aerodynamic models indicate the tail of Chiappeavis produced less lift than that of sympatric ornithuromorphs. This information provides a possible explanation for the absence of widespread aerodynamic tail morphologies in the Enantiornithes. 契氏鸟(Chiappeavis)是首次发现保存有扇状尾羽的反鸟类,显示出尾羽球茎这一结构在较原始的反鸟类中已经发育。详细描述了巨前颌契氏鸟(C. magnapremaxillo)正型标本的骨骼形态学特征。契氏鸟的腭区形态与始祖鸟(Archaeopteryx)相似,而区别于晚白垩世的反鸟类戈壁鸟(Gobipteryx)。即使具有尾羽球茎,鹏鸟类的尾综骨形态也表明该结构发育较差。估算了在契氏鸟中由扇状尾羽所产生的浮力,并与其他早白垩世鸟类进行对比。结果显示,契氏鸟的扇状尾羽所产生的空气浮力小于同时代生活的今鸟型类,这有可能解释了反鸟类中具有空气动力学功能的尾羽形态普遍缺乏的现象。

submitted time 2017-08-10 From cooperative journals:《古脊椎动物学报》 Hits667Downloads289 Comment 0

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