The oldest ceratosaurian (Dinosauria: Theropoda), from the Lower Jurassic of Italy, sheds light on the evolution of the three-fingered hand of birds
The oldest ceratosaurian (Dinosauria: Theropoda), from the Lower Jurassic of Italy, sheds light on the evolution of the three-fingered hand of birds
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DOI：
10.7717/peerj.5976
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发表时间：
2018-12-19
期刊：
影响因子：
2.7
通讯作者：
Cau, Andrea
中科院分区：
文献类型：
--
作者：
Dal Sasso, Cristiano;Maganuco, Simone;Cau, Andrea
鸟类三趾手的同源性仍然是一个有争议的话题，古生物学和发育证据都支持鸟类手指的替代身份模式。晚侏罗世角龙类的沼龙（Limusaurus）有着简化的趾骨形态，被认为支持鸟类的II-III-IV数字身份，以及三指兽脚类（破伤风目）的同源异型转换的复杂模式。本文报道了一种新的大型兽脚亚目食肉动物--萨氏猎腹兽Saltriovenator zanellai gen. et sp. nov.，基于伦巴第（北方意大利）海相Saltrio组（Sinemurian，最低侏罗纪）的部分骨架。埋藏学分析表明，海洋无脊椎动物（第一次记录的古生物遗骸）的骨骼生物侵蚀，并提出了一个复杂的历史的尸体之前，被存放在一个良好的氧气和照明良好的海底。Saltriovenator显示了在四指兽脚亚目和基底破伤风类中看到的马赛克特征。系统发育分析支持姐妹分类群之间的关系，新的意大利兽脚类恐龙和年轻的早侏罗世柏柏尔龙从摩洛哥，在一个血统，这是basalmost角龙。与后来角鼻龙类成员萎缩的手相比，Saltriovenator证明了一个功能齐全的手，非常适合挣扎和抓握，在角鼻龙类中是有竞争力的。沿着鸟茎的祖先状态重建支持2-3-4-1-X和2-3-4-0-X作为角龙类和Tetanurae根部的手指骨公式，证实了鸟类手指同源性的I-II-III模式。因此，利木龙的特殊的手代表了一种仅限于晚期分叉角鼻龙类的派生条件，并且不能帮助阐明破伤风类三指条件的起源。鸟类三趾手的演化可以解释为：在非破伤风目的兽脚亚目恐龙中，牠们逐步地进行侧向简化，而在破伤风目的第四指完全丧失后，牠们的第四根手指的位置会从第四根手指的位置移动到第三根手指的位置，这使得异形龙目、龙类和手盗龙形目的退化第四掌骨会独立地丧失。据估计，Saltriovenator的体长为7.5米，是早侏罗世最大和最强壮的兽脚亚目恐龙，比兽脚亚目恐龙的体重接近1,000公斤早了25百万年。比先前已知的更大和相对更结实的阿韦罗斯特兽脚亚目恐龙的辐射可能是引发早侏罗世蜥脚类恐龙的反亲倾向的因素之一。
The homology of the tridactyl hand of birds is a still debated subject, with both paleontological and developmental evidence used in support of alternative identity patterns in the avian fingers. With its simplified phalangeal morphology, the Late Jurassic ceratosaurian Limusaurus has been argued to support a II-III-IV digital identity in birds and a complex pattern of homeotic transformations in three-fingered (tetanuran) theropods. We report a new large-bodied theropod, Saltriovenator zanellai gen. et sp. nov., based on a partial skeleton from the marine Saltrio Formation (Sinemurian, lowermost Jurassic) of Lombardy (Northern Italy). Taphonomical analyses show bone bioerosion by marine invertebrates (first record for dinosaurian remains) and suggest a complex history for the carcass before being deposited on a well-oxygenated and well-illuminated sea bottom. Saltriovenator shows a mosaic of features seen in four-fingered theropods and in basal tetanurans. Phylogenetic analysis supports sister taxon relationships between the new Italian theropod and the younger Early Jurassic Berberosaurus from Morocco, in a lineage which is the basalmost of Ceratosauria. Compared to the atrophied hand of later members of Ceratosauria, Saltriovenator demonstrates that a fully functional hand, well-adapted for struggling and grasping, was primitively present in ceratosaurians. Ancestral state reconstruction along the avian stem supports 2-3-4-1-X and 2-3-4-0-X as the manual phalangeal formulae at the roots of Ceratosauria and Tetanurae, confirming the I-II-IIIpattern in the homology of the avian fingers. Accordingly, the peculiar hand of Limusaurus represents a derived condition restricted to late-diverging ceratosaurians and cannot help in elucidating the origin of the three-fingered condition of tetanurans. The evolution of the tridactyl hand of birds is explained by step-wise lateral simplification among non-tetanuran theropod dinosaurs, followed by a single primary axis shift from digit position 4 to 3 at the root of Tetanurae once the fourth finger was completely lost, which allowed independent losses of the vestigial fourth metacarpal among aliosaurians, tyrannosauroids, and maniraptoromorphs. With an estimated body length of 7.5 m, Saltriovenator is the largest and most robust theropod from the Early Jurassic, pre-dating the occurrence in theropods of a body mass approaching 1,000 Kg by over 25 My. The radiation of larger and relatively stockier averostran theropods earlier than previously known may represent one of the factors that ignited the trend toward gigantism in Early Jurassic sauropods.