Pintore Romain, Hutchinson John R, Bishop Peter J, Tsai Henry P, Houssaye Alexandra
Mécanismes adaptatifs et évolution (MECADEV) / UMR 7179. CNRS / Muséum National d'Histoire Naturelle, Paris, FR.
Structure and Motion Laboratory, Royal Veterinary College, Hatfield, UK.
Paleobiology. 2024 May;50(2):308-329. doi: 10.1017/pab.2024.6. Epub 2024 May 2.
Theropods are obligate bipedal dinosaurs that appeared 230 million years ago and are still extant as birds. Their history is characterized by extreme variations in body mass, with gigantism evolving convergently between many lineages. However, no quantification of hindlimb functional morphology has shown if these body mass increases led to similar specializations between distinct lineages. Here we studied femoral shape variation across 41 species of theropods (n= 68 specimens) using a high-density 3D geometric morphometric approach. We demonstrated that the heaviest theropods evolved wider epiphyses and a more distally located fourth trochanter, as previously demonstrated in early archosaurs, along with an upturned femoral head and a mediodistal crest that extended proximally along the shaft. Phylogenetically informed analyses highlighted that these traits evolved convergently within six major theropod lineages, regardless of their maximum body mass. Conversely, the most gracile femora were distinct from the rest of the dataset, which we interpret as a femoral specialization to "miniaturization" evolving close to Avialae (bird lineage). Our results support a gradual evolution of known "avian" features, such as the fusion between lesser and greater trochanters and a reduction of the epiphyses' offset, independently from body mass variations, which may relate to a more "avian" type of locomotion (more knee-than hip-driven). The distinction between body mass variations and a more "avian" locomotion is represented by a decoupling in the mediodistal crest morphology, whose biomechanical nature should be studied to better understand the importance of its functional role in gigantism, miniaturization and higher parasagittal abilities.
兽脚亚目恐龙是 obligate 双足恐龙，它们出现在 2.3 亿年前，如今以鸟类的形式仍然存在。它们的历史以体重的极端变化为特征，在许多谱系中巨型化是趋同进化的。然而，没有对后肢功能形态的量化研究表明这些体重增加是否导致了不同谱系之间类似的特化。在这里，我们使用高密度三维几何形态测量方法研究了 41 种兽脚亚目恐龙（n = 68 个标本）的股骨形状变化。我们证明，最重的兽脚亚目恐龙进化出了更宽的骨骺和更位于远端的第四转子，正如早期主龙类中所证明的那样，同时还有一个上翘的股骨头和一个沿骨干向近端延伸的内侧远端嵴。系统发育信息分析强调，这些特征在六个主要兽脚亚目谱系中趋同进化，无论它们的最大体重如何。相反，最纤细的股骨与数据集中的其他部分不同，我们将其解释为接近鸟翼类（鸟类谱系）进化出的对“小型化”的股骨特化。我们的结果支持了已知“鸟类”特征的逐渐进化，比如小转子和大转子之间的融合以及骨骺偏移的减少，这与体重变化无关，这可能与更“鸟类”的运动类型（更多由膝盖而非臀部驱动）有关。体重变化和更“鸟类”的运动之间的区别由内侧远端嵴形态的解耦来体现，其生物力学性质应该被研究，以便更好地理解其在巨型化、小型化和更高矢状面能力中的功能作用的重要性。
