AG Vergleichende Zoologie, Institut für Biologie, Humboldt-Universität zu Berlin, Berlin, 10115, Germany.
UMR 7179 CNRS/MNHN, Département Adaptations du Vivant, Muséum National d'Histoire Naturelle, Paris, 75005, France.
Evolution. 2021 Mar;75(3):641-655. doi: 10.1111/evo.14171. Epub 2021 Jan 22.
Although giraffes maintain the usual mammalian cervical number of seven vertebrae, their first thoracic vertebra (T1) exhibits aberrant anatomy and has been hypothesized to functionally elongate the neck. We test this "functional elongation hypothesis" by combining phylogenetically informed analyses of neck length, three-dimensional (3D) vertebral shape, and of the functional significance of shape differences across a broad sample of ruminants and camelids. Digital bone models of the cervicothoracic transition were subjected to 3D geometric morphometric analysis revealing how the shape of the seventh cervical (C7) has converged in several long-necked species. However, we find a unique "cervicalization" of the giraffe's T1. In contrast, we demonstrate a "thoracalization" of C7 for the European bison. Other giraffids (okapi and extinct Sivatherium) did not exhibit "cervicalized" T1 morphology. Quantitative range of motion (ROM) analysis at the cervicothoracic transition in ruminants and camelids confirms the "functional elongation hypothesis" for the giraffe in terms of increased mobility, especially with regard to dorsoventral flexion/extension. Additionally, other factors related to the unique morphology of the giraffe's cervicothoracic transition such as neck posture and intervertebral stability are discussed and should be considered in future studies of giraffe neck evolution.
尽管长颈鹿的颈椎数量与哺乳动物通常的数量相同，有七个，但它们的第一胸椎（T1）表现出异常的解剖结构，并且有人假设它在功能上使颈部伸长。我们通过结合对广泛的反刍动物和骆驼样本的颈部长度、三维（3D）椎骨形状以及形状差异的功能意义进行系统发育信息丰富的分析，来检验这一“功能伸长假说”。对颈胸过渡的数字骨骼模型进行了 3D 几何形态测量分析，揭示了在几个长颈物种中，第七颈椎（C7）的形状是如何趋同的。然而，我们发现长颈鹿的 T1 存在独特的“颈化”现象。相比之下，我们证明了欧洲野牛的 C7 具有“胸化”现象。其他长颈鹿（霍加狓和已灭绝的犀兽）并没有表现出 T1 的“颈化”形态。在反刍动物和骆驼中，对颈胸过渡处的定量运动范围（ROM）分析证实了长颈鹿在运动性方面的“功能伸长假说”，尤其是在背腹屈伸方面。此外，还讨论了与长颈鹿颈胸过渡的独特形态相关的其他因素，如颈部姿势和椎间盘稳定性，这些因素应在未来对长颈鹿颈部进化的研究中加以考虑。
