Clark E G, Hutchinson J R, Briggs D E G
Department of Earth and Planetary Sciences, Yale University, 210 Whitney Avenue, New Haven, CT 06511, USA.
Structure and Motion Laboratory, Department of Comparative Biomedical Sciences, The Royal Veterinary College, Hawkshead Lane, Hatfield, Hertfordshire AL9 7TA, UK.
R Soc Open Sci. 2020 Dec 23;7(12):201380. doi: 10.1098/rsos.201380. eCollection 2020 Dec.
Living brittle stars (Echinodermata: Ophiuroidea) employ a very different locomotion strategy to that of any other metazoan: five or more arms coordinate powerful strides for rapid movement across the ocean floor. This mode of locomotion is reliant on the unique morphology and arrangement of multifaceted skeletal elements and associated muscles and other soft tissues. The skeleton of many Palaeozoic ophiuroids differs markedly from that in living forms, making it difficult to infer their mode of locomotion and, therefore, to resolve the evolutionary history of locomotion in the group. Here, we present three-dimensional digital renderings of specimens of six ophiuroid taxa from the Lower Devonian Hunsrück Slate: four displaying the arm structure typical of Palaeozoic taxa () and two (, ) with morphologies more similar to those in living forms. The use of three-dimensional digital visualization allows the structure of the arms of specimens of these taxa to be visualized in the round, to our knowledge for the first time. The lack of joint interfaces necessary for musculoskeletally-driven locomotion supports the interpretation that taxa with offset ambulacrals would not be able to conduct this form of locomotion, and probably used podial walking. This approach promises new insights into the phylogeny, functional morphology and ecological role of Palaeozoic brittle stars.
现存的蛇尾纲动物（棘皮动物门：蛇尾纲）采用了一种与其他后生动物截然不同的运动策略：五条或更多的腕臂协同进行有力的大步移动，以便在海床上快速移动。这种运动方式依赖于多面骨骼元素以及相关肌肉和其他软组织的独特形态和排列。许多古生代蛇尾纲动物的骨骼与现存物种的骨骼明显不同，这使得推断它们的运动方式变得困难，进而难以解析该类群运动的进化历史。在此，我们展示了来自下泥盆统洪斯吕克板岩的六个蛇尾纲分类单元标本的三维数字模型：四个呈现出典型古生代分类单元的腕臂结构（），另外两个（，）的形态与现存物种更为相似。据我们所知，三维数字可视化的运用首次使得这些分类单元标本腕臂的结构得以全面呈现。缺乏肌肉骨骼驱动运动所需的关节界面，这支持了一种解释，即具有偏移步带沟的分类单元无法进行这种运动形式，可能采用的是腕足行走。这种方法有望为古生代蛇尾纲动物的系统发育、功能形态学和生态作用带来新的见解。
