Department of Earth and Planetary Sciences, University of California, Davis, One Shields Avenue, Davis, CA, 95616, USA.
Department of Environmental Science and Studies, DePaul University, 1110 West Belden Avenue, Chicago, IL, 60614, USA.
Sci Rep. 2023 Oct 4;13(1):16664. doi: 10.1038/s41598-023-41812-z.
Tunas, lamnid sharks, modern whales, and derived ichthyosaurs converged on the thunniform body plan, with a fusiform body, lunate caudal fin, compressed peduncle, and peduncle joint. This evolutionary convergence has been studied for a long time but little is known about whether all four clades share any skeletal characteristics. Comparisons of vertebral centrum dimensions along the body reveal that the four clades indeed share three skeletal characteristics (e.g., thick vertebral column for its length), while an additional feature is shared by cetaceans, lamnid sharks, and ichthyosaurs and two more by lamnid sharks and ichthyosaurs alone. These vertebral features are all related to the mechanics of thunniform swimming through contributions to posterior concentration of tail-stem oscillation, tail stem stabilization, peduncle joint flexibility, and caudal fin angle fixation. Quantitative identifications of these features in fossil vertebrates would allow an inference of whether they were a thunniform swimmer. Based on measurements in the literature, mosasaurs lacked these features and were probably not thunniform swimmers, whereas a Cretaceous lamniform shark had a mosaic of thunniform and non-thunniform features. The evolution of thunniform swimming appears to be linked with the evolution of prey types and, in part, niche availability through geologic time.
金枪鱼、长尾鲨、现代鲸鱼和衍生的鱼龙都趋同进化成了箭形身体形态，具有梭形身体、新月形尾鳍、压缩的尾柄和尾柄关节。这种进化趋同已经研究了很长时间，但对于这四个类群是否都具有任何骨骼特征知之甚少。对沿身体的椎骨中心尺寸进行比较表明，这四个类群确实具有三个骨骼特征（例如，其长度的脊椎骨很厚），而额外的特征则为鲸类、长尾鲨和鱼龙所共有，另外两个特征仅为长尾鲨和鱼龙所共有。这些椎骨特征都与箭形游泳的力学有关，通过对尾部摆动的尾部集中、尾部稳定、尾柄关节灵活性和尾鳍角度固定的贡献。在化石脊椎动物中对这些特征进行定量识别将可以推断它们是否为箭形游泳者。基于文献中的测量值，沧龙类缺乏这些特征，它们可能不是箭形游泳者，而白垩纪的长尾鲨具有箭形和非箭形特征的镶嵌体。箭形游泳的进化似乎与猎物类型的进化以及在一定程度上与地质时间的生态位可用性有关。
