Lingham-Soliar Theagarten
Nelson Mandela Metropolitan University, Coastal and Marine Research, South Campus, University Way, Port Elizabeth 6001, South Africa
Integr Comp Biol. 2016 Dec;56(6):1323-1336. doi: 10.1093/icb/icw125. Epub 2016 Oct 29.
Among extinct ichthyosaurs the Jurassic forms Ichthyosaurus and Stenopterygius share a number of anatomical specializations with lamnid sharks, characterized in the white shark, Carcharodon carcharias These features allow their inclusion within the mode of high-speed thunniform swimming to which only two other equally distinctive phylogenetic groups belong, tuna and dolphins-a striking testaments to evolutionary convergence. Jurassic ichthyosaurs evolved from reptiles that had returned to the sea (secondarily adapted) about 250 million years ago (MYA) while lamnid sharks evolved about 50 MYA from early cartilaginous fishes (originating ca. 400 MYA). Their shared independently evolved anatomical characteristics are discussed. These include a deep tear-drop body shape that helped initially define members as thunniform swimmers. Later, other critical structural characteristics were discovered such as the crossed-fiber architecture of the skin, high-speed adapted dorsal and caudal fins, a caudal peduncle and series of ligaments to enable transmission of power from the musculature located anteriorly to the caudal fin. Both groups also share a similar chemistry of the dermal fibers, i.e., the scleroprotein collagen.
在已灭绝的鱼龙中，侏罗纪时期的鱼龙属（Ichthyosaurus）和狭翼鱼龙属（Stenopterygius）与鼠鲨科鲨鱼具有许多解剖学上的特化特征，这些特征在大白鲨（噬人鲨，Carcharodon carcharias）身上也有体现。这些特征使它们能够采用高速的鲔鱼式游泳方式，而只有另外两个同样独特的系统发育类群——金枪鱼和海豚——也属于这种游泳方式，这是进化趋同的显著证明。侏罗纪鱼龙是从大约2.5亿年前重返海洋（二次适应）的爬行动物进化而来的，而鼠鲨科鲨鱼则是在大约5000万年前从早期软骨鱼类（起源于约4亿年前）进化而来的。文中讨论了它们共同独立进化出的解剖学特征。这些特征包括深泪滴形的身体形状，这一形状最初有助于将这些成员定义为鲔鱼式游泳者。后来，人们又发现了其他关键的结构特征，如皮肤的交叉纤维结构、适应高速游泳的背鳍和尾鳍、尾柄以及一系列韧带，这些韧带能够将位于身体前部的肌肉组织的力量传递到尾鳍。这两个类群的真皮纤维化学组成也相似，即硬蛋白胶原蛋白。
