de Oliveira Lucas Romero, de Carvalho Marcelo Rodrigues, Soares Mateus Costa
Departamento de Zoologia, Instituto de Biociências, Universidade de São Paulo, São Paulo, SP, Brazil.
J Morphol. 2019 Jan;280(1):68-77. doi: 10.1002/jmor.20916.
Relationships among families of the shark order Carcharhiniformes are still being debated, for example, in relation to the inclusion of hammerhead sharks (Sphyrnidae) and the tiger shark (Galeocerdo cuvier) in the Carcharhinidae. Previous morphological studies have not analyzed the phylogenetic significance of axial caudal muscles among carcharhiniforms. The authors therefore describe here the axial muscles of the tail region (m. epaxialis, m. obliquus superioris, m. obliquus inferioris, m. flexor caudalis) to better understand their anatomical patterns among carcharhiniform sharks. Our results indicate that the family Pentanchidae, recently separated from scyliorhinids (catsharks), present a similar pattern of caudal muscles to catsharks, indicating their close phylogenetic relationship. Triakids, hemigaleids, leptochariids, Galeocerdo cuvier and Prionace glauca have a unique pattern in which the medial axial bundle is bulkier than all other caudal axial muscles. Sphyrnids share with carcharhinids a similar pattern of the m. obliquus superioris (approximately the same size as the dorsal m. epaxialis) and have fibers of the m. flexor caudalis arranged in a V-shape. This chevron-shape is also present on the m. flexor caudalis of hemigaleids and leptochariids. Also, sphyrnids and carcharhinids have more slender septa between muscle bundles (more notably in carcharhinids; the only exception is Galeocerdo cuvier, which has the same pattern as sphyrnids). Although being used here as an outgroup, Squaliformes is unique in lacking one of the epaxial bundles. The authors conclude that the caudal axial musculature has greater phylogenetic potential than previously understood.
真鲨目鲨鱼各科之间的关系仍存在争议，例如，关于双髻鲨科（Sphyrnidae）的锤头鲨和居氏鼬鲨（Galeocerdo cuvier）是否归入真鲨科（Carcharhinidae）。以往的形态学研究尚未分析真鲨目鱼类尾轴肌的系统发育意义。因此，作者在此描述了尾区的轴肌（轴上肌、上斜肌、下斜肌、尾屈肌），以便更好地了解真鲨目鲨鱼的解剖模式。我们的结果表明，最近从猫鲨科（scyliorhinids，猫鲨）分离出来的六鳃鲨科（Pentanchidae）呈现出与猫鲨相似的尾肌模式，表明它们在系统发育上关系密切。三齿鲨科（Triakids）、半沙条鲨科（hemigaleids）、细尾鲨科（leptochariids）、居氏鼬鲨和大青鲨（Prionace glauca）具有独特的模式，即内侧轴束比所有其他尾轴肌更粗壮。双髻鲨科与真鲨科的上斜肌模式相似（大小与背侧轴上肌大致相同），且尾屈肌的纤维呈V形排列。这种人字形也存在于半沙条鲨科和细尾鲨科的尾屈肌上。此外，双髻鲨科和真鲨科在肌束之间有更细长的隔膜（在真鲨科中更明显；唯一的例外是居氏鼬鲨，其模式与双髻鲨科相同）。虽然角鲨目（Squaliformes）在此用作外类群，但它独特之处在于缺少一个轴上束。作者得出结论，尾轴肌肉组织具有比以前所理解的更大的系统发育潜力。
