Paläontologisches Institut und Museum, Universität Zürich, Zürich, Switzerland.
BMC Evol Biol. 2013 Dec 5;13:265. doi: 10.1186/1471-2148-13-265.
A long, slender body plan characterized by an elongate antorbital region and posterior displacement of the unpaired fins has evolved multiple times within ray-finned fishes, and is associated with ambush predation. The axial skeleton of ray-finned fishes is divided into abdominal and caudal regions, considered to be evolutionary modules. In this study, we test whether the convergent evolution of the ambush predator body plan is associated with predictable, regional changes in the axial skeleton, specifically whether the abdominal region is preferentially lengthened relative to the caudal region through the addition of vertebrae. We test this hypothesis in seven clades showing convergent evolution of this body plan, examining abdominal and caudal vertebral counts in over 300 living and fossil species. In four of these clades, we also examined the relationship between the fineness ratio and vertebral regionalization using phylogenetic independent contrasts.
We report that in five of the clades surveyed, Lepisosteidae, Esocidae, Belonidae, Sphyraenidae and Fistulariidae, vertebrae are added preferentially to the abdominal region. In Lepisosteidae, Esocidae, and Belonidae, increasing abdominal vertebral count was also significantly related to increasing fineness ratio, a measure of elongation. Two clades did not preferentially add abdominal vertebrae: Saurichthyidae and Aulostomidae. Both of these groups show the development of a novel caudal region anterior to the insertion of the anal fin, morphologically differentiated from more posterior caudal vertebrae.
The preferential addition of abdominal vertebrae in fishes with an elongate body shape is consistent with the existence of a conservative positioning module formed by the boundary between the abdominal and caudal vertebral regions and the anterior insertion of the anal fin. Dissociation of this module is possible, although less probable than changes in the independently evolving abdominal region. Dissociation of the axial skeleton-median fin module leads to increased regionalization within the caudal vertebral column, something that has evolved several times in bony fishes, and may be homologous with the sacral region of tetrapods. These results suggest that modularity of the axial skeleton may result in somewhat predictable evolutionary outcomes in bony fishes.
在有颌鱼类中，多次进化出一种长而细的身体形态，其特征是眶前区拉长和未配对鳍向后移位，与伏击捕食有关。有颌鱼类的轴骨分为腹部和尾部区域，被认为是进化模块。在这项研究中，我们测试了伏击捕食者身体形态的趋同进化是否与轴骨的可预测的、区域性变化有关，特别是腹部区域是否通过增加椎骨而相对于尾部区域优先延长。我们在 7 个表现出这种身体形态趋同进化的支系中检验了这一假设，检查了 300 多种现存和化石物种的腹部和尾部椎骨数量。在其中的 4 个支系中，我们还使用系统发育独立对比检验了 fineness ratio 和椎骨区域化之间的关系。
我们报告说，在所调查的 5 个支系中，Lepisosteidae、Esocidae、Belonidae、Sphyraenidae 和 Fistulariidae 优先在腹部增加椎骨。在 Lepisosteidae、Esocidae 和 Belonidae 中，腹部椎骨数量的增加与 fineness ratio（衡量伸长的指标）的增加也显著相关。有 2 个支系没有优先增加腹部椎骨：Saurichthyidae 和 Aulostomidae。这两个组都表现出在肛鳍插入点之前的前部形成了一个新的尾部区域，在形态上与更后面的尾部椎骨不同。
在身体形状细长的鱼类中，腹部椎骨的优先增加与由腹部和尾部椎骨区域之间的边界以及肛鳍的前部插入形成的保守定位模块的存在是一致的。尽管这种模块的分离不太可能，但比独立进化的腹部区域更有可能。轴骨-中鳍模块的分离导致尾部椎骨内部的区域化增加，这在硬骨鱼类中已经进化了多次，可能与四足动物的骶骨同源。这些结果表明，轴骨的模块性可能导致硬骨鱼类的一些可预测的进化结果。
