DeBlois Mark C, Motani Ryosuke
Department of Earth and Planetary Sciences, University of California, Davis, California.
J Morphol. 2019 Jun;280(6):908-924. doi: 10.1002/jmor.20992. Epub 2019 Apr 22.
Hydrofoil-shaped limbs (flipper-hydrofoils) have evolved independently several times in secondarily marine tetrapods and generally fall into two functional categories: (1) those that produce the majority of thrust during locomotion (propulsive flipper-hydrofoils); (2) those used primarily to steer and resist destabilizing movements such as yaw, pitch, and roll (controller flipper-hydrofoils). The morphological differences between these two types have been poorly understood. Theoretical and experimental studies on engineered hydrofoils suggest that flapping hydrofoils with a flexible trailing edge are more efficient at producing thrust whereas hydrofoils used in steering and stabilization benefit from a more rigid one. To investigate whether the trailing edge is generally more flexible in propulsive flipper-hydrofoils, we compared the bone distribution along the chord in both flipper types. The propulsive flipper-hydrofoil group consists of the forelimbs of Chelonioidea, Spheniscidae, and Otariidae. The controller flipper-hydrofoil group consists of the forelimbs of Cetacea. We quantified bone distribution from radiographs of species representing more than 50% of all extant genera for each clade. Our results show that the proportion of bone in both groups is similar along the leading edge (0-40% of the chord) but is significantly less along the trailing edge for propulsive flipper-hydrofoils (40-80% of the chord). Both flipper-hydrofoil types have little to no bony tissue along the very edge of the trailing edge (80-100% of the chord). This suggests a relatively flexible trailing edge for propulsive flipper-hydrofoils compared to controller flipper-hydrofoils in line with findings from prior studies. This study presents a morphological correlate for inferring flipper-hydrofoil function in extinct taxa and highlights the importance of a flexible trailing edge in the evolution of propulsive flipper-hydrofoils in marine tetrapods.
水翼状肢体（鳍状水翼）在次生性海洋四足动物中已经独立进化了好几次，通常可分为两种功能类型：（1）在运动过程中产生大部分推力的（推进性鳍状水翼）；（2）主要用于转向和抵抗偏航、俯仰和横滚等不稳定运动的（控制性鳍状水翼）。这两种类型之间的形态差异一直未得到很好的理解。对工程水翼的理论和实验研究表明，后缘灵活的扑翼水翼在产生推力方面更有效，而用于转向和稳定的水翼则受益于更刚性的后缘。为了研究推进性鳍状水翼的后缘是否通常更灵活，我们比较了两种鳍状肢体类型沿弦向的骨骼分布。推进性鳍状水翼组由海龟超科、企鹅科和海狮科的前肢组成。控制性鳍状水翼组由鲸目动物的前肢组成。我们从代表每个分类单元所有现存属50%以上的物种的X光片中量化了骨骼分布。我们的结果表明，两组中沿前缘（弦长的0 - 40%）的骨骼比例相似，但推进性鳍状水翼沿后缘（弦长的40 - 80%）的骨骼比例明显更低。两种鳍状水翼类型在后缘的最边缘（弦长的80 - 100%）几乎没有骨组织。这表明与控制性鳍状水翼相比，推进性鳍状水翼的后缘相对更灵活，这与先前研究的结果一致。这项研究提出了一种形态学关联，用于推断已灭绝类群中鳍状水翼的功能，并强调了灵活后缘在海洋四足动物推进性鳍状水翼进化中的重要性。
