The fin-to-limb transition was marked by the origin of digits and the loss of dermal fin rays. Paleontological research into this transformation has focused on the evolution of the endoskeleton, with little attention paid to fin ray structure and function. To address this knowledge gap, we study the dermal rays of the pectoral fins of 3 key tetrapodomorph taxa-Sauripterus taylori (Rhizodontida), Eusthenopteron foordi (Tristichopteridae), and Tiktaalik roseae (Elpistostegalia)-using computed tomography. These data show several trends in the lineage leading to digited forms, including the consolidation of fin rays (e.g., reduced segmentation and branching), reduction of the fin web, and unexpectedly, the evolution of asymmetry between dorsal and ventral hemitrichia. In Eusthenopteron, dorsal rays cover the preaxial endoskeleton slightly more than ventral rays. In Tiktaalik, dorsal rays fully cover the third and fourth mesomeres, while ventral rays are restricted distal to these elements, suggesting the presence of ventralized musculature at the fin tip analogous to a fleshy "palm." Asymmetry is also observed in cross-sectional areas of dorsal and ventral rays. Eusthenopteron dorsal rays are slightly larger than ventral rays; by contrast, Tiktaalik dorsal rays can be several times larger than ventral rays, and degree of asymmetry appears to be greater at larger sizes. Analysis of extant osteichthyans suggests that cross-sectional asymmetry in the dermal rays of paired fins is plesiomorphic to crown group osteichthyans. The evolution of dermal rays in crownward stem tetrapods reflects adaptation for a fin-supported elevated posture and resistance to substrate-based loading prior to the origin of digits.

中文翻译：

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鳍到肢过渡处的鳍射线图案。

鳍到四肢的过渡以手指的起源和真皮鳍条的丧失为标志。古生物学对这种转变的研究主要集中在内骨骼的进化上，而很少关注鳍条的结构和功能。为了弥补这一知识差距，我们使用计算机断层扫描技术研究了 3 个关键四足形类群——Sauripterus taylori（Rhizodontida）、Eusthenopteron foordi（Tristichopteridae）和 Tiktaalik Roseae（Elpistostegalia）的胸鳍真皮射线。这些数据显示了导致数字形式的谱系的几个趋势，包括鳍条的巩固（例如，减少的分割和分支）、鳍网的减少，以及出乎意料地，背侧和腹侧半毛之间不对称的进化。在真翅目中，背鳍条覆盖轴前内骨骼的程度略多于腹鳍鳍条。在提塔利克鱼中，背鳍条完全覆盖第三和第四中粒，而腹鳍条仅限于这些元件的远端，这表明鳍尖处存在类似于肉质“手掌”的腹侧肌肉组织。在背射线和腹射线的横截面区域中也观察到不对称性。真翅目背鳍条稍大于腹鳍条；相比之下，提塔利克鱼的背鳍条可能比腹鳍条大几倍，并且尺寸越大，不对称程度似乎越大。对现存骨鱼的分析表明，成对鳍的真皮射线的横截面不对称性与冠群骨鱼类似。冠向茎四足动物的真皮射线的进化反映了在手指起源之前对鳍支撑的升高姿势的适应和对基于基底的负载的抵抗力。