Fin ray patterns at the fin to limb transition

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 three 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 area 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 cross sectional asymmetry in the dermal rays of paired fins are 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.

鳍向四肢的演化过渡以指（趾）的起源及真皮鳍条的消失为标志性特征。针对这一演化转变的古生物学研究多聚焦于内骨骼的演化历程，却极少关注鳍条的结构与功能。为填补这一研究空白，我们借助计算机断层扫描（computed tomography），对3个关键四足形类群（tetrapodomorph taxa）的胸鳍真皮鳍条展开研究：分别为泰勒氏蜥鳍鱼（Sauripterus taylori，根齿鱼目Rhizodontida）、真掌鳍鱼（Eusthenopteron foordi，三鳍鱼科Tristichopteridae）以及提塔利克鱼（Tiktaalik roseae，希望螈类Elpistostegalia）。

本次研究数据揭示了指向具指类群的演化支系中存在多项显著特征趋势：包括鳍条的整合（如分节与分支程度降低）、鳍膜的缩减，以及出乎意料的背、腹半鳍条（hemitrichia）不对称性演化。

在真掌鳍鱼中，背侧鳍条对前轴内骨骼的覆盖程度略高于腹侧鳍条。在提塔利克鱼中，背侧鳍条完全覆盖第三及第四中节（mesomeres），而腹侧鳍条仅分布于这些结构的远端区域，这暗示鳍尖处存在类似肉质‘掌部’的腹侧化肌肉组织。

背、腹鳍条的横截面积同样呈现不对称性：真掌鳍鱼的背侧鳍条尺寸略大于腹侧鳍条；相较之下，提塔利克鱼的背侧鳍条体积可达腹侧鳍条的数倍，且不对称程度似乎随个体体型增大而进一步加剧。

对现生硬骨鱼（osteichthyans）的分析表明，成对鳍的真皮鳍条横截面积不对称是冠群硬骨鱼的祖征。

向冠群演化的茎干四足类的真皮鳍条演化历程，反映了其在指（趾）起源之前，为适应鳍支撑的抬高姿态以及抵御基底负载而产生的适应性改变。