The development of paired appendages was a key innovation during evolution and facilitated the aquatic to terrestrial transition of vertebrates. Largely derived from the lateral plate mesoderm (LPM), one hypothesis for the evolution of paired fins invokes derivation from unpaired median fins via a pair of lateral fin folds located between pectoral and pelvic fin territories¹. Whilst unpaired and paired fins exhibit similar structural and molecular characteristics, no definitive evidence exists for paired lateral fin folds in larvae or adults of any extant or extinct species. As unpaired fin core components are regarded as exclusively derived from paraxial mesoderm, any transition presumes both co-option of a fin developmental programme to the LPM and bilateral duplication². Here, we identify that the larval zebrafish unpaired pre-anal fin fold (PAFF) is derived from the LPM and thus may represent a developmental intermediate between median and paired fins. We trace the contribution of LPM to the PAFF in both cyclostomes and gnathostomes, supporting the notion that this is an ancient trait of vertebrates. Finally, we observe that the PAFF can be bifurcated by increasing bone morphogenetic protein signalling, generating LPM-derived paired fin folds. Our work provides evidence that lateral fin folds may have existed as embryonic anlage for elaboration to paired fins.

成对附肢的发育是进化过程中的一项关键创新，促进了脊椎动物从水生到陆生的过渡。^成对鳍的进化主要源自侧板中胚层（LPM），一种关于成对鳍进化的假设是通过位于胸鳍和腹鳍区域之间的一对侧鳍折叠从不成对的中鳍衍生而来。虽然不成对和成对的鳍表现出相似的结构和分子特征，但没有明确的证据表明任何现存或灭绝物种的幼虫或成虫具有成对的侧鳍折叠。由于不成对的鳍核心组件被认为完全源自近轴中胚层，因此任何转变都假定鳍发育程序与 LPM 共同选择以及双边复制²。在这里，我们确定幼虫斑马鱼不成对的肛前鳍褶（PAFF）源自 LPM，因此可能代表中鳍和成对鳍之间的发育中间体。我们追踪了圆口动物和颌口动物中 LPM 对 PAFF 的贡献，支持了这是脊椎动物的一个古老特征的观点。最后，我们观察到 PAFF 可以通过增加骨形态发生蛋白信号传导而分叉，产生 LPM 衍生的成对鳍折叠。我们的工作提供了证据，表明侧鳍褶皱可能作为胚胎原基存在，用于形成配对的鳍。