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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. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) Published version This work was funded by the Industry Aligned Fund (IAF) Agency for Science, Technology and Research (grant to T.J.C. and K.-W.T.); Ministry of Education (MoE) Tier 3 (grant 2016-T3-1-005 to T.J.C., C.W. and H.M.); Ministry of Education (MoE) Tier 1 (grant 2016-T1-001-055 to T.J.C. and C.Z.); Ministry of Education (MoE) Tier 2 (grant MOE-T2EP30221-0008 to C.W.); the Company of Biologists (travelling fellowship to M.J.T.); the National Science Foundation (grants IOS-1853949 to M.C.D. and 2203311 to C.M.); the Swiss National Science Foundation Sinergia (grant CRSII5_180345 to C.M.); the Swiss Bridge Foundation (C.M.); Additional Ventures Single Ventricle Research Fund (SVRF) (grant 1048003 to C.M.); the University of Colorado School of Medicine Anschutz Medical Campus and the Children’s Hospital Colorado Foundation (C.M.); the National Institutes of Health (NIH), National Institute of General Medical Sciences (grants 1T32GM141742-01 to H.R.M. and 3T32GM121742-02S1 to H.R.M.); Australian Research Council (discovery grant DP200103219 to F.J.T. and P.D.C); National Health and Medical Research Council (senior principal research fellow APP1136567 to P.D.C.); and the NIH (grant R35NS111564 to J.S. and M.E.B.).