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.

Australian Regenerative Medicine Institute Monash University Clayton Victoria Australia

Biology Department Wesleyan University Middletown CT USA

CSIRO National Research Collections Australia Australia National Fish Collection Hobart Tasmania Australia

Department of Biological Sciences Faculty of Science National University of Singapore Singapore Singapore

Department of Molecular Life Sciences University of Zurich Zurich Switzerland

Department of Pediatrics Section of Developmental Biology University of Colorado Anschutz Medical Campus Aurora CO USA

Department of Physical and Biological Sciences Western New England University Springfield MA USA

Division of Biology and Biological Engineering California Institute of Technology Pasadena CA USA

Division of Cell Matrix Biology and Regenerative Medicine School of Biological Sciences Faculty of Biology Medicine and Health University of Manchester Manchester UK

Division of Developmental Biology and Medicine School of Medical Sciences Faculty of Biology Medicine and Health University of Manchester Manchester UK

EMBL Australia Victorian Node Monash University Clayton Victoria Australia

Faculty of Fisheries and Protection of Waters University of South Bohemia in Ceske Budejovice Vodnany Czech Republic

Institute of Molecular and Cell Biology A*STAR Singapore Singapore

Lee Kong Chian School of Medicine Nanyang Technological University Singapore Singapore

School of Biosciences University of Sheffield Sheffield UK

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