Fossil evidence for earlier radiation in istiophorid billfishes (Teleostei, Istiophoriformes) uncovered by comparative morphology of the caudal vertebrae

ABSTRACT

Billfishes include some of the largest pelagic teleostean species, but several aspects about their morphology, paleobiology, and evolution remain ambiguous. Their fossil record is fragmentary and mostly represented by rostral and skull remains. Here, we present a comparative study of the caudal vertebral morphology of extant istiophorid species and we use this information to describe two fossil vertebrae from the Gatun and Chagres formations, both from the late Miocene of Panama. The caudal vertebra from the Gatun Fm. is characterized by the presence of a lateral apophysis and identified as Makaira sp., while the vertebra from the Río Indio Member of Chagres Fm. lacks this structure and its morphology indicates a different genus. The estimated total length of the Makaira sp. specimen from Gatun Fm. is about 5.18 m, the largest size calculated for a marlin, while the Rio Indio specimen was about 2.56 m long. Phylogenies show that the radiation of most living istiophorid species seemingly occurred during the Pliocene. However, our results highlight that the genus Makaira and the taxa lacking the lateral apophyses occurred during the late Miocene, showing that the radiation of istiophorids most likely occurred before the Pliocene (ca. 5 Ma).

ACKNOWLEDGMENTS

This study resulted in part from a project, which is funded by the program for young researchers (Grant No. APY–NI10–016A) and the Doctoral and Postdoctoral Scholarships Program of the National Secretary of Science and Technology of Panama (Grant No. BIP–2018–004) as well as the Ricardo Perez S.A and STRI-Tupper Paleontological Fund. Our thanks also go to the Dirección de Recursos Minerales of Panamá for collection permits. We acknowledge the CTPA–STRI paleontological crew for their support during fieldwork, J. Aleman for photography of fossil specimens, and A. Aguirre for help to find rare bibliographic material. We are grateful to J. Clayton, J. Williams, and M. Brett-Surman from the USNM for granting access to the osteological and paleontological collections under their care. Thanks also to P. Béarez from the MNHN and R. Cooke from STRI for access to osteological material of I. platypterus; to M. McGrouther and A. Hay from the AMNH for their valuable help in providing a complete specimen of Is. Indica; to A. Palandačić from the NHMW to grant access to a mounted skeleton of T. belone; and to A. Osorio, J. Martinez, and J. Lezcano for their help during fieldwork in Miguel de la Borda. We dedicate this work to the late Harry Fierstine (1932–2021), who generously shared unpublished data as well his experience and knowledge about istiophorid morphology during the earliest years of the academic formation of C.D., which made this study possible. He will be missed.