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

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).

旗形鱼类（billfishes）包含部分体型最大的远洋硬骨鱼类，但有关其形态学、古生物学与演化的诸多方面仍不明晰。其化石记录零散不全，多仅留存吻部与头骨残骸。本文针对现存旗鱼科（Istiophoridae）物种的尾椎形态开展比较研究，并以此为基础描述两件采自巴拿马晚中新世加顿组与查格雷斯组的化石脊椎骨。采自加顿组的尾椎带有侧突（lateral apophysis），被鉴定为枪旗鱼属未定种（Makaira sp.）；而采自查格雷斯组里奥印第奥段的脊椎骨无该结构，其形态特征指向另一不同属。经估算，加顿组枪旗鱼属未定种标本的总长度约为5.18米，为目前推算出的最大马林鱼体型；里奥印第奥段标本的体长则约为2.56米。系统发育分析显示，多数现存旗鱼科物种的辐射演化似乎发生于上新世。但本研究结果表明，枪旗鱼属以及缺乏侧突的类群在晚中新世时期便已出现，这提示旗鱼科的辐射演化极可能发生在上新世之前（约5 Ma）。