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 <i>Makaira</i> 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 <i>Makaira</i> 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 <i>Makaira</i> 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).

旗鱼类（Billfishes）包含部分体型最大的远洋硬骨鱼类（teleostean），但该类群的形态学、古生物学及演化研究仍存在诸多模糊之处。其化石记录零散残缺，且绝大多数仅以吻部与颅骨遗骸留存。本研究针对现生旗鱼科（Istiophoridae）物种的尾椎形态开展比较分析，并以此为基础描述了两件采自巴拿马晚中新世（late Miocene）的化石脊椎，分别来自加通组（Gatun Formation）与查格雷斯组（Chagres Formation）。采自加通组（Gatun Fm.）的尾椎具有侧突（lateral apophysis）结构，被鉴定为枪鱼属未定种（Makaira sp.）；而采自查格雷斯组（Chagres Fm.）里奥印第奥段（Río Indio Member）的脊椎则无此结构，其形态特征指向另一不同属。经估算，加通组枪鱼属未定种标本的总长度约为5.18米，这是目前计算得到的最大马林鱼体型；而里奥印第奥段标本的体长约为2.56米。系统发育分析（Phylogenies）显示，多数现生旗鱼科物种的适应性辐射似乎发生于上新世（Pliocene）。但本研究结果表明，枪鱼属（Makaira）以及缺乏侧突的分类群（taxa）均起源于晚中新世，这说明旗鱼科的适应性辐射极有可能发生在上新世之前（约500万年前，ca. 5 Ma）。