Towards a unifying systematic scheme of fossil and living billfishes (Teleostei, Istiophoridae)

Extant istiophorids are open ocean apex predators that are extensively studied due to their ecological importance and high values for fisheries. Nevertheless, little is known about their evolution because of a fragmentary fossil record and extremely difficult taxonomy of fossil species. Here, we present a new phylogenetic hypothesis covering fossil and living istiophorids. Our results demonstrate that istiophorid richness is larger than previously assumed, comprising eight genera with 20 species. The phylogenetic analysis shows that istiophorids are grouped into four clades: the Istiophorus clade, which includes the sailfish; the Machairostra clade, which comprises Makaira spp., including two new species from the late Miocene (†Makaira colonense sp. nov. and †Makaira fierstini sp. nov.); the Gracilorostra clade, which comprise all remaining istiophorids with exception of spearfishes and includes two new genera and one new species (†Morgula donosochagrense gen. et sp. nov. and †Spathochoira calvertense gen. et. comb. nov.); and the Tetrapturomorpha clade is composed of the spearfishes and the extinct †Prototetrapturus courcelli gen. et. comb. nov. The family Istiophoridae shows an evolutionary trend toward reduction of the premaxillary thickness and increasing the extension of narial cavities. This reduction is related to an increase of adipose tissues in the rostrum base probably driven by the presence of the oleofera gland, an organ involved in feeding, healing, endothermy and hydrophobic functions. Our phylogeny shows a direct relationship between the rostral and cranial shape explained by body size and feeding behaviour. The larger istiophorids have lateral apophysis and the larger spines of the vertebral column. The spearfishes represent the smaller species of the family, with the extant Tetrapturus spp. first appearing in the late Pliocene. The clade Tetrapturomorpha shows an extreme size reduction over time when compared with species of their sister clade Gracilorostra, demonstrating an evolutionary trend towards size reduction. http://zoobank.org/urn:lsid:zoobank.org:pub:D3D3B15B-36FA-42EB-98AD-FAF369D989EB现存旗鱼科（Istiophoridae）物种均为开阔大洋顶级捕食者，因其生态重要性与极高的渔业价值而被广泛研究。然而，受限于零散的化石记录与极富挑战性的化石物种分类工作，学界对其演化历程仍知之甚少。本研究构建了涵盖化石与现生旗鱼科类群的全新系统发育假说。研究结果表明，旗鱼科的物种丰富度远超此前预期，共计包含8属20种。 系统发育分析显示，旗鱼科可划分为4个演化支（clade）：旗鱼支系（Istiophorus clade），包含旗鱼（sailfish）；枪鳍旗鱼支系（Machairostra clade），涵盖枪鱼属（Makaira spp.），其中包含2个晚中新世的新种：†科隆内枪鱼（†Makaira colonense sp. nov.）与†菲尔斯坦枪鱼（†Makaira fierstini sp. nov.）；细喙旗鱼支系（Gracilorostra clade），包含除四旗鱼类群之外的其余所有旗鱼科类群，另含2个新属及1个新种：†多诺索查格伦新属新种（†Morgula donosochagrense gen. et sp. nov.）以及†卡尔弗特剑形旗鱼新组合（†Spathochoira calvertense gen. et. comb. nov.）；四旗鱼支系（Tetrapturomorpha clade），由四旗鱼类群及已灭绝的†原四旗鱼库尔塞利新组合（†Prototetrapturus courcelli gen. et. comb. nov.）构成。 旗鱼科整体呈现出前上颌骨厚度减小、鼻腔腔扩展的演化趋势。该厚度减小现象与喙部基部脂肪组织的增厚相关，这一变化可能由油脂腺（oleofera gland）的存在所驱动；油脂腺是一种参与摄食、伤口愈合、吸热及疏水功能的器官。本研究的系统发育结果显示，喙部与颅骨形态的差异可由体型大小与摄食行为直接解释。体型更大的旗鱼科物种具有侧突及更发达的脊柱棘突。四旗鱼类群为旗鱼科中体型较小的类群，现生的四旗鱼属（Tetrapturus spp.）物种最早出现于上新世晚期。与姊妹演化支细喙旗鱼支系（Gracilorostra clade）的物种相比，四旗鱼支系随时间推移发生了显著的体型减小，印证了其向小型化演化的趋势。 http://zoobank.org/urn:lsid:zoobank.org:pub:D3D3B15B-36FA-42EB-98AD-FAF369D989EB