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

Abstract

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