Phylogenetic patterns in fossil and living billfishes (Istiophoriformes, Istiophoridae): evidence from the Central Mediterranean

Abstract

Study of the relationships of fossil and living billfishes is crucial for understanding the ecological drivers that control their biodiversity through time and how distributional patterns of extant populations may be affected by current abiotic events. Here we describe six istiophorid species that lived sympatrically in the central Mediterranean Sea during the Late Miocene, based on fossil istiophorids from Italy. Of these, †Pizzikoskerma salentina, †Sicophasma macrocanalata, †Makaira adensa and †Makaira cyclovata are new, while †M. belgica and †M. cf. colonense were described previously. The taxon Istiophorus is reported here for the first time in the Tortonian based on an unidentified species. We reconstructed the evolutionary direction of morphological traits by including the earliest known billfish, †Hemmingwaya sarissa, in our phylogenetic analysis as outgroup. A bill with circular cross‐section, 12 caudal vertebrae, a slim body and elongated first dorsal fin are shown to be plesiomorphic traits, while 13 caudal vertebrae, fusiform body, a shorter and pointed first dorsal fin, lateral process and quadrangular spines on the vertebral centra are traits associated with larger body sizes. The Makaira species described here have trabecular bone in the premaxilla forming the rostrum, an unusual pattern interpreted as an adaptation to reduce weight while simultaneously increasing body size. The fossil billfish assemblage suggests sea‐surface temperatures between 23°C and 24°C, much warmer than the modern central portion of the Mediterranean. The exquisite preservation of one specimen shows a trophic interaction between marlins and barracudas, the first direct evidence of predator–prey relationships in fossil istiophorids.