A New Specimen of Xiphiorhynchus sp. cf. aegyptiacus (Istiophoriformes, Xiphioidei, Xiphiidae) and Billfish Diversity in the Oligocene of South Carolina

Abstract

A partial billfish rostrum from the Chandler Bridge Formation (Early Chattian, Oligocene) near Ladson, South Carolina, U.S.A., is described and identified as Xiphiorhynchus sp. cf. aegyptiacus. The angle of taper, depth to width ratio of the cross section, and other morphological features (including dorsolateral grooves and a planoconvex cross-section), indicate that this specimen (and an earlier published speccimen) is closest in morphology to X. aegyptiacus from the Eocene Birket Qarun Formation of Egypt. This confirms the presence of a second xiphiid in the Chandler Bridge Formation besides the well-documented giant swordfish X. rotundus. This is an unusual example of two Xiphiorhynchus species existing in known sympatry, and strongly contrasting morphologies and morphometrics may point to niche partitioning between the two forms. The occurrence of specimens strongly resembling X. aegyptiacus in the western Atlantic also further substantiates past arguments that easy dispersal across the Atlantic was possible for this genus, and, by extension, that it shared the open-sea, migratory epipelagic lifestyle of modern swordfish. Moreover, the Chandler Bridge Formation boasts the most diverse billfish assemblage in the world, including Xiphiorhynchus sp., cf. X. aegyptiacus, X. rotundus, an early istiophorid, and 4-7 species of blochiid billfish in the genera Aglyptorhynchus and Cylindracanthus.