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.
Highlights
The swordfish family Xiphiidae has a poorly understood evolutionary history
X. eocaenicus in North America (X. eocaenicus was previously only documented from Britain) to argue for a transatlantic distribution of that species, and, by extension, that species of Xiphiorhynchus shared pelagic, migratory cosmopolitan habits with modern swordfish
Most Xiphiorhynchus species have very limited, rarely overlapping spatiotemporal ranges (Agassiz 1844; Cope 1869; van Beneden 1871; Woodward 1901; Leriche 1909; Weiler 1943; Fierstine and Applegate 1974; Fierstine 2006; Fierstine and Pfeil 2009; Fierstine and Weems 2009). This could argue against the South Carolina specimens being X. aegpytiacus, but this temporal and spatial restriction very well may be an artifact of taxonomic oversplitting, or the sheer rarity (Fierstine 2006) of Xiphiorhynchus remains
Summary
The sole extant species, Xiphias gladius, is uncommon as a fossil; all other xiphiid remains fall into the Eocene–Oligocene genus Xiphiorhynchus (Fierstine 2006). Possible Pliocene remains from the southeastern Pacific (Peru) have been suggested to represent Xiphiorhynchus (De Muizon and Devries 1985). Despite this diversity and wide distribution, Xiphiorhynchus is poorly understood; five species are known only from holotype fragments, and only a handful of specimens include any postcrania whatsoever; holotypes for two species are lost (Fierstine 2006). Together with previously published remains, it clarifies the identification of another xiphiid taxon (Xiphiorhynchus sp.) in the region, enhancing our knowledge of the family’s history while raising questions about its ecology and evolution
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