Vertebrate Anatomy Morphology Palaeontology 8:98–104 98 ISSN 2292-1389 A new specimen of Xiphiorhynchus cf. X. aegyptiacus (Istiophoriformes, Xiphioidei, Xiphiidae) and Billfish Diversity in the Oligocene of South Carolina William N. McCuen1,*, Aika S. Ishimori1, and Robert W. Boessenecker1,2 1College of Charleston, Charleston, SC, 29424, USA; [email protected]; [email protected]; [email protected] 2University of California Museum of Paleontology, University of California, Berkeley, CA 94720 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 cf. X. 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 specimen) is closest in morphology to X. aegyptiacus from the Eocene Birket Qarun and Qasr el Sagha formations 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 ex- isting in sympatry, and strongly contrasting morphologies and morphometrics may point to niche partition- ing 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 cf. X. aegyptiacus, X. rotundus, an early istiophorid, and 4–7 species of blochiid billfish in the genera Aglyptorhynchus and Cylindracanthus. INTRODUCTION Here we describe a new specimen (CCNHM-4406) of Xiphiorhynchus from the upper Oligocene Chandler Bridge The swordfish family Xiphiidae has a poorly understood Formation of South Carolina. Together with previously evolutionary history. The sole extant species, Xiphias gla- published remains, it clarifies the identification of another dius, is uncommon as a fossil; all other xiphiid remains fall xiphiid taxon (Xiphiorhynchus sp.) in the region, enhancing into the Eocene–Oligocene genus Xiphiorhynchus (Fierstine our knowledge of the family’s history while raising ques- 2006). Ten species of Xiphiorhynchus have been described tions about its ecology and evolution. from the Atlantic and former Tethys and Paratethys, along with a possible record from Antarctica (Agassiz 1844; Cope 1869; van Beneden 1871; Woodward 1901; Leriche 1909; MATERIALS AND METHODS Weiler 1929; Fierstine and Applegate 1974; Fierstine and Identification Methods: Thanks to compilation of ex- Pfeil 2009). Possible Pliocene remains from the southeast- tensive morphometric data on billfish rostra by species, it is ern Pacific (Peru) have also been suggested to represent possible to identify even very fragmentary rostra. We used Xiphiorhynchus (De Muizon and Devries 1985). Despite the datasets of Fierstine and Starnes (2005) and Fierstine this diversity and wide distribution, Xiphiorhynchus is poor- and Weems (2009) to evaluate CCNHM-4406 in terms of ly understood; five species are known only from holotype angle of taper ( , the angle between lateral margins) and fragments, and only a handful of specimens include any cross-sectional depth to width ratio (D/W); ratios involving postcrania whatsoever; moreover, holotypes for two species the complete rostrumα could not be used because the distal are lost (Fierstine 2006). portion of our specimen is missing. Published 12 July, 2020 *corresponding author. © 2020 by the authors; submitted May 7, 2020; revisions received June 15, 2020; accepted June 18, 2020. Handling editor: Alison Murray. DOI 10.18435/vamp29367 Vertebrate Anatomy Morphology Palaeontology is an open access journal http://ejournals.library.ualberta.ca/index.php/VAMP Article copyright by the author(s). This open access work is distributed under a Creative Commons Attribution 4.0 International (CC By 4.0) License, meaning you must give appropriate credit, provide a link to the license, and indicate if changes were made. You may do so in any reasonable manner, but not in any way that suggests the licensor endorses you or your use. No additional restrictions — You may not apply legal terms or technological measures that legally restrict others from doing anything the license permits. McCuen et al. — Xiphiorhynchus sp. from Oligocene of USA Institutional Abbreviations: CCNHM, Mace Brown and Lemon 1984). The upper sandy unit entirely lacks Museum of Natural History, at the College of Charleston, calcareous material like the remaining Chandler Bridge Charleston, South Carolina, USA; ChM, Charleston Formation and is decidedly less fossiliferous than typical Museum, Charleston, South Carolina, USA. beds 1–2 at the same locality; however, fossil marine verte- brate taxa typical for the Chandler Bridge Formation occur SYSTEMATIC PALEONTOLOGY within this unit, including Carcharhinus gibbesi, Physogaleus aduncas, Hemipristis serra, Plinthicus stenodon, Rhinoptera Class ACTINOPTERYGII Cope, 1887 sp., Cylindracanthus, Carolinachelys, and the giant dolphin Division TELEOSTEI, Müller, 1844 “Genus Y” (Tab. 1). However, some unusual taxa typical Order ISTIOPHORIFORMES Betancur-R et al. 2013 for younger Miocene deposits also occur, including teeth Suborder XIPHIOIDEI Rafinesque, 1815 resembling Galeocerdo ‘casei’, Carcharhinus leucas, and eur- Family XIPHIIDAE Rafinesque, 1815 hinodelphinid and squalodelphinid odontocetes. Subfamily XIPHIORHYNCHINAE Regan, 1909 This upper sandy unit differs from the Oligocene– Genus Xiphiorhynchus van Beneden, 1871 Miocene Edisto Formation and Parachucla Formation by Xiphiorhynchus cf. X. aegyptiacus consisting entirely of clean, quartzose sandstone rather than calcarenite and notably lacks a basal phosphatic Locality and Geological Setting: Chandler bonebed. All Oligocene–Miocene formations in the Bridge Formation, Chattian (late Oligocene), McKewn Charleston Embayment are expressed with a basal phos- Subdivision, North Charleston, Dorchester County, South phatic bonebed and disconformity (Weems and Lemon Carolina, U.S.A. 1984; Weems et al. 2014). Instead, bed 2 of the Chandler The Chandler Bridge Formation is typically 1–1.5 meters thick and exposed in the vicinity of Summerville, South Table 1. Faunal list for the upper sandy unit of the Chandler Carolina. It is an unlithified, noncalcareous, and patchy Bridge Formation deposit consisting of four beds: olive phosphatic silt (bed 0), light yellowish brown silty quartz sandstone (bed Chondrichthyes 1), brown phosphatic sandstone (bed 2), and light olive Lamniformes gray clayey quartz sandstone (bed 3; Katuna et al. 1997). Carcharocles angustidens Deposition of the Chandler Bridge Formation was initiated Isurus sp. in a marine shelf environment, transitioning to foreshore Carcharhiniformes Carcharhinus gibbesi and estuarine conditions and then a mixed estuarine/ Carcharhinus leucas fluvial environment (Katuna et al. 1997). Microfossils Galeocerdo 'casei' further support this assessment; marine dinoflagellates are Hemipristis serra common in the lower beds, only to be replaced by pollen Physogaleus contortus and plant debris in bed 2 (Katuna et al. 1997). However, Myliobatiformes marine vertebrate fossils are common throughout the unit, Plinthicus stenodon particularly beds 0–2 (Katuna et al. 1997; Cicimurri and Rhinoptera sp. Knight 2009), suggesting continuous marine deposition. Osteichthyes The Chandler Bridge formation unconformably overlies Istiophoriformes the lower Oligocene Ashley Formation, which represents a Cylindracanthus sp. Xiphiorhynchus cf. X. aegyptiacus deeper water (mid-outer shelf) environment but bears an Testudines essentially similar fossil fauna (Fierstine and Weems 2009). Chelonioidea CCNHM-4406 was collected from an unusual section of Carolinachelys wilsoni the Chandler Bridge Formation exposed within two storm- Aves water pond excavations in the McKewn Homes subdividi- Odontopterygiformes sion in Ladson, South Carolina. This section included a 1 Pelagornis sp. m thick typical section of the Chandler Bridge Formation Suliformes (including beds 1–2, but lacking beds 0 and 3) overlain by Sulidae n. gen. n. sp. 1–1.5 meters of unconsolidated, massively bedded, fine to Cetacea very fine grained (and occasionally silty) quartz sand with Odontoceti Agorophius sp. scattered vertical cylindrical burrows and 1–6 cm diameter Agorophiidae n. gen. (Genus Y) discoidal quartz pebbles. This upper sandy unit is in turn cf. Eurhinodelphinidae overlain by the Pleistocene Ten Mile Hill beds (Weems cf. Squalodelphinidae 99 Vertebrate Anatomy Morphology Palaeontology 8:98–104 Bridge Formation smoothly grades into the upper sandy Table 2. Mean angle of taper ( ) and depth to width (D/W) unit without a clear erosional surface mantled with a ratios of specimens of Xiphiodei after Fierstine and Voigt phosphate pebble and fossil bearing lag deposit like other (1996) and Fierstine and Starnes⍺ (2005), with measure- boundaries. Therefore, at present, it is most parsimonious ments for CCNHM-4406. ( values for extant species are unpublished and unavailable.) to consider this bed to belong to the Chandler Bridge ⍺ Formation, which is already known to be heterogeneous Specimen