98Vertebrate Anatomy Morphology Palaeontology 8:98–104 ISSN 2292-1389 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. INTRODUCTION The swordfish family Xiphiidae has a poorly understood evolutionary history. The sole extant species, Xiphias gla- dius, is uncommon as a fossil; all other xiphiid remains fall into the Eocene–Oligocene genus Xiphiorhynchus (Fierstine 2006). Ten species of Xiphiorhynchus have been described 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; Weiler 1929; Fierstine and Applegate 1974; Fierstine and Pfeil 2009). Possible Pliocene remains from the southeast- ern Pacific (Peru) have also been suggested to represent Xiphiorhynchus (De Muizon and Devries 1985). Despite this diversity and wide distribution, Xiphiorhynchus is poor- ly understood; five species are known only from holotype fragments, and only a handful of specimens include any postcrania whatsoever; moreover, holotypes for two species are lost (Fierstine 2006). 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; mccuennn@g.cofc.edu; ishimorias@g.cofc.edu; boesseneckerrw@cofc.edu 2University of California Museum of Paleontology, University of California, Berkeley, CA 94720 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 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. Here we describe a new specimen (CCNHM-4406) of Xiphiorhynchus from the upper Oligocene Chandler Bridge Formation of South Carolina. 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 ques- tions about its ecology and evolution. MATERIALS AND METHODS Identification Methods: Thanks to compilation of ex- tensive morphometric data on billfish rostra by species, it is possible to identify even very fragmentary rostra. We used the datasets of Fierstine and Starnes (2005) and Fierstine and Weems (2009) to evaluate CCNHM-4406 in terms of angle of taper (α, the angle between lateral margins) and cross-sectional depth to width ratio (D/W); ratios involving the complete rostrum could not be used because the distal portion of our specimen is missing. McCuen et al. — Xiphiorhynchus sp. from Oligocene of USA 99 Institutional Abbreviations: CCNHM, Mace Brown Museum of Natural History, at the College of Charleston, Charleston, South Carolina, USA; ChM, Charleston Museum, Charleston, South Carolina, USA. SYSTEMATIC PALEONTOLOGY Class ACTINOPTERYGII Cope, 1887 Division TELEOSTEI, Müller, 1844 Order ISTIOPHORIFORMES Betancur-R et al. 2013 Suborder XIPHIOIDEI Rafinesque, 1815 Family XIPHIIDAE Rafinesque, 1815 Subfamily XIPHIORHYNCHINAE Regan, 1909 Genus Xiphiorhynchus van Beneden, 1871 Xiphiorhynchus cf. X. aegyptiacus Locality and Geological Setting: Chandler Bridge Formation, Chattian (late Oligocene), McKewn Subdivision, North Charleston, Dorchester County, South Carolina, U.S.A. The Chandler Bridge Formation is typically 1–1.5 meters thick and exposed in the vicinity of Summerville, South Carolina. It is an unlithified, noncalcareous, and patchy deposit consisting of four beds: olive phosphatic silt (bed 0), light yellowish brown silty quartz sandstone (bed 1), brown phosphatic sandstone (bed 2), and light olive gray clayey quartz sandstone (bed 3; Katuna et al. 1997). Deposition of the Chandler Bridge Formation was initiated in a marine shelf environment, transitioning to foreshore and estuarine conditions and then a mixed estuarine/ fluvial environment (Katuna et al. 1997). Microfossils further support this assessment; marine dinoflagellates are common in the lower beds, only to be replaced by pollen and plant debris in bed 2 (Katuna et al. 1997). However, marine vertebrate fossils are common throughout the unit, particularly beds 0–2 (Katuna et al. 1997; Cicimurri and Knight 2009), suggesting continuous marine deposition. The Chandler Bridge formation unconformably overlies the lower Oligocene Ashley Formation, which represents a deeper water (mid-outer shelf ) environment but bears an essentially similar fossil fauna (Fierstine and Weems 2009). CCNHM-4406 was collected from an unusual section of the Chandler Bridge Formation exposed within two storm- water pond excavations in the McKewn Homes subdividi- sion in Ladson, South Carolina. This section included a 1 m thick typical section of the Chandler Bridge Formation (including beds 1–2, but lacking beds 0 and 3) overlain by 1–1.5 meters of unconsolidated, massively bedded, fine to very fine grained (and occasionally silty) quartz sand with scattered vertical cylindrical burrows and 1–6 cm diameter discoidal quartz pebbles. This upper sandy unit is in turn overlain by the Pleistocene Ten Mile Hill beds (Weems and Lemon 1984). The upper sandy unit entirely lacks calcareous material like the remaining Chandler Bridge Formation and is decidedly less fossiliferous than typical beds 1–2 at the same locality; however, fossil marine verte- brate taxa typical for the Chandler Bridge Formation occur within this unit, including Carcharhinus gibbesi, Physogaleus aduncas, Hemipristis serra, Plinthicus stenodon, Rhinoptera sp., Cylindracanthus, Carolinachelys, and the giant dolphin “Genus Y” (Tab. 1). However, some unusual taxa typical for younger Miocene deposits also occur, including teeth resembling Galeocerdo ‘casei’, Carcharhinus leucas, and eur- hinodelphinid and squalodelphinid odontocetes. This upper sandy unit differs from the Oligocene– Miocene Edisto Formation and Parachucla Formation by consisting entirely of clean, quartzose sandstone rather than calcarenite and notably lacks a basal phosphatic bonebed. All Oligocene–Miocene formations in the Charleston Embayment are expressed with a basal phos- phatic bonebed and disconformity (Weems and Lemon 1984; Weems et al. 2014). Instead, bed 2 of the Chandler Table 1. Faunal list for the upper sandy unit of the Chandler Bridge Formation Chondrichthyes Lamniformes Carcharocles angustidens Isurus sp. Carcharhiniformes Carcharhinus gibbesi Carcharhinus leucas Galeocerdo 'casei' Hemipristis serra Physogaleus contortus Myliobatiformes Plinthicus stenodon Rhinoptera sp. Osteichthyes Istiophoriformes Cylindracanthus sp. Xiphiorhynchus cf. X. aegyptiacus Testudines Chelonioidea Carolinachelys wilsoni Aves Odontopterygiformes Pelagornis sp. Suliformes Sulidae n. gen. n. sp. Cetacea Odontoceti Agorophius sp. Agorophiidae n. gen. (Genus Y) cf. Eurhinodelphinidae cf. Squalodelphinidae Vertebrate Anatomy Morphology Palaeontology 8:98–104 100 Bridge Formation smoothly grades into the upper sandy unit without a clear erosional surface mantled with a phosphate pebble and fossil bearing lag deposit like other boundaries. Therefore, at present, it is most parsimonious to consider this bed to belong to the Chandler Bridge Formation, which is already known to be heterogeneous with lateral changes in lithology and varies from expos- ure to exposure by the presence or lack of particular beds (chiefly bed 0 and bed 3; Sanders et al. 1982; Katuna et al. 1997). At this locality, this upper unit appears to grade laterally into a blueish gray siltstone resembling bed 3 – although we hesitate to identify this upper stratum as bed 3 (or a new bed) for the time being. Regardless, this lateral facies change from the upper stratum to bed 3 strongly suggests that the two are equivalent in age. The Chandler Bridge Formation has yielded dinoflagel- lates corresponding to zones NP 24–25 (29.6–23.1 Ma) and 87Sr/86Sr ratios from fossil oysters (bed unknown; 24.7–24.5 Ma). In accordance with 87Sr/86Sr dates of 23.5 Ma from the overlying Edisto Formation, an age of 24.7–23.5 Ma is assigned to the Chandler Bridge Formation (Boessenecker and Fordyce 2016). It is unclear whether these dates apply readily to this upper unit as this layer may be younger than the typical beds. The Edisto and Parachucla formations do not crop out extensively and are rarely identified in subsurface auger holes (Weems et al. 2014), and only one small exposure of each unit is present in the Charleston embayment, 20 and 28 km (respectively) due west of this locality. The stratum overlying this upper sandy unit is Pleistocene. For the time being, no evidence suggests that this upper unit is younger than Oligocene and, pending further study, a latest Oligocene age is as- signed. We note that matrix associated with two skulls of the archaic dolphin Agorophius sp. reported from a nearby exposure (Coosaw Preserve Subdivision) by Boessenecker and Geisler (2018) match this lithology and reassign the stratum that these specimens were collected from (bed 2) to this upper sandy unit. Description: Billfish rostra, exemplified by the dramat- ic ‘sword’ of swordfish, marlin etc., consist of extremely elongate, fused premaxillae that bifurcate posteriorly before meeting the rest of the skull while tapering to a sharp point distally (in istiophorids, but not xiphiids, the prenasals also extend into the rostrum’s proximal portion). Hollow longitudinal nutrient canals also run the length of a xiphioid rostrum, the number and arrangement of which varies among species and families (Fierstine 2006). CCNHM-4406 (Fig. 1A–C; Tab. 2) is a partial rostrum, acutely triangular in dorsal and ventral views, approximate- ly 11.5 cm long. The cleft between the posterior ends of the premaxillae (where they diverge proximally) is wider and extends farther forward on the ventral side than the dorsal side. On both sides, the cleft extends farther anteriorly along the midline as a shallow groove, with the groove on the dorsal surface deeper than on the ventral side. Both grooves gradually shallow and terminate before reaching the distal end of the specimen. Breakage at both ends of the specimen reveals the longitudinal nutrient canals typical for billfish rostra (Fierstine and Voight 1996), but their full number and pattern are not evident, seemingly owing to suboptimal preservation and the natural obscurity of some canals that occurs in some specimens (compare cross sections from Fierstine and Weems 2009:figs. 25D, E, 26D; Fierstine and Starnes 2005:fig. 4). Almost the entire dorsal and lateral portions have been broken off the hollow proximal section of the right premaxilla. On the left side, enough of the dorsolateral surface is preserved that a wide, shallow groove that reaches to the anterior tip of the fossil is apparent. The ventral surface is much more flattened than the dorsal, giving a planoconvex cross-section (Fig. 1D). CCNHM-4406 was found to have an ⍺-value of 10.3°, and a D/W value of 0.53 (Tab. 2). Identification and Comparisons: Despite in- cluding most of the proximal portion of the rostrum, CCNHM-4406 shows no suturing to indicate extension of the prenasals into the posterior half of the rostrum (charac- teristic of Istiophoridae, see above, Fierstine 2006). It also Table 2. Mean angle of taper (⍺) and depth to width (D/W) ratios of specimens of Xiphiodei after Fierstine and Voigt (1996) and Fierstine and Starnes (2005), with measure- ments for CCNHM-4406. (⍺ values for extant species are unpublished and unavailable.) Specimen type D/W ⍺ CCNHM-4406 Xiphiorhynchus cf. X. aegyptiacus 0.53 10.3 X. aegyptiacus 0.45 11.5 X. eocaenicus 0.62 16.0 X. elegans 0.63 5.0 X. kimblalocki 0.82 10.0 X. rotundus 0.84 19.0 X. priscus (range) 0.69–0.79 3.0–12.0 ChM PV8317 (Xiphiorhynchus indet.) 0.45 10.5 Istiophorus platypterus (mean and range) 0.68 (0.58–0.78) - Tetrapturus albidus (mean and range) 0.62(0.56–0.66) - Makaira nigricans (mean and range) 0.70 (0.59–0.80) - Xiphias gladius (mean and range) 0.34 (0.29–0.39) - McCuen et al. — Xiphiorhynchus sp. from Oligocene of USA 101 Figure 1. Partial rostrum of Xiphiorhynchus sp. cf. X. aegyptiacus (CCHM-4406), Chandler Bridge Formation, late Oligocene, South Carolina. A, dorsal view. B, ventral view. C, left lateral view D, Anterior view. E, Posterior view. Anterior to right for A–C. More clearly visible nutrient canals and the dorsolateral grooving are labelled. Vertebrate Anatomy Morphology Palaeontology 8:98–104 102 has a more dorsoventrally flattened cross-section (lower D/W ratio) than the round proportions found in that family, although less flattened than the modern swordfish (Xiphias gladius, see Table 2). CCNHM-4406 is also distinct from Xiphiorhynchus rotundus (D/W = 0.84, ⍺ = 19.0°, see Table 2), a xiphiid well-documented from the Chandler Bridge Formation by Fierstine and Weems (2009). In addition to distinct pro- portion values, the adult size of X. rotundus (up to around 5 m in total length, based on estimates from isolated verte- brae; Fierstine and Weems 2009) is also much greater than this individual (although CCNHM-4406 may be a juven- ile, considering its small size and relatively poorly ossified, porous bone texture). However, Fierstine and Weems also reported another, very different specimen of Xiphiorhynchus from the Chandler Bridge, ChM PV8317, a rostrum only slightly larger than CCNHM-4406. With a D/W of 0.53 and ⍺ of 10.3°, this specimen not only has similar proportions to CCNHM-4406, it also shares the same planoconvex cross section. They noted close similarity of that specimen in these features to the geochronologically older species Xiphiorhynchus aegyptiacus from the late Eocene of Egypt (Tab. 2), but hesitated to refer it to that species owing to poor preservation, difference in time (7–10 my), and geographic separation between localities (Fierstine and Weems 2009). Besides the traits they noted, X. aegyp- tiacus and ChM PV8317 also share a shallow groove on each dorsolateral surface of the rostrum (noted above for CCNHM-4406, compare rostra from Fierstine and Weems 2009:fig. 26A; Fierstine and Gingerich 2009:fig. 2B). In view of these similarities, it is likely that ChM PV8317 and our specimen, CCNHM-4406, are conspecifics of a poorly understood billfish form from the late Oligocene of South Carolina, possibly a geochronologically late occur- rence of X. aegyptiacus (representing a major range and temporal extension for that species), or an as-yet unnamed close relative. We therefore identify CCNHM-4406 as Xiphiorhynchus cf. X. aegyptiacus, noting that more com- plete specimens are needed to further clarify species-level identification. DISCUSSION Ecology and Distribution of Xiphiorhynchus: CCNHM-4406 and ChM PV8317 may extend the known geographic and geochronologic range of X. aegyptiacus, rais- ing implications for the ecology of Xiphiorhynchus. Fierstine and Starnes (2005) used the occurrence of Xiphiorhynchus sp. cf. 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. The presence of X. aegyptiacus in North America would also support this conclusion. However, 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 tem- poral and spatial restriction very well may be an artifact of taxonomic oversplitting, or the sheer rarity (Fierstine 2006) of Xiphiorhynchus remains (causing most species to only be known from very few localities). Interestingly, Xiphiorhynchus aegyptiacus and this extreme- ly similar North American form show some of the lowest known D/W values in the genus, coming closest to those of modern Xiphias (Tab. 2). This would have affected the function of the rostrum as a hunting weapon, the increas- ingly popular primary explanation for rostral elongation in xiphioids. Rostral morphology and the exact behavioral tactics involved in its employment show variations between extant species (Habegger et al. 2015, Habegger et al. 2020, Hansen et al. 2020). Biomechanics studies (Habegger et al. 2015, Habegger et al. 2020) seem to indicate that rostral flattening in living swordfish helps facilitate rapid lateral slashes to stun or even severely damage small, schooling prey fish, a behavior supported by finds of fish cut in half in swordfish stomachs. The combination of thinness in lateral view with a relatively much greater width would streamline the rostrum for such motion, while still ensur- ing flexural stiffness, strength against lateral stresses, and a narrow surface of impact for maximum prey damage. In contrast to the more swordfish-like morphology found in X. aegyptiacus, X. rotundus has much higher D/W values, comparable to modern Istiophoridae (Tab. 2). The same studies on extant billfish rostral function concluded that the round cross-section of istiophorid rostra is better optimized for multi-directional (as opposed to strictly lateral) swiping, as well as stabbing motions. Both tactics are supported by field observations of istiophorid hunt- ing and stomach contents (Habegger et al. 2015, 2020). Considering the role of the rostrum in foraging, it is very possible that the markedly different morphologies in these two forms of Xiphiorhynchus reflect differences in hunting tactics as well, perhaps even analogous to those between their modern relatives. Billfish Ecology and Diversity in the Oligocene of South Carolina: The successive Ashley and Chandler Bridge formations of South Carolina represent the rich- est locality (in terms of diversity) in the world for fossil billfish. Besides Xiphiorhynchus cf. X. aegyptiacus, and McCuen et al. — Xiphiorhynchus sp. from Oligocene of USA 103 the giant swordfish Xiphiorhynchus rotundus, Fierstine and Weems (2009) noted that these rocks have yielded at least three (possibly as many as six) species of the large blochiid Aglyptorhynchus, the small purported blochiid Cylindracanthus, and fragments of the earliest definite istio- phorid. The next most speciose billfish assemblage is that of the lower Eocene London Clay (UK), with six species (Friedman et al. 2015). It is unclear whether this unparal- leled diversity of fossil billfish reflects time averaging of fossiliferous shallow marine deposits or genuine diversity. In parallel, unusually speciose assemblages of cetaceans and sea turtles (Boessenecker and Geisler 2018, and references therein; R.W. Boessenecker pers. observ.) are reported from these Oligocene strata. Billfish remains, even if just isolated fin rays, are frequently encountered in the Ashley and Chandler Bridge formations (R.W. Boessenecker, pers. observ.), suggesting not just high diversity, but high abun- dance in the area as well. Many of these billfish fossils (for instance, CCNHM-4406) are preserved in shallow marine deposits (see also Fierstine and Weems 2009), which is contrary to expectations if ancient billfish were deep-water pelagic migrants. However, other authors note that carcass- es drifting with the currents, movement of remains long distances in the stomach contents of large predators, and even rostra breaking off in the bodies of impaled animals and being transported thereafter have been observed for modern billfish and cannot be totally discounted for fossils unless they are fairly complete and well-articulated (Fierstine and Starnes 2005). Although time-averaging and even long-distance post- mortem transport may help explain billfish richness and diversity in these rocks, other possible factors include niche partitioning or habitat segregation. If postmortem transport was important to Ashley and Chandler Bridge formation taphonomy, it raises the possibility of billfish being transported from slightly different, nearby habitats to a single final resting place. However, particularly if the billfishes of this assemblage shared, at least to some ex- tent, the wide-roaming, epipelagic habits of their modern counterparts, as argued above, it is likely they overlapped and competed directly. In this scenario, fine niche parti- tioning would explain the diversity observed. Against this backdrop, the differences between forms of Xiphiorhynchus noted above take on a new interest, as does the bizarre morphology of Aglyptorhynchus. Unlike any extant bill- fish, Aglyptorhynchus had a mobile rostrum, which, com- plemented by its equally elongate mandible, would have allowed a uniquely wide gape. Moreover, at least one of the species present here, A. robustus, had a ball-and-socket articulation between the first vertebra and the occipital condyle, possibly related to hunting strategy and otherwise almost unknown in teleosts (Fierstine and Weems 2009). Like Xiphiorhynchus, the Aglyptorhynchus species in this assemblage show variations in rostral morphology (Fierstine and Weems 2009) that could also have ecological relevance (e.g., higher D/W values in A. palmeri than in A. alsand- ersi). For now, detailed interpretations of such data must remain speculative, but we hope some of the above points regarding contrasting morphologies may help guide future studies of the factors that have shaped xiphioid diversity. The Oligocene rocks of South Carolina have proven an ex- cellent source of billfish fossils, outstanding in the world for species richness; future research here promises to continue to clarify the murky evolutionary history of billfishes. ACKNOWLEDGEMENTS This study would not have been possible without the generosity of S. Hildenbrandt, who donated the specimen. Thanks to B. Doster and DR Horton Construction for locality access. We thank S. Boessenecker (CCNHM) for providing access to specimens under her care. 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