Description of Loancorhynchus Catrillancai Holotype General remarks— Loancorhynchus catrillancai is three-dimensionally preserved. Besides the rostrum, all the available postorbital elements are well-preserved and none of them is severely deformed. Both operculi even preserve their convexity. The rostrum is partially crushed dorsoventrally. Its anteriormost fragment is slightly recurved to the right side. On the contrary, the left dentary (the most complete) is recurved to the left side, which indicates that both deformations were caused likely by taphonomic condition instead of occurring in the living animal. During its preparation, it was evident that the skull was naturally dissarticulated and embedded in a soft, reddish, micaceous sandstone (consistent with those observed in the base of the studied section; see Geologic Setting). The length of the cranial elements in anatomic position, represents approx. 60 cm. Based on the affinities of Loancorhynchus catrillancai with Blochiidae, the skull could reach a conservative length of 90 cm, assuming a rostrum with a blunt tip instead a gradually reduced, sharp tip. The body length of Loancorhynchus catrillancai can be estimated based on the proportions of complete skeletons of Blochius longirostris, where the skull is ca. one third of the whole skeleton (Fierstine & Monsch, 2002: fig. 1). Then, Loancorhynchus catrillancai body length can be estimated in 2.7 m. Premaxillaries—The premaxillaries form most of the anteriormost part of the rostrum.The anterior tip of the rostrum is lost; however, this was likely formed by the union of both premaxillaries, suggested by their large participation in the anteriormost available rostrum fragment. In the latter, premaxillaries are strongly fused in the mid and ventral part, being interrupted in the dorsal midline by a thin anterior extension of the prenasals(?). Both fused premaxillaries show a dorsoventrally compressed oval cross-section. Nutrient canali are not obseved, probably due to taphonomic conditions. Profuse villiform teeth cover all the ventral and lateral surface of both premaxillaries, over the whole length of the preserved rostrum (Figs. 3A and 3B). The posterior end of each premaxillary bears profuse craniocaudal striations. Two main interior canali are visible in the anterior fragment of the rostrum (Figs. 3E–3G). Smaller canali are also present, being paired with respect to the rostrum midline. Maxillaries—A small fragment of the left maxillary is preserved (Fig. 4). This lies crushed under the left premaxillary. This bone is elongated with a sub-squared cross-section. Based on the available fragment, the lateral exposure of each maxillary seems to be precluded by the posterior extension of the premaxillaries. Prenasals(?)—In the anteriormost available rostrum fragment, two thin bones in its dorsal midline are here interpreted as both prenasals(?) sensu Fierstine (1990) (Fig. 4). In cross-section, their contact with the premaxillaries is obscured due to preservation. The posteriormost available rostrum fragment preserves the posterior part of both prenasals(?).These laterally diverge and they have a posterolateral notch (Fig. 4D), which marks the anterior margin of the orbit. Then, the anteroventral orbit is laterally conformed by the posterior extension of the premaxillary, and ventrally, by the posterior extension of the maxillary. The posterior end of both prenasals(?) have strong folds and sulci over their dorsal surface. Dermethmoid—In the posterior part of the rostrum, a partially open sutural contact is visible asides the dorsal midline. This marks the contact of the prenasals(?) with the dermethmoid, which is placed in the midline and tapers anteriorly. Dorsally, the dermethmoid bears strong folds and sulci (Fig. 4). Its posterior part is missing. Hyomandibular—Only the left hyomandibular is available. This element is complete and well preserved (Figs. 5A–5D). In lateral (external) view, this shows a prominent pterotic facet which is medially recurved. In the same view, the sphenotic facet is high and diverged from the pterotic facet under ca. 90°. A large opercular process is extended over the posterior margin of the hyomandibular. In posterior view, this process appears as diagonally oriented with respect to the vertical. Over the lateral (external) surface, the hyomandibular bears a dorsoventral scar, consistent with the preopercular groove. In both anterior and internal views, there is a large foramen under the pterotic facet, here interpreted as the foramen for the truncus hyoideomandibularis of the facial nerve (VII). Metapterygoid—This bone remains strongly articulated to the hyomandibular (Figs. 5A–5D), although, its suture is still visible from the internal and lateral views. This element diagonally overlaps the hyomndibular. Laterally, it contacts with the latter through a suture adjacent to the preopercular groove. In internal view, the dorsal part of the metapterygoid is comparatively broader than its external exposure. Its ventral margin has a squared outline. Frontal—The left frontal is preserved (Figs. 5E and 5F), having the medial and posterior margins missing. This element has a posterorlateral process. This indicates that the skull becomes laterally broader immediately behind the orbit. In ventral (internal) view, the frontal shows a thick ridge that broadens posteriorly. Operculum—Only the left operculum is preserved (Fig. 5G) This shows the articular process for the hyomandibular. It has a well-marked convexity immediately ventral to the articulation. Subopercular—Only the right subopercular is preserved (Fig. 5H). This seems to be complete. It has a dorsal rounded contour. This element remains articulated to the right interopercular. interopercular—It is crushed together with the subopercular due to taphonomic conditions, but it can be distinguished as a bony element separated from the subopercular (Fig. 5H). It has a triangular outline with a convex posterior margin. Dentaries—Fragments of both dentaries are preserved (Fig. 6). The left dentary is the most complete. However, its anterior part as well as its posterior end are both missing. The right dentary is represented by two separated fragments. The complete occlusal and laterodorsal surface of the left dentary are covered by profuse, small villiform teeth. Villiform teeth are also present in the two fragments of the right dentary. The left dentary is very straight, having a slight lateral divergence in its posteriormost preserved end. Two separated fragments of the right dentary can be attached to the left dentary in anatomic position. This shows that the jaw had a large symphysis. The anterior cross-section of the preserved jaw has a sub-squared outline. The anterior part of both dentaries narrows anteriorly. This suggests that the length of the missing anterior part could be approximately 5 cm. Both mandibular rami diverge posteriorly. Based on this divergence, a temptative position of the jaw elements with respect to the rostrum can be estimated. Even if this is not accurate, it is clear that the lower jaw is much shorter than the rostrum. Fin rays—Remains of least nine fin rays were recovered. All of them were included in a single block. A large incomplete fragment has an articular head, recurved with respect to the shaft. It also has a small bulk in its anterior margin. These features are present in the first pectoral ray of Xiphias gladius, reason by why it is identified as the latter element.

Conclusions Loancorhynchus catrillancai represents the unique Middle Eocene xiphioid known to date in the southeastern Pacific. Its first available elements (rostrum and dentaries) were initially considered as an indeterminate xiphiorhynchine (Friedman & Otero, 2009), while its stratigraphic provenance remained dubious since 1935, when this specimen was recovered. New preparation revealed basal features found among blochiids, xiphiorhynchines and hemingwayids. Also, derived traits exclusively known in Neogene xiphiines were found in SGO.PV.6634. Based on these facts, this research reassesses the taxonomical determination of SGO.PV.6634, being now identified as a new genus and species, Loancorhynchus catrillancai. This new genus and species represents the first record of the clade Blochiidae in the southern hemisphere. The new occurrence of a Middle Eocene swordfish now in the southeastern Pacific, helps to fill the Eocene-Middle Miocene gap in the xiphioid fossil record. It also represents the fourth record of a Paleogene billfish in the Southern Hemisphere, showing that this group already reached a wide distribution along the Southern Hemisphere previous to the Neogene, with Eocene records known in New Zealand (Gottfried, Fordyce & Rust, 2013; Campbell et al., 2013), Antarctica (Cione, Reguero & Elliot, 2001) and the new record here presented, now in South America.