Holotype.

MDT-PV 2, an originally articulated cranial and cervical skeleton consisting of the nearly complete skull, the partial axis associated with its rib from the right side and articulated with the cranial part of the third cervical vertebra, a fragment of the fifth cervical vertebra, the nearly complete sixth cervical vertebra and its right rib, the partial seventh cervical vertebra, and a section of ossified cervical tendon.

Etymology.

Sarmiento, for the Patagonian town and the administrative department in which it is located, the latter of which has yielded numerous Cretaceous dinosaur fossils; saurus, Greek, ‘lizard.’ The specific name honors the late Dr. Eduardo Musacchio, a model scientist and educator at the Universidad Nacional de la Patagonia San Juan Bosco in Comodoro Rivadavia, Argentina.

Locality and horizon.

Estancia Laguna Palacios (44°54'11.6'' S, 69°22'56.7'' W), Sierra Nevada Anticline, Golfo San Jorge Basin, south-central Chubut Province, central Patagonia, Argentina (Fig 1). Uppermost section of the Lower Member of the Upper Cretaceous Bajo Barreal Formation, Chubut Group. The specimen was found in situ in a tuffaceous sandstone that is regarded as Cenomanian—Turonian in age [69,72,76–78,80].

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Fig 1. Map of Chubut Province, central Patagonia, Argentina, showing location of the Estancia Laguna Palacios, the type locality of Sarmientosaurus musacchioi gen. et sp. nov. (modified from Ibiricu et al. [232]).

https://doi.org/10.1371/journal.pone.0151661.g001

Diagnosis.

Basal lithostrotian titanosaurian sauropod diagnosed by the following autapomorphies: (1) maximum (rostroventral—caudodorsal) diameter of orbit nearly 40% rostrocaudal length of cranium (as measured from tip of snout to occipital condyle); (2) complex maxilla—lacrimal articulation, with ascending ramus of maxilla embedded in and bordered laterally and medially by lacrimal dorsal process; (3) medial edge of caudal sector of maxillary ascending ramus bordering bony nasal aperture with low but well-defined ridge; (4) ‘tongue-like’ ventral process of quadratojugal that overlaps quadrate caudally; (5) separate foramina for all three branches of the trigeminal nerve; (6) absence of median venous canal connecting infundibular region to ventral part of brainstem; (7) premaxillary teeth subvertical, maxillary teeth procumbent, and dentary teeth recumbent; (8) middle cervical vertebrae with ‘strut-like’ (as opposed to ‘sheet-like’) centroprezygapophyseal laminae; (9) extremely elongate and slender ossified tendon extending along cervical series ventrolateral to vertebrae and ribs.

Preservation.

The cranium, mandible, and all preserved cervical vertebrae and ribs of the new titanosaur were originally found in articulation (Fig 2). Nevertheless, during the course of laboratory preparation, we were only able to recover the skull, parts of the articulated axis and third cervical vertebra, most of the sixth and seventh cervical vertebrae, and pieces of the fifth cervical vertebra and the second and sixth cervical ribs from the right side. Unfortunately, the remainder of the collected vertebrae (the atlas and cervical four) and ribs were too poorly preserved and damaged by weathering to be salvageable.

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Fig 2. Disposition of the type specimen of Sarmientosaurus musacchioi gen. et sp. nov. (MDT-PV 2) upon discovery.

(A) Articulated skull in ventral view, showing close association of ossified cervical tendon (arrow) with occipital region of cranium. (B, C) Two views of articulated skull and partial cervical series in ventral view, showing considerable craniocaudal extent and consistently narrow diameter of ossified cervical tendon (arrows). (D) Relationship of a cervical rib (white arrow) with the ossified cervical tendon (black arrow).

https://doi.org/10.1371/journal.pone.0151661.g002

In the field, we observed a very slender, dark, cylindrical structure situated adjacent to and oriented parallel to the right ventrolateral area of the articulated cervical vertebrae and ribs (Fig 2). This structure extended from near the right occipital region of the skull and past several vertebrae without changing diameter. Although we observed the structure on only the right side of the specimen, we assume that it was bilaterally symmetrical in the living animal. Therefore, given that the right side of the specimen is generally better preserved than the left, the equivalent structure on the left side presumably eroded away prior to discovery.

Given the extraordinary length attained by the cervical ribs of some sauropods (e.g., mamenchisaurids [91,92]), including other titanosauriforms (e.g., Giraffatitan [93], Sauroposeidon [94]), it is conceivable that this structure might represent the caudal end of one of these ribs. Nevertheless, as observed in the field, the structure maintained its same, diminutive diameter alongside several cervical vertebrae, and its cranial extreme was situated immediately caudal to the skull, morphologies that are inconsistent with known sauropod cervical ribs. Furthermore, because the skull, cervical vertebrae, and ribs were all fully articulated, the identification of this structure as a displaced cervical rib shaft seems unlikely. We therefore interpret the structure as an ossified tendon that is distinct from the cervical ribs.

Unlike the situation in Nemegtosaurus [10,11] and Tapuiasaurus [14], the skull of Sarmientosaurus was not strongly affected by taphonomic distortion. Instead, the skull is only moderately deformed in its caudodorsal and dorsal areas. Pressure applied to these regions apparently caused the jugal processes of both postorbitals to slide slightly rostrally over the postorbital processes of the corresponding jugals. Nevertheless, these modest alterations demonstrate that the caudal part of the skull was not significantly rostrally displaced relative to more rostral regions. There is no evidence of dorsoventral compression of the snout; indeed, in this area of the skull, only the dorsal parts of the premaxilla and maxilla are damaged, presumably due to pre-diagenetic erosion. The sixth and seventh cervical vertebrae have suffered some lateral deformation, which has mainly affected parts of the neural arches such as the prezygapophyses.

During the excavation of the Sarmientosaurus holotype, an abelisaurid tooth was discovered only a few centimeters from the occipital region of the skull, raising the possibility that this titanosaurian specimen was scavenged by this theropod. This is, however, ambiguous, as the Sarmientosaurus bones do not exhibit tooth marks or other feeding traces.

Anatomical Terminology.

In our description of the dentition of Sarmientosaurus, we employ the terms used by García and Cerda [61].

Premaxilla.

Both premaxillae of Sarmientosaurus are preserved (Figs 3–7; S1 Fig; S1, S2, S3 and S4 Movies. They articulate rostromedially, rendering the rostral end of the snout convex in lateral view, as in Abydosaurus, Europasaurus, Giraffatitan, Nemegtosaurus, Quaesitosaurus, and Tapuiasaurus. This contrasts the morphologies in Malawisaurus (Jacobs et al. [102]:fig. 1a; Gomani [32]:fig. 4a) and Narambuenatitan (Filippi et al. [40]:fig. 4a), where the more vertical nasal process lends the rostral margin of the premaxilla a taller, straighter lateral profile. The premaxilla of Euhelopus seems intermediate between these convex and subvertical conditions [101]. The premaxillae of Sarmientosaurus continue caudodorsally to the bony nasal apertures; only the left premaxilla preserves the rostral margin of the aperture, however. Along this margin, the area of the interpremaxillary articulation shows remnants of a sagittal crest that probably corresponds to the rostroventral base of the internarial bar. The premaxillae articulate with the maxillae caudally; in life, they would presumably have also contacted the nasals caudodorsally.

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Fig 3. Cranium of Sarmientosaurus musacchioi gen. et sp. nov. (MDT-PV 2).

Photographs (A, C) and interpretive drawing (B) in right lateral (A, B) and left lateral (C) views. Abbreviations see text. Scale bar = 10 cm.

https://doi.org/10.1371/journal.pone.0151661.g003

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Fig 4. Cranium of Sarmientosaurus musacchioi gen. et sp. nov. (MDT-PV 2).

Photographs (A, C, E) and interpretive drawings (B, D, F) in rostral (A, B), caudal (C, D), and caudodorsal (E, F) views. Abbreviations see text. Scale bars = 10 cm.

https://doi.org/10.1371/journal.pone.0151661.g004

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Fig 5. Cranium of Sarmientosaurus musacchioi gen. et sp. nov. (MDT-PV 2).

Photographs (A, C) and interpretive drawings (B, D) in dorsal (A, B) and ventral (C, D) views. Abbreviations see text. Scale bars = 10 cm.

https://doi.org/10.1371/journal.pone.0151661.g005

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Fig 6. Cranium of Sarmientosaurus musacchioi gen. et sp. nov. (MDT-PV 2).

Computed tomography-based digital visualization in right lateral (A), left lateral (B), rostral (C), caudal (D), dorsal (E), and ventral (F) views. Scale bar = 10 cm.

https://doi.org/10.1371/journal.pone.0151661.g006

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Fig 7. Snout and narial region of Sarmientosaurus musacchioi gen. et sp. nov. (MDT-PV 2).

Computed tomography-based digital visualization in dorsal view, showing locations of preantorbital, rostral maxillary, and subnarial (blue star) foramina (A), and left bony nasal aperture (blue overlay) and narial fossa (black line) (B). Abbreviations see text. Scale bar = 10 cm.

https://doi.org/10.1371/journal.pone.0151661.g007

The rostroventral end of the suture between the premaxilla and maxilla is clearly discernible toward the tip of the snout, far rostral to the bony nasal aperture. In Malawisaurus [32,102], Narambuenatitan [40], and probably ISI R K 27/528 [31,102], by contrast, this suture lies ventral to the rostral end of the nasal aperture, indicating that these apertures were not retracted in these titanosaurs. Nevertheless, this condition may well have varied through titanosaurian ontogeny, as unretracted bony nasal apertures are also present in the embryonic skulls from Auca Mahuevo [15–17].

Remnants of the narial fossa of Sarmientosaurus are evident in more dorsal areas of the premaxillary—maxillary contact. This fossa takes the form of a slight depression of the snout, the perimeter of which has been damaged by erosion. The narial fossa is better preserved on the right side, where it can be seen to reach rostrally to the region of the premaxillary—maxillary contact (Fig 7). Thus, the narial fossa was much larger than the bony nasal aperture, as is the case in most sauropods [103,104]. A comparable narial fossa is present in Abydosaurus and Giraffatitan. The lateral surface of each premaxilla is rostrocaudally short and shows small, irregular traces, some of which may be artifacts of the erosion that has affected more dorsal regions of these bones. There is a bony lamina lateral to the premaxillary teeth that is also present in the maxilla. The suture between the premaxilla and maxilla should bear the subnarial foramen, an aperture that transmitted blood vessels between the narial region and palate. This foramen is found in virtually all saurischians [105] and is enlarged in many sauropods [95,103,104]. The relevant region is not well preserved in MDT-PV 2, but it is present on the left side. Here, gaps in the preserved bone fragments indicate the likely position of the subnarial foramen (Fig 7), which would be consistent with that in other sauropods. Given that inferred position and the extent of the narial fossa, it is likely that, in life, Sarmientosaurus had a rostrally positioned fleshy nostril as has been reconstructed for other sauropods [104].

Each premaxilla bears four alveoli, as in all other sauropods. Medially, the ventral margin of the premaxilla exhibits a continuous ridge situated close to the teeth, which is contiguous with a similarly-positioned ridge on the maxilla.

Maxilla.

The maxilla of Sarmientosaurus (Figs 3–8; S1 Fig; S1, S2, S3 and S4 Movies) is a stout, rostrocaudally elongate element. Its gently convex lateral surface is pierced by neurovascular foramina that open into prominent grooves, rendering the surface slightly undulatory; these grooves are mediolaterally oriented, as in Nemegtosaurus [10,11] and to a lesser extent in Giraffatitan [95]. The holotypic maxilla fragment of Campylodoniscus—which, like Sarmientosaurus, was recovered from the Upper Cretaceous Chubut Group of southern Chubut Province—appears to be proportionally taller than that of the new taxon (see Huene [9]:82). Furthermore, Campylodoniscus was regarded as a nomen vanum by Bonaparte and Gasparini [106] and a nomen dubium by Powell [7]. In Sarmientosaurus, the ascending ramus of the maxilla forms a bar that separates the bony nasal aperture from the antorbital fenestra. The ramus arises near the rostrocaudal midline of the maxilla, further caudally than the ascending rami of Euhelopus [101], Narambuenatitan, ISI R K 27/528, UNPSJB-PV 583 (an isolated titanosaur maxilla from the Bajo Barreal Formation [53]), and especially Rapetosaurus, but similar to the conditions in Abydosaurus, Nemegtosaurus, and Tapuiasaurus.

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Fig 8. Palate of Sarmientosaurus musacchioi gen. et sp. nov. (MDT-PV 2).

Computed tomography-based digital visualization in ventral view indicating palatal bones (ectopterygoids, palatines, pterygoids, and vomers) and the right suborbital fenestra. Abbreviations see text. Scale bar = 10 cm.

https://doi.org/10.1371/journal.pone.0151661.g008

The phylogenetic distribution of the preantorbital fenestra, a large accessory opening in the maxilla that is characteristic of Diplodocidae [107–109], was widened considerably when Wilson and Sereno [103] homologized a neurovascular foramen that occurs in various sauropods with the definitive preantorbital fenestra of diplodocids. In taxa such as Camarasaurus, Europasaurus, and Giraffatitan, the homologous structure is a relatively inconspicuous foramen, such that the term ‘preantorbital fenestra’ does not seem appropriate, even if the homologous foramen is elaborated into a large opening in other taxa. Although Witmer [109] and Wilson and Sereno [103] regarded the preantorbital fenestra of diplodocids as relating to the pneumaticity associated with the antorbital cavity, more recent studies [110] have suggested that the structure is vascular in origin. Derived lithostrotian titanosaurs such as Nemegtosaurus [11], Rapetosaurus [13], and Tapuiasaurus [14] apparently converged on diplodocids in expanding this neurovascular foramen into a relatively large opening that is here termed the preantorbital foramen.

Sarmientosaurus lacks a true preantorbital foramen or fenestra, but probably possesses the homologous neurovascular foramen. Criteria for establishing the homologies of these openings have not previously been established, but include the following: (1) the foramen/fenestra is located dorsal to the maxillary palatal shelf, where it communicates with the canal for the maxillary neurovascular bundle (traceable in computed tomographic [CT] scan data); (2) the foramen/fenestra is in the vicinity of the suborbital fenestra, where the palatine and ectopterygoid unite with the maxillary palatal shelf; and (3) the foramen/fenestra is generally just caudal to the alveolar tooth chamber (and this chamber, housing the replacement teeth, may extend somewhat caudal to the most distal [= caudal] erupted tooth position). Applying these criteria to Sarmientosaurus reveals a credible (albeit poorly preserved) foramen on each side of the cranium that we regard as the homolog of the preantorbital foramen/fenestra (Figs 3–7). This small, otherwise unremarkable opening is located ventral to the rostral end of the antorbital fenestra, similar to its position in Giraffatitan [95] and the Auca Mahuevo embryos [17]. In Abydosaurus [98] and especially Nemegtosaurus [11], the preantorbital foramen or its homolog is positioned more rostrally on the snout, whereas in Euhelopus (Wilson and Upchurch [111]:fig. 6) and an immature specimen of Camarasaurus (CM 11338; Wilson and Sereno [103]:fig. 7a), it is situated more caudally. In Rapetosaurus, the preantorbital foramen is placed comparatively rostrally, on the rostroventral base of the jugal process of the maxilla, ventrolateral to the rostral end of the greatly enlarged antorbital fenestra [13]. The extremely large preantorbital foramen of Tapuiasaurus is also placed rostrally relative to that of Sarmientosaurus [14]. The homolog of the preantorbital foramen/fenestra is reduced or absent in adult individuals of Camarasaurus (Wilson and Sereno [103]:46).

Wilson and Sereno (1998) highlighted another neurovascular feature in sauropods: the rostral (= anterior) maxillary foramen, which opens within the narial fossa caudal (or lateral, in diplodocids) to the subnarial foramen. In a sense, this structure is a counterpart to the preantorbital foramen/fenestra in that both are associated with the canal for the maxillary neurovascular bundle and transmitted branches thereof in life [110]. The CT scan data of Sarmientosaurus clearly show (especially on the right side of the cranium) the course of the maxillary neurovascular bundle through the maxilla and where this bundle gives off the branch that leads to the rostral maxillary foramen before continuing rostrally through the bone. The rostral maxillary foramen opens medially into the narial fossa just inside the rim of the fossa, near the base of the maxillary ascending ramus. As preserved, the foramen is modest in size, comparable in relative scale to that observed in Camarasaurus (CM 11338) and Giraffatitan (MB R.2223.1).

Caudally, the ventral edge of the maxilla of Sarmientosaurus forms a marked, roughly semicircular embayment (the ‘postdental emargination’ of Gallina and Apesteguía [24]:fig. 7) that is also present in other titanosaurians such as Nemegtosaurus [11], Rapetosaurus [13], Tapuiasaurus [14], and the Auca Mahuevo embryos [17], and even in the brachiosaurid Giraffatitan [95] and the basal macronarian Europasaurus [97]. The development of this structure in Sarmientosaurus is intermediate between the incipient conditions in Europasaurus, Giraffatitan, and Nemegtosaurus (see Wilson [11]:fig. 16a) and the much larger, deeper embayments of Tapuiasaurus and especially Rapetosaurus. The maxilla articulates with the jugal caudoventrally and the lacrimal and prefrontal caudodorsally. Caudal to the tooth row, it exhibits a robust caudoventral process with a jugal articulation that is more elongate than that in Nemegtosaurus, Rapetosaurus, and probably Tapuiasaurus [14]. The medial edge of the maxillary ascending ramus bordering the bony nasal aperture has a low but well-defined ridge that we tentatively consider an autapomorphy of Sarmientosaurus, although a similar or the same structure may also be incipiently developed in Tapuiasaurus (see Zaher et al. [14]:fig. 1c).

The medial surface of the maxilla is longitudinally concave and exhibits the same continuous bony flange that is present in the premaxilla. There are 12 teeth in the left maxilla of the Sarmientosaurus holotype and 11 in the right. The tooth row encompasses 64% of the length of the maxilla, a condition that is intermediate between those in Camarasaurus and Giraffatitan (75%) on one hand and Abydosaurus (52%), Tapuiasaurus (46%), and Nemegtosaurus (34%) on the other. The relatively long tooth row in Sarmientosaurus may relate to the plesiomorphic (i.e., unexpanded) condition of the homolog of the preantorbital foramen as well as the intermediate condition of the ‘postdental emargination.’ In other words, the restriction of the teeth to the rostral end of the snout in more advanced titanosaurs may be correlated with the increased development of both these features.

Nasal.

The nasal (Figs 3–7; S1 Fig; S1, S2, S3 and S4 Movies) is a planar bone that is roughly quadrangular in dorsal view. It is rostrocaudally longest medially, at the internasal articulation, and extends rostrally as a process that presumably would have articulated with the ascending ramus of the premaxilla to form the missing internarial bar. The curved rostrolateral edge of the nasal forms the caudal border of the bony nasal aperture, and continues to expand caudally at an approximately straight lateral margin that borders the prefrontal and frontal. The caudal margin of the nasal has a straight, mediolaterally-oriented suture with the frontal. The caudal ends of both nasals are damaged near their contact with the frontals.

Lacrimal.

The lacrimal (Figs 3–7; S1 Fig; S1, S2, S3 and S4 Movies) is a dorsoventrally elongate and gently caudodorsally-inclined bone that separates the antorbital fenestra from the orbit. As observed in rostral view, the lacrimal is oriented dorsomedially—ventrolaterally (i.e., its dorsal end is positioned slightly more medially than its ventral end in the articulated skull). The lacrimal is expanded rostroventrally at its contact with the jugal. Caudodorsally, the lacrimal articulates with the ascending ramus of the maxilla, the prefrontal, and the nasal. Its dorsal end possesses a very subtle rostral process that is comparable to those of Giraffatitan and Nemegtosaurus but much less developed than in Abydosaurus, Rapetosaurus, Tapuiasaurus, and especially Bonitasaura. The maxilla—lacrimal articulation is complex in that the ascending ramus of the maxilla is embedded in and bordered laterally and medially by the dorsal process of the lacrimal, a feature that we provisionally regard as an autapomorphy of Sarmientosaurus, though a similar or the same morphology may also be present in Tapuiasaurus (see Zaher et al. [14]:fig. 1c). Unlike in most macronarians (e.g., Abydosaurus, Camarasaurus, Giraffatitan, Nemegtosaurus, Rapetosaurus, and as reconstructed for Bonitasaura), the dorsal terminus of the lacrimal is not well exposed in lateral view due to a contact between the maxilla and prefrontal. The lateral surface of the lacrimal is relatively smooth compared to those of the premaxilla and maxilla.

Prefrontal.

The prefrontal of Sarmientosaurus (Figs 3–7; S1 Fig; S1, S2, S3 and S4 Movies) is crescentic and rostrocaudally elongate in dorsal view. Its dorsal surface is convex and its ventral surface is smoothly concave. The prefrontal articulates with the maxilla and lacrimal rostrally and rostromedially, the nasal caudomedially, and the frontal caudally. The ventral surface forms the rostrodorsal margin of the orbit. Both prefrontals are well preserved, and their lateral surfaces are somewhat rugose, as in Rinconsaurus [45], though not nearly as much so as in Nemegtosaurus [11]. The prefrontal has an elongate rostral process, a feature that is absent in Abydosaurus, Camarasaurus, and Giraffatitan but shared with Nemegtosaurus, Rapetosaurus, and Tapuiasaurus; Zaher et al. [14] considered this a feature of nemegtosaurid titanosaurs. In Sarmientosaurus, the rostral process is triangular in dorsal view, with the concave medial margin articulating with the nasal and maxilla and the convex lateral edge forming part of the rostrodorsal sector of the orbit. The prefrontal is dorsoventrally thick laterally and becomes even thicker medially.

Frontal.

Both frontals (Figs 3–7; S1 Fig; S1, S2, S3 and S4 Movies) have suffered strong mediolateral deformation. This, coupled with the presence of cracks and rugosities on their dorsal surfaces, precludes us from determining whether these bones are coossified or simply firmly sutured. As our CT data do not provide evidence to resolve this matter, we will describe both frontals as a single unit. Together, these bones comprise a transversely wide surface that extends between the orbits but that is much shorter in rostrocaudal dimension (transverse width to rostrocaudal length ratio equals 3.5 to 1). The frontals are bordered by the nasals and prefrontals rostrally, the parietals caudally, and the postorbitals laterally, and they also contribute to the dorsal margins of the orbits. Unlike in Bonitasaura [24], Nemegtosaurus and Quaesitosaurus [11], and Rapetosaurus [13], the orbital margin of the frontal of Sarmientosaurus is smooth, not ornamented. The frontal also appears to lack the rostrolateral process present in Ampelosaurus [18,19], the median ‘dome’ of several titanosaurs (e.g., Antarctosaurus, Bonatitan, Bonitasaura, Rapetosaurus, Saltasaurus, the isolated braincase MGPIFD-GR 118), and the more lateral dorsal prominences of Bonitasaura and Saltasaurus [24]. The rostral and caudal sutures of the frontals are poorly preserved; as such, those indicated in Figs 3–5 are probable but not definitive. Nevertheless, the frontals appear to be proportionally rostrocaudally shorter than those of some other titanosauriforms (e.g., Ampelosaurus, Phuwiangosaurus, Saltasaurus). Ventrally there is a gap, bounded laterally by the orbitosphenoids, where the aperture for the olfactory tracts opens.

Parietal.

The boundaries of the parietal (Figs 4–6; S1 Fig; S1, S2, S3 and S4 Movies) are difficult to establish due to the extreme fusion and deformation suffered by part of the caudal region of the skull. CT data show that the central interparietal/interfrontal zone forms a single triangular surface, the truncated apex of which arises from the parietal—supraoccipital contact and the base from the nasofrontal suture. Detailed examination of the cracked dorsal surface of the skull roof allows the identification of a probable frontoparietal suture, which suggests that the parietal contributes to the supratemporal fenestra. In contrast to the conditions in most other macronarians for which this region of the skull is known (e.g., Ampelosaurus, Antarctosaurus, Bonatitan, Bonitasaura, Camarasaurus, Isisaurus, Jainosaurus, Malawisaurus, Nemegtosaurus, the isolated titanosaur braincases FGGUB 1007, MGPIFD-GR 118, and MML-194), the supratemporal fenestrae of Sarmientosaurus are separated by only a short distance, as in Europasaurus, Rapetosaurus, and especially Giraffatitan and USNM 5730 (a partial skull referred to Brachiosaurus [112]). Unlike in many of these forms (e.g., Ampelosaurus, Antarctosaurus, Bonatitan, Bonitasaura, Jainosaurus, Nemegtosaurus, MGPIFD-GR 118), the long axes of these fenestrae are oriented approximately perpendicular to the sagittal plane instead of being aligned rostromedially—caudolaterally. In these regards, Sarmientosaurus is intermediate between brachiosaurids (i.e., Giraffatitan, USNM 5730) and derived lithostrotians. This compression and reorientation of the supratemporal fenestrae coincides with the lateral reorientation of the orbits in titanosaurs, both of which presumably evolved in response to the expansion of the nasal vestibule.

There is no visible suture between the parietals, but together they have a wing-like contour comparable to that observed in Camarasaurus and proportionally wider than those of Giraffatitan, Nemegtosaurus, and Rapetosaurus. The parietal contacts the postorbital laterally, the supraoccipital caudoventrally, and the otoccipital more ventrally. There is no parietal foramen, in contrast to the condition in sauropods such as Shunosaurus [113] and some diplodocoids (see [114]). The parietal of Sarmientosaurus lacks the bizarre dorsal excrescences of the isolated Transylvanian braincase FGGUB 1007 [52].

Postorbital.

The postorbital (Figs 3–7; S1 Fig; S1, S2, S3 and S4 Movies of Sarmientosaurus has the form of a caudally-reclined ‘T.’ Its thick, convex caudodorsal ramus contributes to the dorsal margins of the orbit and infratemporal fenestra. The longer ventral ramus is rostroventrally directed to contact the dorsal process of the jugal, and it forms most of the boundary between the infratemporal fenestra and orbit. Both postorbitals are well-preserved, though they have lost contact with their respective jugals; furthermore, the end of the ventral ramus of the left postorbital has rotated laterally. The lateral surface of the postorbital is fairly smooth. The ventral ramus is thickened at its rostroventral end that articulates with the jugal. Its flattened rostral surface is slightly concave in lateral view, whereas its caudolateral side is convex. The jugal articular surface of the postorbital is oriented caudomedially. The caudodorsal ramus is an expansion in the form of a convex anvil. It is pierced by a small but well-defined, caudolaterally-opening vascular foramen near the dorsal margin of the infratemporal fenestra and the dorsal base of the ventral ramus. The same or a very similar postorbital foramen occurs in the North American titanosauriform Abydosaurus (see Chure et al. [98]:figs. 3b, 4b; D’Emic [115], appendix 4); this structure has so far been documented only in that taxon and Sarmientosaurus, though it may occur in other sauropods (L.M.W., pers. obs.). There is no evidence of the ornamentation of the orbital margin of the caudodorsal ramus that occurs in Nemegtosaurus and Quaesitosaurus [10,11,43]. As best observed in rostromedial view, there are two small, probably vascular foramina situated close together at the rostroventral end of the right postorbital. Their presence on the left postorbital cannot be verified due to damage. The postorbital of Phuwiangosaurus differs from that of Sarmientosaurus in having a dorsoventrally thinner caudodorsal ramus, the rostral projection of which is much longer than its caudal counterpart (see Suteethorn et al. [41]:fig. 6). The rostral projection also appears much longer than the caudal projection in the recently described Argentinean titanosaur Quetecsaurus [44].

Jugal.

Both jugals (Figs 3 and 5–7; S1 Fig; S1, S2, S3 and S4 Movies) are well preserved, but their surfaces exhibit small cracks caused by erosion. The jugal of Sarmientosaurus is a roughly ‘L’-shaped bone that is rostrocaudally longer and differently shaped than the jugals of most other macronarians, including Camarasaurus, Euhelopus, Giraffatitan, Malawisaurus, Nemegtosaurus, and Rapetosaurus. The jugals of Tapuiasaurus [14] and the taxon represented by the Auca Mahuevo embryos [17] are similarly elongate but very different in shape, being tetraradiate rather than triradiate and comparatively dorsoventrally thick. Within Titanosauriformes, the jugal of Sarmientosaurus most closely resembles that of Abydosaurus, though the caudoventral ramus is sharply pointed and considerably longer in the latter taxon (see Chure et al. [98]:fig. 3a, b). In Sarmientosaurus, the subvertical dorsal ramus of the jugal contacts the postorbital and separates the ventral ends of the orbit and infratemporal fenestra. The rostroventral ramus is rostrodorsally projected and has a sigmoid contact with the caudal end of the maxilla. Ventrally, near its caudal end, the rostroventral ramus makes a slight contribution to the large lateral embayment caudal to the tooth row. The jugal also forms the ventral margin of the orbit and the caudoventral corner of the antorbital fenestra, and contacts the lacrimal rostrodorsally. The dorsal and rostroventral jugal rami meet at a nearly right angle. The rostroventral ramus is laminar throughout its extent, and is dorsoventrally expanded at its contact with the lacrimal. More caudally, between the orbit and the caudalmost sector of the maxilla, the rostroventral ramus of the jugal narrows and intersects the dorsal ramus. The caudoventral ramus of the jugal is much shorter than the rostroventral ramus. The jugal has been disarticulated from the postorbital on both sides of the skull; specifically, the articular end of the right postorbital is free and has been displaced laterally relative to the dorsal articular end of the jugal, and part of the postorbital overlaps the jugal on the left side. The articular surface of the right postorbital is rostromedially oriented and longitudinally twisted, suggesting that rostrocaudal pressures suffered by the skull during diagenesis have caused the bilateral displacement between the postorbitals and jugals.

Squamosal.

The squamosal (Figs 3, 4 and 6; S1 Fig; S1, S2, S3 and S4 Movies) is a rostrocaudally elongate and dorsoventrally oriented bone. Along with the quadratojugal, it forms most of the caudoventral margin of the infratemporal fenestra. Whether or not the squamosal participates in the supratemporal fenestra is not totally clear, although it probably does not given the seemingly considerable distance between these structures. The squamosal is excluded from the supratemporal fenestra in Phuwiangosaurus [41] and the derived titanosaurs Nemegtosaurus, Quaesitosaurus, and Tapuiasaurus. In Sarmientosaurus, the squamosal articulates with the quadratojugal ventrally, the postorbital caudodorsally, and the quadrate medially. Its sutural contacts are not clearly delimited, despite the fact that the right squamosal is fairly well preserved; the left is damaged laterally and caudally. The lateral surface of the right squamosal is fractured and cracked. The main body of the squamosal is flexed, forming a rostromedially-oriented convexity near its contact with the postorbital. This convexity divides two regions: a long rostrolateral sector that is wide and laminar at its contact with the quadratojugal, and a shorter caudomedial sector that has a concave surface and that is bordered medially by the quadrate. The squamosal is slightly sigmoid in lateral view.

Quadratojugal.

The right quadratojugal (Figs 3–6; S1 Fig; S1, S2, S3 and S4 Movies) is preserved. It is an ‘L’-shaped bone with a dorsal process that is angled slightly caudally and that is shorter than the ventral process; the latter is directed rostrodorsally to contact the jugal. The quadratojugal forms the rostroventral border of the infratemporal fenestra. It articulates with the jugal rostrodorsally, the squamosal caudodorsally, and the quadrate medially. Its contact with the squamosal is difficult to define due to fracturing of the region in question, although that contact clearly occurs in a rostromedial plane. The rostral section of the ventral ramus that contacts the jugal is slightly cracked. A small bone fragment adhered to the lateral surface of the caudodorsal part of the coronoid eminence of the right mandibular ramus appears to be the rostralmost tip of the ventral ramus of the right quadratojugal. In contrast to Nemegtosaurus [10,11], Quaesitosaurus [43], and especially Tapuiasaurus [14], the ventral edge of the ventral ramus is uniformly convex rather than sinuous in lateral view, and its rostral end is not ventrally expanded. In Sarmientosaurus, there may have been some rostral displacement of the quadratojugal relative to the jugal, but if so, it does not appear to have been significant. As observed in ventral view, the ventral process of the quadratojugal is projected dorsomedially, forming a concave medial surface. In caudal view, the subvertical dorsal ramus of the ‘L’ comprises the lateral border of the quadrate fossa; ventrally, this same margin is caudomedially projected as a ‘tongue-like’ process that caudally overlaps the quadrate. In Tapuiasaurus, by contrast, this process is not present, but the quadratojugal of that Brazilian titanosaur does possess a well-developed, acute caudoventral flange that does not occur in Sarmientosaurus [14]. The tongue-like process appears to be absent in other macronarians (e.g., Camarasaurus, Giraffatitan, Nemegtosaurus, Quaesitosaurus) as well; as such, we regard it as an autapomorphy of Sarmientosaurus.

Palatine.

Although both palatines are incomplete, the right is better preserved than the left (Figs 3, 5, 6 and 8; S1 Fig; S1, S2, S3 and S4 Movies). The right palatine preserves part of the lateral region, primarily the elongate, rostrolaterally-directed maxillary process. The entire medial section of the bone where it articulates with the pterygoid has been lost, whereas the left palatine preserves most of the pterygoid contact. The body of the maxillary process is dorsomedially inclined and roughly tubular. Its rostrolateral contact with the ectopterygoid is preserved, as is its more caudolateral contact with the rostral end of the pterygoid, although all of these bones are somewhat disarticulated. The rostral end of the maxillary process is fractured into pieces on both sides, but enough is preserved to suggest that its contact with the palatal process of the maxilla is typical for sauropods in that the palatine underlaps the maxilla ventrally. Likewise, the arrangement of the maxillary process of the palatine, the ectopterygoid, and the pterygoid around the suborbital (= postpalatine) fenestra also resembles that of other sauropods in that this fenestra is small and bounded rostrally by the palatine, caudally by the ectopterygoid (with the pterygoid nearby), and laterally by the palatal process of the maxilla. The maxillary process of the palatine narrows and expands again caudomedially as it approaches the pterygoid. As shown on the left side, although somewhat damaged and disarticulated, the pterygoid contact of the palatine is expanded and articulates in the fork between the medial vomerine ramus and the lateral transverse ramus of the pterygoid, as is common in sauropods [95,96,103].

Pterygoid.

The pterygoid (Figs 3, 5, 6 and 8; S1 Fig; S1, S2, S3 and S4 Movies) is the largest bone of the palatal complex. Neither pterygoid is complete, but enough is preserved of both to offer a reasonably comprehensive description. In general, the pterygoid of Sarmientosaurus is typical for sauropods in that the bone has a complex shape, with three main processes—the quadrate, vomerine, and transverse rami—arising from the highly arched body. The pterygoid body, which is better preserved on the left side, is expansive, forming a distinct ventral fossa (the postchoanal fossa) that faces rostromedially. The quadrate ramus, also better preserved on the left pterygoid, branches off the caudolateral corner of the body and twists into a more vertical plane as it attaches to the medial aspect of the reciprocal pterygoid ramus of the quadrate. The vomerine ramus passes dorsomedially, contacting its counterpart at the midline to form a thin triangular wedge that approaches the roof of the nasal cavity, where it nearly contacts the subnarial processes of the premaxilla and maxilla. Rostrally, the vomerine rami pass medial to the paired vomers. Near the juncture of the vomerine and quadrate rami, the body of the pterygoid forms a shallow but distinct, caudomedially facing pocket for the articulation of the basipterygoid process.

The transverse ramus of the pterygoid is preserved on both sides, but is better preserved on the right side. As is true for most sauropods, the transverse ramus is slender and swept far rostrally, carrying the ectopterygoid with it. The lateral end of the transverse ramus is slender and curves ventrally. The ectopterygoid attaches broadly to its rostral surface, just caudal to the suborbital fenestra. Together, these two bones form the ‘pterygoid flange,’ which is a strong, transverse projection in many other archosaurs, but is a relatively delicate structure in Sarmientosaurus and most other sauropods. As noted above, the palatine articulates with the body of the pterygoid rostrally, between the vomerine and transverse rami of the latter.

Vomer.

The vomers (Figs 5, 6 and 8; S1 Fig; S1, S2, S3 and S4 Movies) are delicate, paired bones that are damaged and best observed in the CT images. They are thin laminae of bone that attach to the lateral and ventral portions of the vomerine rami of the pterygoids. As noted above, along with the pterygoids, they likely contacted the ventral surfaces of the subnarial processes of the premaxillae and maxillae. This is also the case in Camarasaurus and Diplodocus, and undoubtedly in other sauropods as well, though this part of the cranium is not well understood in most taxa.

Ectopterygoid.

The ectopterygoids (Figs 5, 6 and 8; S1 Fig; S1, S2, S3 and S4 Movies) are essentially complete but remain largely embedded in matrix. Of the two, the right is the more clearly visible. The ectopterygoid is a relatively simple, slender bone that forms the caudal border of the suborbital fenestra. Its lateral end contacts the medial surface of the maxilla. The rostral end of the ectopterygoid passes ventrally and curves to articulate on the rostral face of the transverse ramus of the pterygoid, such that they collectively form the ‘pterygoid flange,’ as noted above. The lateral, medial, and ventromedial surfaces of the ectopterygoid are smooth. The ectopterygoid of Sarmientosaurus is much shorter rostrocaudally than the bones that Wilson [11] identified as ectopterygoids in Nemegtosaurus and Quaesitosaurus.

Quadrate.

The right quadrate is, in most regions, better preserved than the left (Figs 3–6 and 8; S1 Fig; S1, S2, S3 and S4 Movies). It articulates with the pterygoid rostromedially, the squamosal caudodorsally, the quadratojugal laterally, and the articular ventrally, forming the jaw joint. Although the rostroventral region of both quadrates is damaged, both preserve at least part of the articulation with the pterygoid, which is better preserved on the left side. On the right quadrate, caudally, the edge of the vertical lamina that articulates laterally with the quadratojugal and ventrally with the articular is eroded. The right quadrate is fairly smooth in areas where its surface is intact. Its long axis is oriented caudodorsally relative to that of the skull. The quadrate is obscured from rostral view by the other bones it contacts. It is best observed in caudal view, where the damaged head that articulates with the squamosal rostrolaterally may be seen at its dorsal end. Other structures of the right quadrate evident in caudal view include the subvertical and plate-like medial edge, the thick expansion that terminates ventrally in the mandibular articulation, and the quadrate fossa, which is delimited medially by the quadrate and laterally by the quadratojugal. The quadrate fossa of Sarmientosaurus faces caudolaterally, as in Nemegtosaurus, Quaesitosaurus, and Rapetosaurus, though it is apparently not as laterally oriented as it is in these taxa. The quadrate fossa is comparable in shape to those of most other titanosaurs (e.g., Narambuenatitan, Nemegtosaurus, Quaesitosaurus, Rapetosaurus), but is seemingly more dorsoventrally elongate than in Phuwiangosaurus [41] and especially Malawisaurus; in the latter African titanosaur, the quadrate fossa appears nearly as wide mediolaterally as it is tall (see Gomani [32]:fig. 5d). The ventral extreme of the right quadrate preserves part of the medial condyle, which is mediolaterally wider than that of Narambuenatitan (see Filippi et al. [40]:fig. 5a). The pterygoid ramus of the right quadrate and its damaged contact with the pterygoid are also evident in caudal view. The pterygoid ramus is mediolaterally thick. The quadrate—squamosal articulation is laterally expanded. The part of the quadrate that is situated more caudally than the squamosal is visible in lateral view. According to Zaher et al. [14], caudolateral exposure of the quadrate is a feature of nemegtosaurids; its occurrence in Sarmientosaurus broadens the distribution of this character to include more basal titanosaurians as well. In overall morphology, the quadrate of the new Patagonian taxon is similar to those of Quaesitosaurus and Rapetosaurus.

Supraoccipital.

The supraoccipital (Figs 4–6; S1 Fig; S1, S2, S3 and S4 Movies) is a massive, subhexagonal element that is exposed only in caudal, dorsal, and ventral views. It exhibits a prominent sagittal nuchal crest that, in caudoventral view, resembles a large spool flanked by two wing-like sectors. The supraoccipital is bordered by the parietals rostrodorsally and laterally and the otoccipitals ventrally, forming the dorsal margin of the foramen magnum. The surface of the supraoccipital is fairly smooth. The caudodorsal end of the sagittal nuchal crest displays a marked bony flange that is damaged. There is a smooth triangular depression in the caudoventral region of the crest, the apex of which reaches half the dorsoventral height of the crest. The wings of the supraoccipital are markedly longitudinally concave and slightly elevated at their lateral contact with the parietals. The ventral contact with the otoccipital is aligned dorsolaterally—ventromedially. In caudal view, the supraoccipital of Sarmientosaurus resembles those of Camarasaurus and Giraffatitan in its general form and relationships to other bones. In Nemegtosaurus, the sagittal nuchal crest is much smaller and shaped differently, but the wing-like sectors are similar to those of the new Patagonian taxon. The protuberance at the dorsal end of the sagittal nuchal crest (= the ‘supraoccipital protuberance’ of García et al. [59] and Filippi et al. [40], among others) of Antarctosaurus, Bonatitan, Jainosaurus, Malawisaurus, Narambuenatitan, Rapetosaurus, and the isolated Patagonian titanosaur braincases MML-194 and MUCPv-334 is prominent and mound-like in caudal view. In Sarmientosaurus and Isisaurus [27], conversely, this protuberance appears more quadrangular, and in Ampelosaurus it is only weakly developed [18,19]. The supraoccipital protuberance of Phuwiangosaurus seems unusually narrow transversely [41]. The supraoccipitals of Bonatitan, Rapetosaurus, MML-194, and probably Muyelensaurus (see Calvo et al. [39]:fig. 4) have a midline groove on their caudal aspect, a structure that is absent in other titanosaurs.

Otoccipital (= exoccipital—opisthotic).

The exoccipital and opisthotic (Figs 4–6; S1 Fig; S1, S2, S3 and S4 Movies) are fused, as in other sauropods [96,113,116] and indeed in most other diapsids, such that this compound element is often termed the otoccipital. The surface of this complex is marked by small cracks and grooves that obscure its margins in some areas, although the CT images provide clear information in this respect. The surface of the caudal section is pitted by erosion. The otoccipitals are massive, and extend as paired wing-like, mediolaterally elongate and gently arched paroccipital processes laterally flanking the foramen magnum, which is circumscribed by a marked ridge. This lateral elongation is comparable to that present in many titanosauriforms (e.g., Ampelosaurus, Bonatitan, Giraffatitan, Jainosaurus, Malawisaurus, Muyelensaurus, Narambuenatitan, Quaesitosaurus, Rapetosaurus), but differs from the conditions in Antarctosaurus, Nemegtosaurus, and Saltasaurus, in which the ventral margins of the medial bases of the paroccipital processes angle more steeply ventrolaterally. The otoccipital is bordered by the laterosphenoid rostromedially, the prootic rostroventrally, the supraoccipital dorsally, the parietals dorsolaterally, and the basioccipital ventrally. The opisthotic portion of the bone forms a smooth concavity approaching the prootic; the contact between these bones on the right side of the skull is a sinuous, dorsoventrally-oriented suture. A prominent feature of the otoccipital is the metotic fissure (= vagal, jugular foramen), which is semilunar in contour and constitutes the largest opening on the lateral wall of the braincase. The glossopharyngeal, vagus, and accessory nerves (cranial nerves IX–XI) all would have traversed the metotic fissure. The fenestra cochleae (round window of the inner ear; see below) opens into the metotic fissure about halfway along the length of the latter. The fenestra vestibuli (oval window of the inner ear) is just rostrodorsal to the metotic fissure, between the prootic and the opisthotic portion of the otoccipital.

Dorsal to the metotic fissure, the otosphenoidal crest (= crista prootica) is present as a sharp ridge. The ventralmost edge of the wing of the otoccipital caudally defines a semilunar recess on either side of the foramen magnum. The foramen magnum is ovate and taller than wide, as in many macronarians (e.g., Antarctosaurus, Bonatitan, Camarasaurus, Giraffatitan, Jainosaurus, Nemegtosaurus, Pitekunsaurus, Quaesitosaurus, Vahiny), but not Malawisaurus, Muyelensaurus, Narambuenatitan, Rapetosaurus, and Saltasaurus, in which these dimensions are subequal, and Ampelosaurus, in which the foramen magnum is subtriangular and slightly wider than tall [18,19]. On both sides of the foramen magnum there are two openings that transmitted branches of cranial nerve XII (the hypoglossal nerve), a condition that also occurs in more basal sauropods such as Spinophorosaurus [117], Camarasaurus [118], and Giraffatitan [95,119]. Only one hypoglossal foramen is reported in most titanosaurs, such as Ampelosaurus sp. [19], Bonatitan [20,22], Rapetosaurus, MGPIFD-GR 118 [58], and MCF-PVPH-765 [60], and the isolated titanosauriform braincase TMM 40435 [120], although two hypoglossal foramina per side are present in CCMGE 628/12457, an isolated titanosaur braincase from the Upper Cretaceous of Uzbekistan [63]. No proatlantal facets are evident lateral to the foramen magnum of Sarmientosaurus, in contrast to the condition in two braincases referred to Isisaurus [27,29] and a generically indeterminate titanosaur braincase from the Maastrichtian of France [50].

Prootic.

The left prootic is better preserved than the right (Fig 6; S1 Fig; S1, S2, S3 and S4 Movies). As noted by Madsen et al. [96], the prootic is one of the most difficult cranial bones to study in sauropods—especially in an articulated skull—due to its location in the braincase and the difficulty in defining its boundaries with surrounding bones. The prootic of Sarmientosaurus is laterally positioned in the braincase. Caudally, it borders the supraoccipital and the opisthotic portion of the otoccipital, but the suture with the former is not visible. A foramen for the facial nerve can be seen on both sides, at the approximate midpoint of the otosphenoidal crest. Medially, dorsal to the metotic fissure, there is a groove for the maxillary branch of the trigeminal nerve that courses rostrocaudally through a large foramen. Immediately caudal to the maxillary nerve foramen is the foramen for the mandibular branch of the trigeminal nerve. Medially, the trigeminal nerve foramen is a single aperture between the prootic and laterosphenoid, but laterally it branches such that maxillary and mandibular nerves emerge through separate foramina. Moreover, the ophthalmic branch of the trigeminal nerve also diverges within the braincase wall to emerge within the laterosphenoid. Thus, Sarmientosaurus is presently unique within Sauropoda in having separate foramina, bilaterally, for all three branches of the trigeminal nerve, as is discussed further in the “Cranial Endocast” section below. The prootic articulates with the basisphenoid and basioccipital ventrally and the parietal dorsally, but its precise margins cannot be determined in these areas. The rostrodorsal surface of the prootic is the best exposed; it is pockmarked and traversed by cracks. The rostral section of the bone is laminar and relatively wide. The otosphenoidal crest caudolaterally defines a recess which lies dorsal to the metotic fissure of the otoccipital.

Laterosphenoid—orbitosphenoid.

The suture between the laterosphenoid and orbitosphenoid (Figs 3, 4 and 6; S1 Fig; S1, S2, S3 and S4 Movies) is not easily visible, as is often the case in sauropods [96], but is marked by a line of foramina (see below), as in most sauropods [118] and indeed most other dinosaurs [121]. The laterosphenoid is situated caudolateral to the orbitosphenoid, and both bones contact the frontal dorsally. The laterosphenoid is bordered by the otoccipital caudally and the prootic caudally and ventrally, whereas the orbitosphenoid contacts the basisphenoid rostroventrally. In rostral view, the laterosphenoid forms a slightly concave, wing-shaped lamina, and its caudolateral margin comprises the crista antotica. Rostromedially, each laterosphenoid is continuous with its corresponding orbitosphenoid. The rostromedial union of the paired orbitosphenoids forms the ventrally sharp, somewhat sigmoid convergent rostral end of the braincase. The slightly curved section of the rostral end of the conjoined orbitosphenoids accommodates the rostral face of the dorsal end of the hypophyseal fossa. Dorsally, this rostral surface of the orbitosphenoid gradually expands to form the aperture for the olfactory tract (although this area is covered by sediment, the structures are visible in the CT data; see “Cranial Endocast” section below). Near the rostral vertex of the orbitosphenoids, the foramen for the optic nerve (cranial nerve II) is visible in rostral and lateral views. As noted above, the suture between the orbitosphenoid and laterosphenoid is marked by a row of foramina, which transmitted, from dorsal to ventral, the orbitocerebral vein, trochlear nerve, oculomotor nerve, and abducens nerve (see “Cranial Endocast” section below). Caudally, at the height of the oculomotor nerve foramen and at the contact between the laterosphenoid and prootic, there are a series of openings that were described above (with the prootic) that pertain to the three branches of the trigeminal nerve; the margins of these foramina are poorly defined due to erosion, but the canals are clear and present on both sides in the CT data. Dorsal to the trigeminal nerve foramen, and in approximately the same plane as the trochlear nerve foramen, there is another foramen that corresponds to the canal for the transversotrigeminal (rostral middle cerebral) vein.

Basicranium.

The basicranium (Figs 4–6 and 8; S1 Fig; S1, S2, S3 and S4 Movies) forms the floor of the braincase, and consists of the coossified basioccipital, basisphenoid, and parasphenoid. Rostrally, the basioccipital articulates with the otoccipital and is fused with the basisphenoid. The neck of the occipital condyle has been preserved, and it suggests that the condyle was dorsoventrally flattened and roughly trapezoidal in section. The condyle appears to have been substantially wider than the foramen magnum, as in many macronarians (e.g., Antarctosaurus, Camarasaurus, Isisaurus, Jainosaurus, Lirainosaurus, Muyelensaurus, Narambuenatitan, Nemegtosaurus, Phuwiangosaurus, Quaesitosaurus, Tambatitanis, Vahiny, MGPIFD-GR 118); this condition is especially pronounced in Giraffatitan. In several titanosaurs (e.g., Ampelosaurus, Bonatitan, Malawisaurus, Pitekunsaurus, Rapetosaurus, MML-194), by contrast, the occipital condyle and foramen magnum appear subequal in width. When the skull roof of Sarmientosaurus was held horizontally, the occipital condyle projected caudoventrally, which is consistent with evidence from the inner ear (see “Cranial Endocast” section below) that suggests that the alert posture of the head was with the snout pointing strongly downward (see below). A similarly downward orientation of the head has also been reconstructed for some diplodocoids [118,122,123].

The basal tubera of the basioccipital are thick, as in Bonatitan, Camarasaurus, Giraffatitan, Rapetosaurus, the isolated titanosaurian braincase MML-194 [59], and most other macronarians, in sharp contrast to their rostrocaudally thin, sheet-like morphology in Saltasaurus [7,46]. They lack the peculiar ‘notch’ and pendant lateral process present in Jainosaurus [28,30] and Vahiny [48], and the foramen found in Lirainosaurus [35]. The condylar region is linked to the basal tubera by two thick, low, ventrolaterally-oriented ridges that delimit a deep, roughly quadrangular space that García et al. [59] termed the subcondylar recess. A similar condition appears to occur in Malawisaurus (Gomani [32]:fig. 6a), Muyelensaurus (Calvo et al. [39]:fig. 4), and the Uzbekistan titanosaur CCMGE 628/12457 [63]. Conversely, the subcondylar recess is shallow or absent in Jainosaurus, Lirainosaurus, Mongolosaurus [38], Saltasaurus, Tambatitanis, Vahiny, and MUCPv-334 [57]. No small pits or foramina are evident between the basal tubera of Sarmientosaurus, in contrast to the conditions in Mongolosaurus, Nemegtosaurus, Phuwiangosaurus, Quaesitosaurus, Rapetosaurus, Saltasaurus, Tambatitanis, and MUCPv-334. The basal tubera are united rostrally by a thinner, curved lamina, with a dorsally-directed concavity defining the hypophyseal fossa as a dome-shaped pit. Unlike those of Rapetosaurus, the basal tubera are not separated by a deep, V-shaped notch [13]. They diverge at the midline at an angle of approximately 35°, as in Giraffatitan, Bonatitan, MML-194, and other titanosauriforms. As in other lithostrotians [38,115,124], their combined width is clearly much greater than (approximately twice) that of the occipital condyle, although only the neck of the latter is preserved. In contrast to Nemegtosaurus, Rapetosaurus, and Tapuiasaurus, however, the basal tubera of Sarmientosaurus do not contact the quadrates. In overall morphology, they seem to most closely resemble those of the slightly younger northern Patagonian titanosaur Muyelensaurus (see Calvo et al. [39]:fig. 4).

The basipterygoid processes are rostroventrally projected, proportionally slightly mediolaterally expanded, and subcylindrical in cross section. Their bases are positioned well rostral to the basal tubera, and are ‘set off’ from these latter structures by the well-developed lamina that connects the tubera. The canals for the cerebral carotid arteries enter the braincase at the base of the basipterygoid process, just rostral to the basal tuber. The basipterygoid processes are subparallel, as in Malawisaurus [32], Muyelensaurus [39], Nemegtosaurus, Phuwiangosaurus [41], Pitekunsaurus [42], Quaesitosaurus [11], Rapetosaurus, Tambatitanis [47], and probably MUCPv-334 [57], unlike the conditions in Antarctosaurus, Bonatitan, Camarasaurus, Giraffatitan, Jainosaurus, and probably Vahiny, in which these processes diverge more widely. The dorsal bases of the basipterygoid processes of Sarmientosaurus are closely spaced, separated only by a distance approximately equal in transverse width to one of these processes, as in Lirainosaurus [35], Malawisaurus, Mongolosaurus [37,38], Phuwiangosaurus, Rapetosaurus, and probably MUCPv-334. In Antarctosaurus, Camarasaurus, Giraffatitan, Muyelensaurus, Narambuenatitan, Pitekunsaurus, Tambatitanis, Vahiny, and MML-194, by contrast, the bases of these processes are placed further apart. The basipterygoid processes of Sarmientosaurus are much longer than their strikingly short counterparts in ISI R 467, a titanosaur braincase from India (see Chatterjee and Rudra [28]:fig. 12) that may be referable to Isisaurus [29], and much narrower than the unusually broad, plate-like basipterygoid processes of Saltasaurus [7,46]. Unlike in Rapetosaurus and Tapuiasaurus, there is no midline sagittal crest between the basipterygoid processes of Sarmientosaurus.

As in most diapsids, the parasphenoid of Sarmientosaurus is completely fused to the basisphenoid. The cultriform process of the parasphenoid is damaged and largely obscured by matrix but is present as a vertical median plate of bone located rostral to the pituitary fossa. There is an elongate ventral recess between the two basipterygoid processes at the base of the cultriform process.

Cranial nerves and vasculature.

The cranial nerves (CN) and vascular canals are clearly visible and typical of many sauropods [118,134] (Fig 10). As with most fossil sauropsid endocasts, traces of the olfactory nerves (CN I) themselves are not preserved. The optic nerve (CN II) enters the endocast just dorsal to the infundibular region of the pituitary. The oculomotor (CN III) and trochlear (CN IV) nerves exit through separate foramina in the infundibular region at the juncture of the orbitosphenoid and laterosphenoid, ventral to the foramen for the orbitocerebral vein. Earlier works [131,135–137] had regarded the largest and most dorsal of the foramina located in the orbitosphenoid—laterosphenoid suture as transmitting the trochlear nerve, but Witmer et al. [118] reidentified this aperture as venous and named it the orbitocerebral vein foramen. The presence of all three foramina (oculomotor nerve, trochlear nerve, and orbitocerebral vein) on both sides of MDT-PV 2 lends further support to this hypothesis, given that all cranial nerves are accounted for in this specimen. The fact that the orbitocerebral vein opening leads to a known venous structure (the rostral dural expansion) confirms this identification. All three of these structures extend more laterally than rostrally, consistent with the trend of the orbits to migrate caudally in derived titanosaurians, presumably in connection with the caudal expansion of the narial region. The cerebral carotid arteries enter near the ventral limit of the pituitary fossa. A pair of sphenopalatine arteries exit halfway up the pituitary.

The trigeminal nerve (CN V) exits caudal to the infundibular region, ventral to the transverse sinus. Unlike any other described sauropod, the ophthalmic (CN V₁), maxillary (CN V₂), and mandibular (CN V₃) branches exit the braincase through separate foramina. Sarmientosaurus is unique within Sauropoda in having separate foramina for any of these branches, let alone all three (Fig 10). Moreover, we are interpreting a clearly discernible structure that arises from the left ophthalmic nerve as the supraorbital nerve, a branch rarely seen in any dinosaur group, although we cannot rule out the hypothesis that it is instead a canal for the trigeminal vein and artery. The abducens nerve (CN VI) exits the endocast at the pontine flexure and passes lateral to the pituitary fossa without penetrating it, an apomorphic condition diagnostic of titanosaurs [19,21,63]. This character is likely related to the retraction of the orbits in titanosaurs such that they face directly laterally, as reflected in the lateral orientation of the oculomotor and trochlear nerve canals and the abducens canal taking a more lateral course. As in other sauropsids, the facial nerve (CN VII) exits the endocranial cavity in common with two branches of the vestibulocochlear nerve (CN VIII), extending ventrolaterally. The more dorsal branch of the vestibulocochlear nerve enters the vestibular labyrinth on the medial side, ventral to the ampulla for the rostral semicircular canal, and likely received input from the pars vestibuli of the inner ear. The more ventral branch enters the cochlear fossa. The vagal (= jugular, metotic) foramen exhibits the typical reptilian conformation, likely transmitting the glossopharyngeal (CN IX), vagus (CN X), and spinal accessory (CN XI) nerves. As it passes the inner ear, it accepts the perilymphatic foramen (fenestra cochleae) from the cochlear fossa. Caudally, there are two rootlets for the hypoglossal nerves (CN XII).

Inner ear.

The inner ear of Sarmientosaurus (Fig 11) displays features both primitive and intermediate among sauropod evolution on the line to derived Titanosauria. As a whole, the vestibular labyrinth is intermediate in size between the disparately large labyrinth of Giraffatitan and the proportionally much smaller sizes seen in more advanced titanosaurs (Fig 12). The semicircular canals are more slender than those of more basal taxa. In particular, the lateral canal is remarkably long and slender in comparison to other sauropods, which may indicate increased sensitivity in the mediolateral plane. Vertebrate semicircular canals sense angular acceleration and turning movements of the head in connection with neural mechanisms to coordinate eye, head, and neck movements for gaze stabilization (see [118,129,132] and references therein). Thus, the elongate lateral canal of Sarmientosaurus (Fig 12G) may reflect behaviors that emphasized lateral scanning movements of the eyes and head. This stands in contrast to the interepretation of the dramatically short lateral semicircular canals of other sauropods, especially other titanosaurs, as suggesting a behavioral emphasis on dorsoventral movments of the head, which is consistent with inferred feeding behaviors [118]. While still the longest of the three canals, the rostral canal is reduced in length relative to the others, more so than in Giraffatitan, but not to the extent seen in some other titanosaurs in which the rostral and caudal canals tend to be subequal in length. Interestingly, the angle between the rostral and caudal canals averages 63°, whereas these canals tend to be more orthogonal in bracketing taxa (Fig 12; see also [19,21]). As a 90° angle (i.e., orthogonality) in Sarmientosaurus would cause the rostral canal to breach the endocast, perhaps the reduced angle is an intermediate step in response to the shifting of the orbit caudolaterally and the concomitant compression of the temporal region. The smaller labyrinth of more derived titanosaurs is more orthogonal, and it is possible that the relaxed packaging constraints of its small size allowed a return to orthogonality.

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Fig 11. Left endosseous inner ear labyrinth of Sarmientosaurus musacchioi gen. et sp. nov. (MDT-PV 2).

Computed tomography-based digital visualization in lateral (A), caudal (B), and dorsal (C) views. Abbreviations see text. Scale bar = 1 cm.

https://doi.org/10.1371/journal.pone.0151661.g011

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Fig 12. Comparison of left endosseous inner ear labyrinths of Sarmientosaurus musacchioi gen et sp. nov. (MDT-PV 2) and other saurischian dinosaurs in a phylogenetic context.

Images are computed tomography-based digital visualizations in dorsal (top row) and lateral (bottom row) views. (A) The basal theropod Herrerasaurus ischigualastensis (MCZ 7063). (B) The basal sauropodomorph Plateosaurus engelhardti (MB R.1937). (C) The basal eusauropod Spinophorosaurus nigerensis (GCP-CV-4229). (D) The derived diplodocoid Diplodocus longus (CM 3452). (E) The basal macronarian Camarasaurus lentus (CM 11338). (F) The basal titanosauriform Giraffatitan brancai (MB R.2180.22.1–4). (G) The basal lithostrotian titanosaur Sarmientosaurus musacchioi (MDT-PV 2). (H) An unnamed derived lithostrotian titanosaur from Uzbekistan (CCMGE 628/12457). (I) The derived lithostrotian titanosaur Jainosaurus septentrionalis (ISI R162). A, C, D, E, F, I modified from Knoll et al. [117]; H modified from Sues et al. [63]. Scale bar = 2 cm.

https://doi.org/10.1371/journal.pone.0151661.g012

The endolymphatic canal communicates with the endocranial cavity at the base of the common crus. The fenestra vestibuli (fenestra ovalis) and fenestra cochleae (fenestra pseudorotundum, perilymphaticum) are both displaced ventrally, as seen in other sauropods, leaving the possibility for a large utricular space relative to non-neosauropod outgroups. This finding suggests that sensing linear acceleration was important to these animals. The cochlea (= lagena of some other authors) is located ventral to the fenestra vestibuli and is angled more strongly rostrally than in most other sauropods. The cochlea is similar in relative length to those seen in Spinophorosaurus, Camarasaurus, and Giraffatitan, but much longer than the cochleae preserved in diplodocids and more advanced titanosaurs (Fig 12). Given that the cochlea houses the hearing organ (i.e., the neuroepithelium of the basilar papilla), the relatively elongate cochlea of Sarmientosaurus may suggest a greater reliance on airborne sounds and potentially lower frequency hearing than in other titanosaurs (see [118,129]). When the skull of Sarmientosaurus is aligned with the lateral canal to Earth horizontal in the alert posture [118,132], the skull becomes downturned roughly 46° relative to the tooth row, with the occipital condyle being nearly horizontal (Fig 13).

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Fig 13. Computed tomography-based digital visualizations of the skull of Sarmientosaurus musacchioi gen. et sp. nov. (MDT-PV 2) in the ‘alert’ posture.

(A) Skull rendered semitransparent in left lateral view, revealing the cranial endocast. (B) Skull rendered solid in left lateral view. (C) Skull rendered solid in left rostrolateral view. With the lateral semicircular canals oriented roughly with Earth horizontal, the skull is oriented with the tooth row 46° below horizontal. The eyeball is restored to provide a sense of the impact of the the strongly downturned ‘alert’ head posture on the visual field. Although the bony scleral ring was not preserved in Sarmientosaurus, its dimensions were based on scaling the complete ring of the titanosaur Nemegtosaurus mongoliensis (ZPAL MgD-I/9) and using known osteological correlates preserved n MDT-PV 2. Scale bar = 5 cm.

https://doi.org/10.1371/journal.pone.0151661.g013

Dentary.

The dentary (Figs 14–16; S1 Fig; S6, S7 and S8 Movies) is the largest bone of the mandible, and contacts the opposing dentary at the symphysis, the splenial and coronoid caudomedially, the surangular caudodorsally, and the angular caudoventrally. The dorsal and ventral edges of the dentary are subparallel. In Rapetosaurus and especially Malawisaurus, however, the dorsal margin of the dentary is straight but the ventral margin is concave in medial and lateral views. As noted above, the symphysis is subvertical and nearly perpendicular to the long axis of the mandibular ramus, and its contour is subrectangular. It is mediolaterally thick, more so than in Nemegtosaurus and the ventral half of the dentary of Brasilotitan (see Machado et al. [25]:fig. 2e). The caudal contact with the surangular is very broad and sinuous, and extends from the rostral base of the tall coronoid eminence to the contact with the angular along a rostrally convex path. The caudoventral contact with the angular is short and caudodorsally oriented. There is no indication of the ‘accessory process’ of the dentary caudoventral process that occurs in Abydosaurus and Giraffatitan [98]. On the medial surface, the Meckelian groove extends along the ventral edge of the dentary, with part of its caudal portion bordering the splenial. It is not possible to verify whether the groove reaches the symphysis as in Nemegtosaurus, but in its caudal course along the dentary it rises and curves gently. The suture of the dentary with the splenial is best viewed on the left mandibular ramus, where the latter bone is mostly missing. The lateral surface of the dentary exhibits vascular foramina that are smaller than those in the maxilla and less profuse than those in Nemegtosaurus. Nine dental foramina, which correspond to the first nine tooth positions, are visible on the medial surface of the right dentary, adjacent to the dorsal edge of the bone. The coronoid bone covers the dental foramina medially, at least as far rostrally as the seventh dentary tooth, as seen on the left side. The dental foramina rostral to the coronoid are damaged in the left dentary.

Each dentary contains 13 tooth positions, with the largest teeth being closest to the symphysis as in Ampelosaurus atacis (see Le Loeuff [18]:fig. 4.3c), Bonitasaura, Camarasaurus, Giraffatitan, Nemegtosaurus, and Rapetosaurus. The tooth row begins immediately caudal to the symphysis, and the portion of the dentary caudal to the tooth row does not display a sharp ridge (termed the ‘guillotine crest’ by Apesteguía [23] and the ‘posterior crest’ by Gallina and Apesteguía [24]) as in Bonitasaura, Brasilotitan, probably Antarctosaurus and Rapetosaurus [25], and possibly the little-known African titanosaur Karongasaurus (see Gomani [32]:27). In dorsal view, the dental series curves gently rostromedially, as in Euhelopus, but not as much as in Bonitasaura, Karongasaurus, Nemegtosaurus (see Nowiński [10]:pl. 14, fig. 1a), Rapetosaurus, and especially Antarctosaurus and Brasilotitan. This contrasts the straighter dorsal profile of Camarasaurus.

Surangular.

The surangular (Figs 14–16; S1 Fig; S6, S7 and S8 Movies) is much more completely preserved in the right mandibular ramus, although its contact with the dentary is better preserved on the left side. Its lateral aspect contacts the dentary rostrally and the angular ventrally. Its dorsal surface is slightly damaged on the right side but the remainder is well preserved. The central sector of the surangular is slightly laterally convex. The bone is very mediolaterally narrow in this region, which corresponds to the presence of the adductor fossa on its medial side. This fossa is filled with sediment but CT images reveal that it is rectangular in section with straight borders. The coronoid eminence is made up entirely by the surangular. The rostral surangular foramen is well preserved and present on both sides, with its proportional size and position most closely resembling the conditions in Camarasaurus [96,103] and Tapuiasaurus [14]. In Abydosaurus [98] and Giraffatitan [103], by contrast, the rostral surangular foramen appears to be placed slightly more dorsally, and in Nemegtosaurus [11] and Rapetosaurus [11,13] it is larger and more caudally situated. The caudal surangular foramen of Sarmientosaurus is preserved on the right side, in a caudal position similar to that in most macronarians (e.g., Camarasaurus, Giraffatitan, Nemegtosaurus), with the exception of Tapuiasaurus in which it is more rostrally placed. The medial surface of the surangular contacts the dentary rostrodorsally. Here, most of the surangular is occupied by the large adductor fossa, which housed mandibular adductor musculature and is shaped like a rostrocaudally elongate ellipse.

Angular.

The angular (Figs 14–16; S1 Fig; S6, S7 and S8 Movies) is exposed laterally and medially. Laterally, its rostral margin contacts the dentary via a straight, caudodorsally-angled suture, whereas its dorsal edge meets the surangular along a suture that is slightly dorsally convex in lateral view. As in the surangular, the lateral surface of the angular is slightly convex and marked by fine longitudinal grooves. The lateral exposure of the angular is more dorsoventrally extensive than in Camarasaurus, Nemegtosaurus, Quaesitosaurus, and especially Giraffatitan and Rapetosaurus, more closely resembling Abydosaurus and Tapuiasaurus in this regard. Unlike in this latter Brazilian titanosaur, the caudal half of the angular is not turned markedly ventrally (see Zaher et al. [14]:fig. 1a, b). Medially, the dorsal portion of the angular is sheathed by the splenial, such that, in medial view, the angular and prearticular are separated by the splenial throughout much of their lengths. The angular participates in the adductor fossa, especially internally, as revealed by the CT data, but it is excluded from the medial border of this fossa by the splenial.

Splenial.

The splenial (Figs 14–16; S1 Fig; S6, S7 and S8 Movies) is centrally located on the medial side of the mandible. As noted above, it is largely absent on the left mandibular ramus, and thus its description here is drawn from the right side. The splenial articulates with the dentary, coronoid, prearticular, and angular. Most of the edges of the bone are poorly defined and difficult to discern via gross external examination, but the CT data do allow its margins to be identified. The splenial contacts the dentary rostrally via an indistinct, generally dorsoventrally-arrayed suture. The dorsal contact of the splenial with the coronoid is also not clearly visible grossly, but it can be traced in the CT data. The splenial covers the rostral portion of the adductor fossa medially and overlaps the ventral half of the coronoid. The visible contact of the splenial with the prearticular is a nearly straight suture that is directed caudoventrally from the ventromedial border of the adductor fossa. The suture with the angular is also largely straight, and it is oriented nearly horizontally. The splenial covers the dorsal portion of the angular medially ventral to the adductor fossa and extends far caudally as a pointed splint between the angular and prearticular. The caudal termination of the Meckelian groove is in the ventral area of the splenial. The putative splenial of Malawisaurus exhibits some resemblance to that of Sarmientosaurus (see Gomani [32]:fig. 7d, e).

Prearticular—articular.

As revealed by CT data obtained from the right mandibular ramus, the prearticular and articular appear to be coossified (Figs 14–16; S1 Fig; S6, S7 and S8 Movies). The portion of this compound element that is comprised by the prearticular is laminar and subvertically oriented, with a wide ventral base and a dorsal convexity, the caudal end of which is slightly sigmoid. The prearticular borders the coronoid rostrodorsally, the splenial rostroventrally, and the angular caudoventrally. The medial surface of the prearticular is smooth, whereas the lateral surface is obscured by sediment. The caudal contact of this bone with the angular is the best preserved of all mandibular sutures in Sarmientosaurus, and extends obliquely caudoventrally. The dorsomedial surface of the fused articular is rugose, as in Camarasaurus [96] and Giraffatitan [95]. Although it is incomplete caudally, enough remains of the articular of Sarmientosaurus to suggest that it was rostrocaudally longer than that of Camarasaurus but shorter than that of Giraffatitan.

Coronoid.

Sarmientosaurus possesses a rostrocaudally elongate, dorsoventrally narrow, strap-like bone at the dorsal edge of the medial surface of the mandibular ramus, medial to the distalmost 6–7 teeth (Figs 14–16; S1 Fig; S6, S7 and S8 Movies). Madsen et al. [96] regarded this element as the intercoronoid, but we follow Wilson [11] in considering it to be the coronoid (see Wilson [11] for a discussion of the homology and nomenclature of this element). The coronoid of Sarmientosaurus is a mediolaterally thin bone that sheaths the alveolar region of the dentary dorsally (including the distal dental foramina); as preserved, it extends as far rostrally as the ninth dentary tooth on the right side and the seventh tooth on the left side. The ventral portion of the coronoid is itself obscured by the splenial. Caudally, the coronoid extends ventrally along the rostral margin of the adductor fossa to make a slight contact with the rostrodorsal tip of the prearticular. As might be expected in a basal lithostrotian, the coronoid of Sarmientosaurus is rostrocaudally shorter than those of Camarasaurus and Giraffatitan but longer than that of Nemegtosaurus (provided that the rostral terminus of the coronoid of the latter has been correctly identified [11]).

Tooth replacement.

CT images of the premaxilla, maxilla, and dentary (Fig 20; S2, S3, S4, S6, S7 and S8 Movies) reveal that Sarmientosaurus has two replacement teeth per alveolus, except in the fourth right maxillary alveolus, which houses three germ teeth. Tooth replacement in the new taxon was of the ‘wave of alternating replacement’ type, as also described for Nemegtosaurus [10,11] and an indeterminate titanosaur dentary (MCSPv-061 [61]). As previously recognized in isolated titanosaurian dentigerous elements [55,61], during development, the germ teeth of Sarmientosaurus moved labially from their origin near the medial wall of the alveolus until they finally erupted.

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Fig 20. Tooth replacement in Sarmientosaurus musacchioi gen. et sp. nov. (MDT-PV 2) as revealed by computed tomography.

(A) Transverse section through maxillae showing three replacement teeth in the right maxilla (on the left side of the image) and two in the left. (B) Transverse section through dentaries showing replacement teeth. Scale bars = 10 cm.

https://doi.org/10.1371/journal.pone.0151661.g020

Microwear.

Numerous scanning electron microscope (SEM) images were taken of the first right maxillary and the fifth right dentary teeth of Sarmientosaurus, to examine their wear facets and to assess the new taxon for the grooves that have been recorded in the enamel of other sauropodomorphs [61,123,143,149]. SEM images of the maxillary tooth (Fig 21) show thick and thin grooves that sometimes intersect but that are generally oriented parallel to the long axis of the wear facet. The dentary tooth (Figs 22 and 23) shows a similar pattern in the area of the mesial groove and the labial surface, although the intersecting grooves may be slightly more dense than those in the maxillary tooth. The apical facet of the dentary tooth also exhibits densely-packed groups of pits of different depths and perimeters, as in specimens studied by García and Cerda [61]. The general orientation of the grooves suggests the existence of orthal jaw movement, as proposed by Calvo [140] for titanosauriforms.

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Fig 21. Scanning electron microscope images of first right maxillary tooth of Sarmientosaurus musacchioi gen. et sp. nov. (MDT-PV 2).

(A) Lingual wear facet (enlarged 18 times). (B, C) Distal wear facet, showing microwear oriented generally parallel to the long axis of the tooth (enlarged 120 and 220 times, respectively). (D) Lingual wear facet, showing microwear oriented generally parallel to the long axis of the tooth (enlarged 430 times). Scale bars = 1 mm in A; 100 μm in B–C; 50 μm in D.

https://doi.org/10.1371/journal.pone.0151661.g021

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Fig 22. Scanning electron microscope images of mesial groove of fifth right dentary tooth of Sarmientosaurus musacchioi gen. et sp. nov. (MDT-PV 2).

(A) Enlarged 20 times. (B) Enlarged 150 times. (C) Enlarged 220 times. Scale bars = 1 mm in A; 100 μm in B–C.

https://doi.org/10.1371/journal.pone.0151661.g022

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Fig 23. Scanning electron microscope images of fifth right dentary tooth of Sarmientosaurus musacchioi gen. et sp. nov. (MDT-PV 2).

(A, B) Lingual surface (enlarged 22 and 130 times, respectively). (C) Apical facet (enlarged 85 times). (D) Labial zone (enlarged 450 times). Scale bars = 1 mm in A; 100 μm in B; 200 μm in C; 50 μm in D.

https://doi.org/10.1371/journal.pone.0151661.g023

An association between grooves and pits in the enamel and dentine and the diet of a given taxon has been proposed by multiple authors [123,149–151]. Nevertheless, other workers [61,152] have proposed that the origin of these traces may instead be related to the accidental ingestion of sediment associated with plant fodder, and therefore not strongly indicative of diet.

Axis.

The caudal articular surface of the centrum of the axis (Fig 24) is markedly concave, a feature that can be observed because the cranial articular condyle of the third cervical vertebral centrum is partially eroded and displaced from its contact with the axial centrum. Although it is incomplete cranially, the axis appears to have been dorsoventrally low and craniocaudally elongate, as is the case in the other preserved cervical vertebrae of Sarmientosaurus. There is no evidence of lateral pneumatic fossae (‘pleurocoels’). A cervical rib fragment approximately 6 cm in total length is preserved as two pieces adhered to the right ventrolateral area of the centrum. The region of the centrum caudal to the ventral base of the neural arch is proportionally long. The preserved portion of the neural arch is subtriangular and wedge-shaped in lateral view. The neural spine is inclined caudodorsally at roughly 50° from the horizontal, as in the titanosauriform axis from the Late Cretaceous of India described by Wilson and Mohabey ([155]; 196/CRP/GSI/05). This exceeds the 32° angle present in Futalognkosaurus (see Calvo et al. [156]:fig. 5) but is less than the 70° angle in Saltasaurus (Powell [46]:fig. 4a). The left centropostzygapophyseal lamina is stout, convex, and subcircular in section. The postzygapophyses project well beyond the caudal edge of the neural spine, more so than in the axis of many other sauropods, including other titanosaurs (see, for example, Wilson and Mohabey [155]:fig. 4). Their articular surfaces are caudolaterally oriented.

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Fig 24. Articulated partial axis and cervical vertebra 3 of Sarmientosaurus musacchioi gen. et sp. nov.

Photographs (A, C) and interpretive drawing (B) in right lateral (A, B) and dorsal (C) views. Abbreviations see text. Scale bar = 10 cm.

https://doi.org/10.1371/journal.pone.0151661.g024

Third cervical vertebra.

The cranial end of the third cervical vertebra (Fig 24) is preserved in partial articulation with the axis. The preserved portion includes part of the cranial articular condyle, which is conical and delineated from the remainder of the centrum by a ridge. The neural canal opens just dorsal to the centrum. In lateral view, the robust, cranially-projecting prezygapophyses are roughly triangular and subhorizontal. This last condition contrasts with that in the putative third cervical vertebra of Malawisaurus (MAL-243 [32]), in which the prezygapophyses are oriented craniodorsally at an angle of approximately 30°. Craniodorsally-angled prezygapophyses also present in the third cervical vertebrae of Erketu [157], Maxakalisaurus [36], Mongolosaurus [37,38], and probably Rapetosaurus (see Curry Rogers [154]:fig. 7c). The prezygapophyseal articular facets are flat and craniomedially oriented to articulate with the postzygapophyses of the axis. The robust cranial ends of the prezygodiapophyseal and spinoprezygapophyseal laminae extend caudally from the prezygapophyses.

Fifth cervical vertebra.

Only a diminutive fragment of the centrum of the fifth cervical vertebra (Fig 25) is preserved, affixed to the cranial articular condyle of the sixth cervical. Its broken margins reveal that its internal tissue structure was camellate.

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Fig 25. Cervical vertebra 6 of Sarmientosaurus musacchioi gen. et sp. nov. (MDT-PV 2).

Photographs (A, C, E, G, I, J), interpretive drawings (B, D, F, H), and computed tomography image (K) in left lateral (A, B), right lateral (C, D), dorsal (E, F), cranial (G, H), ventral (I), and caudal (J) views. (K) Transverse section showing extent of lateral pneumatic fossa of centrum (arrow). Abbreviations see text. Scale bars = 10 cm in A–J; 5 cm in K.

https://doi.org/10.1371/journal.pone.0151661.g025

Sixth cervical vertebra.

The sixth cervical vertebra of Sarmientosaurus (Fig 25) has a very low and elongate, opisthocoelous centrum (see Table 4). Many areas of the centrum are damaged, including the cranial half of the left lateral surface, the entire right lateral surface, and the central part of the ventral surface, in the area where the missing left cervical rib would have attached. In the neural arch, the dorsal and ventral extremes of the neural spine are damaged, the postzygodiapophyseal laminae are distorted, and the left postzygapophysis is not preserved.

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Table 4. Measurements (mm) of cervical vertebrae of MDT-PV 2, the holotype of Sarmientosaurus musacchioi gen. et sp. nov.

Abbreviation: C, cervical vertebra. * = element incomplete, measurement as preserved.

https://doi.org/10.1371/journal.pone.0151661.t004

The cranial articular condyle is subspherical, slightly wider than high, and smaller than that of the seventh cervical vertebra. As mentioned above, a small piece of the caudal end of the fifth cervical centrum is attached to its cranial surface. The caudal articular cotyle of the sixth cervical vertebra is higher than wide and only gently concave, presumably because its rim is eroded. Remnants of the incomplete parapophysis and diapophysis are preserved on the left lateral side of the centrum. Between them, a small lateral pneumatic fossa (‘pleurocoel’) is present as a craniocaudally elongate depression that is partially covered by sediment. It is larger than that present in the seventh cervical vertebra. Unlike the condition in Rapetosaurus [154], it is bordered dorsally by a low crest. Cross sections generated by CT scans (Fig 25K) reveal this fossa to be a deep, transversely rectangular cavity; it is, however, not as extensive as that of the seventh cervical vertebra. The preserved surface of the centrum is cracked and pitted by erosion. The neural arch is low, with stout, cranially-projecting prezygapophyses. The right prezygapophysis is oriented nearly horizontally relative to the centrum, but the left is deformed and rotated dorsally. The subcircular articular facet is craniomedially oriented and surpasses the cranial articular condyle of the centrum by a few centimeters. The prezygapophyses are supported ventrally by structures that we interpret as modified centroprezygapophyseal laminae. Interestingly, instead of comprising continuous sheets of bone as in other sauropods, these structures are perforated caudally, rendering them similar to ‘pillars’ or ‘struts’ that are roughly ellipsoidal in cross section. Although this condition could conceivably be due to the taphonomic loss of a hypothetical, mediolaterally thin caudal portion of a more ‘standard’ centroprezygapophyseal lamina, we regard this possibility as unlikely given the presence of a closely comparable ‘pillar’ in the seventh cervical vertebra as well (see below). Consequently, we consider ‘centroprezygapophyseal pillars’ in the middle cervical vertebrae to be a diagnostic feature of Sarmientosaurus. Parts of the low spinoprezygapophyseal laminae are preserved dorsally. The right postzygapophysis has a subcircular, ventrolaterally-oriented articular facet, and the incomplete neural spine was seemingly low and undivided. The postzygodiapophyseal laminae are also incomplete. The spinopostzygapophyseal laminae caudally delimit a cavity, the spinopostzygapophyseal fossa of Wilson et al. [158], that is wide ventrally but narrows dorsally.

The low and elongate morphology of this Sarmientosaurus cervical vertebra (Table 4) is comparable to that present in some other titanosauriform taxa, such as Erketu [157,159], Malawisaurus [32], Phuwiangosaurus [41,160], Sauroposeidon [94,161], Trigonosaurus [162], and ‘Peirópolis Series A’ [7,153]. In this vertebra of the new Patagonian titanosaur, the ratio between the craniocaudal length of the centrum and the dorsoventral height of its caudal articular cotyle (the ‘elongation index’ as defined by Wedel et al. [94]) is 4.15. Similarly, when Upchurch’s [142] original definition of the ‘elongation index’ (centrum length divided by transverse cotyle width) is applied, the value for the sixth cervical of Sarmientosaurus is 5.44. These values are comparatively high (see Ksepka and Norell [157]), similar to those of the middle cervical vertebrae of Tapuiasaurus [14]. The neural arch of the sixth cervical vertebra of Sarmientosaurus is also substantially lower than those of similarly-positioned vertebrae of many other titanosaurs, such as Atsinganosaurus [163], Isisaurus [164], Maxakalisaurus [36], and Muyelensaurus [39].

Seventh cervical vertebra.

The caudal part of the centrum of the seventh cervical vertebra (Fig 26) is missing, and the zygapophyses and part of the neural spine are damaged. The centrum has a convex cranial articular condyle and deeply excavated lateral faces, and its ventral surface is markedly concave transversely. On the left lateral surface there is a small, craniocaudally elongate pneumatic fossa that is partitioned by a lamina into a cranial portion and a slightly larger caudal portion. On the right side of the centrum, the external margin of the pneumatic fossa is eroded. Nevertheless, CT images demonstrate that the centrum has a minimum transverse thickness of only ~6.5 mm here; as such, in this region, the centrum consists only of a median lamina that separates the two large pneumatic fossae. A similar morphology is present in a cervical vertebra of a juvenile individual of Phuwiangosaurus [165] and in Erketu, an especially long-necked somphospondylan from the Early Cretaceous of Mongolia. In this latter taxon, the lateral surfaces of each cervical centrum are excavated by well-developed pneumatic fossae that reduce the centrum to a thin median septum [157,159].

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Fig 26. Cervical vertebra 7 of Sarmientosaurus musacchioi gen. et sp. nov. (MDT-PV 2).

Photographs (A–E, G) and interpretive drawing (F) in left lateral (A), right lateral (B), dorsal (C), ventral (D), cranial (E, F), and caudal (G) views. Abbreviations see text. Scale bars = 10 cm.

https://doi.org/10.1371/journal.pone.0151661.g026

Only traces of the parapophyses are preserved in the seventh cervical vertebra of Sarmientosaurus. The zygapophyses are also incomplete. The more complete right prezygapophysis is stoutly constructed, and, as in the sixth cervical vertebra, it is supported ventrally by a ‘centroprezygapophyseal pillar.’ Parts of the robust prezygodiapophyseal lamina and the postzygodiapophyseal lamina are preserved on the right side of the neural arch. The preserved portions of the postzygapophyses are similar in construction to those of the sixth cervical vertebra. The neural spine seems to be considerably more caudally situated than in comparably-positioned cervical vertebrae of most other titanosaurs (e.g., Atsinganosaurus, Futalognkosaurus, Isisaurus, Malawisaurus, Maxakalisaurus, Muyelensaurus, Rinconsaurus, Saltasaurus, Uberabatitan).

Cervical ribs.

As noted above, much of the caudal process of the right cervical rib of the axis is preserved and separated into two fragments (Fig 24A and 24B). The right rib of the sixth cervical vertebra is best observed in medial view (Fig 25). As seen in right lateral view, parts of its tuberculum and capitulum are preserved, and both of these structures are delicately constructed. The caudal process of the rib is thin and tubular. The cranial process narrows cranially but its tip is not preserved.

Ossified cervical tendon.

An enigmatic, cable-like structure was observed in situ at the type locality of Sarmientosaurus, ventrolateral to the right side of the cervical vertebrae and ribs. The structure originated immediately caudal to the skull and coursed along multiple vertebrae, maintaining a constant diameter of only ~3 mm throughout its length (Figs 2, 27A and 27B). This delicate structure is clearly distinct from the cervical ribs, which are much thicker and occupied a different position (Fig 27B). The structure is nearly oval in section and its external surface is rugose and striated (Fig 27A). As seen in thin section, its microstructure is composed of bone with numerous Haversian canals and longitudinally oriented fibers (Fig 27C and 27D). These fibers are comparable to the fibroblasts of tendinous tissue [166] that are found in the tendons of some extant birds (Fig 27E). Consequently, we regard this peculiar structure of Sarmientosaurus as an ossified cervical tendon. Sauropod cervical ribs are known to be derived from ossified tendons [166,167], raising the possibility that the structure corresponds to the distal part of one or more of these ribs. Nevertheless, unquestionable cervical ribs attached to the axis and all more caudal cervical vertebrae were also observed in the quarry; these were thicker and positioned differently, and they each decreased in diameter caudally. Furthermore, as above, the enigmatic tendon of Sarmientosaurus originated adjacent to the occipital region of the skull and extended for several meters without interruption or an appreciable change in diameter.

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Fig 27. Ossified cervical tendon of Sarmientosaurus musacchioi gen. et sp. nov. (MDT-PV 2).

(A) Section of tendon. (B) Field photograph showing relative dispositions of the camellate tissue of a cervical vertebra (hatched arrow), a cervical rib (white arrow) and the cervical tendon (black arrows) upon discovery. (C) Longitudinal thin-section of tendon showing longitudinal structures compatible with fibroblasts (near the top of the thin-section). (D) Transverse thin-section showing Haversian tissue. (E) Longitudinal thin-section (crossed polars, 100×) of tendon of Podilymbus (grebe) showing fibroblasts (modified from Organ and Adams [174]). Abbreviation see text. Scale bars = 5 cm in A; 1 mm in C–D; 0.1 mm in E.

https://doi.org/10.1371/journal.pone.0151661.g027

Except Sarmientosaurus, no sauropods (or non-avian dinosaurs in general) are known to have ventrolaterally-situated, ossified cervical tendons; nevertheless, the position and morphology of the structure in question is consistent with this hypothesized identification. One line of evidence that may support this identity is the fact that many modern birds have very elongate cervical muscles, and that some of these birds have ossified tendons situated deep within the neck musculature. The latter occurs in Gruidae (cranes; e.g., Anthropoides paradiseus, Grus antigone), in which there are many ossified cervical tendons; nevertheless, in these birds, the tendons extend the length of one or only a few presacral centra (Fig 28). The enigmatic tendon of Sarmientosaurus could conceivably be part of an insertion tendon of M. rectus capitis anterior (ventralis), which connects the cranial cervical centra with the basal tubera of the skull in extant diapsids [168,169]. In modern taxa, however, this tendon is not ossified, and it is usually wide rather than cable-like as in MDT-PV 2 (T. Tsuihiji, pers. comm.). Another possibility is that the structure is an insertion tendon of M. longus colli ventralis or Mm. intertransversarii, but not extending along the entire neck (J. Harris, pers. comm.). Histologically, the tendon shows significant secondary remodeling, suggesting that it ossified early in ontogeny, probably long before the sauropod attained its maximum size. Moreover, ontogenetic studies of other archosaurs (e.g., ornithischian dinosaurs) indicate that tendons often remain ossified throughout the ontogeny of these animals. As such, the ossification of this tendon in Sarmientosaurus is probably not attributable to the seemingly advanced ontogenetic stage of the holotype (I. Cerda, pers. comm.).

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Fig 28. Ossified tendons in presacral vertebrae of extant Gruidae (cranes).

(A) Grus antigone (ANS 24782), articulated cervical vertebrae 6–9 and associated ossified tendons in left lateral view. (B) Grus antigone (ANS 24782), articulated cranial thoracic vertebrae and associated tendons in dorsal view. (C) Anthropoides paradiseus (ANS 37018), thoracic vertebra 1 and associated tendons in dorsal view. Photos courtesy Jerry Harris, Dixie State University. Scale bar = 5 cm.

https://doi.org/10.1371/journal.pone.0151661.g028

In sum, several facts support the hypothesis that the enigmatic structure in the neck of Sarmientosaurus is an ossified tendon that is distinct from the cervical ribs. The structure maintained a diameter of only ~3 mm through a length of at least 3 m, and is histologically consistent with ossified tendon. Furthermore, unquestionable cervical ribs are associated with the axis and sixth cervical vertebra on the right side. The function of this presumed ossified tendon is unknown.

Proportions of orbit.

In Sarmientosaurus, the maximum diameter of the orbit is nearly 40% the rostrocaudal length of the cranium (with the latter measured from the tip of the snout to the occipital condyle). By contrast, in the basal macronarian Camarasaurus, the maximum orbital diameter is only 26% of cranial length. In the brachiosaurid Giraffatitan, this value is 21%, and in the Brazilian lithostrotian titanosaur Tapuiasaurus it is 20%. With a maximum diameter of about 35% total cranial length, only the orbit of the Early Cretaceous titanosauriform Abydosaurus approaches the proportionally large size seen in Sarmientosaurus.

Maxilla—lacrimal articulation.

Sarmientosaurus has a complex articulation between the maxilla and the lacrimal: the ascending ramus of the maxilla is embedded in and bordered medially and laterally by the dorsal process of the lacrimal. This unusual configuration is not present in Camarasaurus, in which the maxillary ascending ramus contacts the nasal and lacrimal, nor in Giraffatitan, where this ramus mostly articulates with the nasal, and to a lesser extent, the lacrimal. In the Madagascan titanosaur Rapetosaurus, the ascending ramus appears to overlap the rostral half of the lacrimal laterally and dorsally [13]. In Tapuiasaurus, however, the lacrimal appears to have a complex relationship with the maxillary ascending ramus that is reminiscent of that in Sarmientosaurus. The left ascending ramus (the right is damaged) is bordered by the lacrimal both laterally and medially, although the medial exposure of the latter (on the lateral margin of the bony nasal aperture) is proportionally much greater than in Sarmientosaurus. Nevertheless, this region of the skull is significantly distorted in Tapuiasaurus, and as such its true morphology remains in doubt.

Narial ridge.

In MDT-PV 2, the medial edge of the ascending ramus of the maxilla exhibits a low but distinct ridge that borders the bony nasal aperture. This narial ridge is not present in Abydosaurus, Camarasaurus, Giraffatitan, or Rapetosaurus. Tapuiasaurus appears to show a subtle ridge in the same or a closely similar position as that in Sarmientosaurus (see Zaher et al. [14]:fig. 1c), but further observations of the holotype of this Brazilian titanosaur (MZSP-PV 807) are required to ascertain this. The ascending ramus of the maxilla and the margin of the bony nasal aperture are not preserved in Nemegtosaurus.

Caudoventral process of quadratojugal.

The quadratojugal of Sarmientosaurus is distinctive in that its caudoventral corner is attenuated into a caudomedially-projecting, rounded, ‘tongue-like’ process that caudally overlaps the ventrolateral extreme of the quadrate and comprises the ventrolateral margin of the quadrate fossa. This process is not observed in most other macronarians (e.g., Camarasaurus, Giraffatitan, Nemegtosaurus, Quaesitosaurus); in these taxa, no part of the quadrate is obscured from caudal view by the quadratojugal. A comparable process may be present in Tapuiasaurus and possibly Rapetosaurus (see Zaher et al. [14]:5), but if so, it is significantly less developed than in Sarmientosaurus (see Curry Rogers and Forster [13]:fig. 17b; Zaher et al. [14]:fig. 1d). We therefore consider this ‘tongue-like’ quadratojugal process to be an autapomorphy of the new Patagonian titanosaur.

Tooth inclination.

The only known specimen of Sarmientosaurus has noticeably procumbent maxillary teeth and recumbent dentary teeth. This combination of tooth inclinations is not observed in any other sauropod, although procumbent maxillary and dentary teeth are known in the diplodocid Diplodocus and the macronarians Camarasaurus and Euhelopus [99,101,111]. It could be argued that the combination of tooth orientations seen in MDT-PV 2 is an artifact of lithostatic pressure applied to the skull. Nevertheless, it is difficult to envision a mode of taphonomic deformation that (1) would cause most maxillary and dentary teeth to ‘lean’ mesially and distally, respectively, while simultaneously keeping the premaxillary and mesialmost maxillary teeth subvertical, and that (2) would not cause obvious mediolateral distortion or damage to the tooth crowns.

Centroprezygapophyseal ‘struts’ in cervical vertebrae.

The middle cervical vertebrae of Sarmientosaurus are unique among Sauropoda in having centroprezygapophyseal laminae that have been modified into structures resembling ‘struts’ or ‘pillars.’ In Camarasaurus, the centroprezygapophyseal laminae are continuous bony sheets that connect the ventral surfaces of the prezygapophyses to the dorsal surface of the centrum. This is also the case in the non-titanosaurian titanosauriforms Erketu and Sauroposeidon [170,171] and the titanosaurs Malawisaurus, Rapetosaurus, Saltasaurus, and Trigonosaurus. In the brachiosaurid Giraffatitan [172], the cranial sector of the centroprezygapophyseal lamina is solid; an infraprezygapophyseal fossa is present more caudally, but the lamina is never perforated to form a ‘strut’ or ‘pillar’ as in Sarmientosaurus.

Ossified cervical tendon.

The new Patagonian titanosaur appears to possess a very elongate, ossified tendinous structure that extends along the cervical series, ventrolateral to the vertebrae and ribs. Ossified tendons are common in various groups of non-avian and avian dinosaurs [173,174], including sauropods. Bony rods found in sauropod quarries have traditionally been regarded as the ends of the elongate cervical ribs of these animals. In their studies of the histogenesis of sauropod cervical ribs, Cerda [166] and Klein et al. [167] concluded that the elongate portion of the rib arose from the ossification of a tendinous element. Nevertheless, during the recovery of MDT-PV 2, the two of us who were present (R.D.F.M. and G.A.C.) observed a dark, cylindrical structure that originated approximately 5 cm from the right quadrate and extended uninterrupted for several meters while maintaining a constant diameter of only ~3 mm. Simultaneously, we observed ‘typical’ ribs attached to the axis and sixth cervical vertebra, which were much thicker and lighter in color. The enigmatic ossified cervical tendon of Sarmientosaurus is presently unique among Dinosauria.