Osteology

MCCM-HUE-8741 (Figures 1, 2, S1, S2, S3) was discovered in August 2007 in the lowest part of the fossiliferous succession (G1; see [1]). It is of overall small size (length in the median axis: 100.8 mm; maximal width of the left, best preserved lateral half: 64.3 mm). Almost no sutures are visible. This is probably due to the fact that this is a mature titanosaurian in which the bones have largely fused together. Difficulties in discriminating sutures are exacerbated by the iron oxides that have penetrated the bone, concealing the bony surface and forming hard concretions in places that cannot be removed without jeopardizing the integrity of the specimen. Portions of the ventral half of the braincase (i.e., a small part of the basioccipital and some of the basisphenoid-parasphenoid) are missing. As a result, structures such as the basipterygoid processes cannot be appraised in any manner. The orbitosphenoid, which perhaps was incompletely ossified, is poorly preserved and has sunk into the cranial cavity. Nevertheless, the specimen does not appear to have suffered significantly from taphonomic deformation, as demonstrated, for instance, by its unaltered bilateral symmetry.

Frontal. The left frontal is complete. The rostrolateral corner of the right frontal is missing, but the break suggests that this happened during the excavation. The lateral margin of the frontal is remarkably sinuous, with two processes: one rostrolaterally and the other more caudolaterally. The rostral border of the rostrolateral process shows a large groove, for the articulation of the prefrontal, whereas the caudolateral process possibly articulated with the postorbital. The rostral margin is also pointed (close to the central axis), although in a much more subtle way. The dorsal surface of each frontal is uneven: it is convex along the central axis in the caudal half and concave elsewhere except in the zone of the rostrolateral process, where it is approximately flat. The ventral side of the frontal is marked by a large hemispheric depression whose lateral margin shapes into the two above-mentioned processes, which together constitute the roof of the orbit. The rostromedial part of the ventral surface is separated from this depression by a strong crest that runs caudally from the rostrolateral zone toward the midline of the united frontals, separating the orbital and narial portions of the braincase. The left frontal is 57.3 mm long and 64.3 mm wide. The frontals of MCCM-HUE-8741 differ greatly from those of the Transylvanian titanosaurian braincase FGGUB 1007 ([15]: fig. 15). In the latter specimen, the lateral margin of the frontal is roughly straight, whereas in the Lo Hueco specimen the participation of the frontal in the orbital margin distinctly stands out laterally. Also, in the Transylvanian specimen, the frontals are oriented strongly ventrally from the articulation with the parietal. The ventral orientation of the frontals in MCCM-HUE-8741 is less prominent and, in particular, its dorsal surface does not show any strong ventral curvature. Although closer in morphology, the frontal of MCCM-HUE-8741 is also distinct from that of the specimen from Fox-Amphoux FAM 03.064 ([14]: figs. 2–5). Specifically, the latter has a continuously convex rostral edge and a straight lateral margin. In contrast, the frontal of Ampelosaurus atacis ([11]: fig. 4.2) resembles that of the specimen from Lo Hueco. It shows, in particular, the extensive contribution to the roof of the orbit and its correlated hemispheric depression on the ventral surface. However, the frontal of this species is not identical to that of MCCM-HUE-8741. For instance, the caudal border of the orbital roof is much more ventral in A. atacis ([11]: fig. 4.2E) than in the specimen from Lo Hueco and its lateral margin is not embayed ([11]: fig. 4.2A–B). The frontal of MCCM-HUE-8741 is clearly distinct from that of Nemegtosaurus mongoliensis ([6]: fig. 7), which has a fairly flat dorsal surface and mostly convex lateral margin marked with discrete transverse wrinkles. The frontal of N. mongoliensis ([6]: fig. 7) also bears a rostromedial depression, which is absent in MCCM-HUE-8741. The latter concavity is also present in Quaesitosaurus orientalis [6].

Parietal. The dorsal margin of the conjoined parietals is marked by a ω-shaped crest. The midpoint of this prominence contacts the supraoccipital caudally, whereas laterally the parietal sends two occipital wings. These extensions are not fully preserved, but they would have bordered the upper temporal fenestrae caudally, at least in their medial half. The caudal border of each occipital wing would have constituted the dorsal margin of the post-temporal fenestrae, but there is no evidence of these openings, suggesting they were either absent or situated laterally to the occiput as preserved. An aperture of angular outline is visible on the midline of the cranial roof near the frontoparietal contact. Its position is consistent with its identification as a pineal foramen (but see below). As preserved, the parietal is 79.6 mm wide. The parietals of MCCM-HUE-8741 are clearly different from the unfused ones of the juvenile titanosaurian braincase FGGUB 1007 ([15]: fig. 15). The latter are extremely unusual in bearing rostromedially low, rounded outgrowths. In contrast, the conjoined parietals of Ampelosaurus atacis ([11]: fig. 4.2A) look similar in morphology to that of MCCM-HUE-8741. Le Loeuff ([11]:119–120) noted similarities between the parietal of A. atacis and that of Antarctosaurus wichmannianus, which does resemble that of the specimen from Lo Hueco in its arcuate dorsal crest ([21]:pl. 28 fig. 2, [22]:pls 63, 64 fig. e). The parietal of MCCM-HUE-8741 is also very close to that of the specimen from Fox-Amphoux ([14]: figs. 2–5), which possibly bore a similarly shaped crest. The parietal of MCCM-HUE-8741 is distinct from that of Nemegtosaurus mongoliensis ([6]: fig. 7, [17]: fig. 4a, pl. 13 fig. 1), which bears a prominent dorsal crest that is not nearly as biarcuate. The conjoined parietals of N. mongoliensis also show a short, flat median suture ([6]: fig. 7).

Supraoccipital. The precise morphology of the supraoccipital cannot be ascertained due to imperfect preservation. It appears to have been strongly convex and may have born a median nuchal crest (for ligament insertion), at least in its more dorsal part. It is presently pierced by two irregular apertures dorsally in its suture with the parietal. The participation of the supraoccipital to the foramen magnum cannot be known for sure, but it did not exceed the most dorsal quarter to judge from the position of the proatlas facets, which are typically borne by the exoccipitals. The supraoccipital is only 10.1 mm deep (dorsoventrally) and 16.5 mm long (rostrocaudally). The supraoccipital of MCCM-HUE-8741 resembles that of the fragmentary titanosaur braincase described by Le Loeuff et al. [12] which has a massive nuchal crest, though this is a fairly widely distributed character in dinosaurs in general and in sauropods in particular. A strong nuchal crest is also present in the supraoccipital of Ampelosaurus atacis ([11]: fig. 4.2D).

Otoccipital. There is no way to distinguish the exoccipital from the opisthotic, and they presumably are co-ossified into a single complex (otoccipital), as is typical in archosaurs. Each otoccipital no doubt makes up most of the lateral margin of the foramen magnum. It is marked by a small protuberance in its dorsomedial area: the facet for the proatlas articulation. This bulge distorts the edge of the foramen magnum and thereby gives the latter a pyriform outline (21.1×19.1 mm). Whereas the medial otoccipital is strongly convex, the paroccipital process has a rather flat occipital surface. The latter, which is fusiform in section, is oriented ventrally and slightly caudally. Its state of preservation distally does not allow affirming if it originally bore a non-articulating ventral processes as in many titanosaurs, such as Rapetosaurus krausei ([3]: fig. 19) and Saltasaurus loricatus ([22]: fig. 19). The proximoventral margin of the otoccipital bears a groove-like depression, whose ventral border is the tuberal crest (crista tuberalis). CT scan data suggest that this furrow accommodated the single hypoglossal nerve (XII) as it left the braincase. The tuberal crest is extremely sharp and prominent. CT data reveal that it overhangs proximally the jugular foramen (foramen jugulare), which formed the exit of the vagoaccessory nerves (X–XI), and that the oval window (fenestra ovalis = fenestra vestibuli) opens just rostral to the latter. The better-preserved left otoccipital is 49.4 mm wide. Compared with that of MCCM-HUE-8741, the paroccipital process of the braincase presented by Le Loeuff et al. [12] is much stouter. Thus, it is much higher at its base than the foramen magnum is wide ([12]:unnumb. pl.), whereas the height of the paroccipital process of the specimen from Lo Hueco is roughly similar to the transverse diameter of the foramen magnum. The specimen described by Le Loeuff et al. [12] bears a small protuberance at about midheight of the exoccipital. This somewhat recalls that seen in MCCM-HUE-8741, though it is not situated on the margin of the foramen magnum but slightly more distally. Similar protuberances are also found in the Dongargaon specimen ([23]: fig. 2) from the Maastrichtian of India and that from Balochistan ([24]: fig. 2C), Maastrichtian of Pakistan, as well as in other sauropods (e.g., Spinophorosaurus nigerensis [25]: fig. 3B, S1, S2, S3). In contrast, no such feature is seen on the otoccipital of Lirainosaurus astibiae ([9]: fig. 2A), Ampelosaurus atacis ([11]: fig. 4.2D), and the specimen reported by Allain ([13]:pl. 6 fig. 1A). The paroccipital process of the latter specimen is oriented also more strongly caudally than that of the Lo Hueco specimen. The otoccipital of MCCM-HUE-8741 is clearly distinct from that of Nemegtosaurus mongoliensis ([6]: fig. 7, [17]: fig. 5a), whose paroccipital process is proportionally higher (dorsoventrally) and more ventrally inclined. In addition, the otoccipital of N. mongoliensis ([6]: fig. 7) does not bear any noticeable proatlas facet. It displays, however, a prominent ridge that extends from the dorsolateral margin of the foramen magnum onto the paroccipital process, but subsides at the midlength of it. A comparable elongate prominence is present in Quaesitosaurus orientalis ([6]: fig. 18), whose paroccipital process has a tapered prong distoventrally ([6]: fig. 18). In both N. mongoliensis and Q. orientalis, the foramen magnum is oval with the long axis oriented dorsoventrally ([2]: fig. 2a, [6]: figs. 9, 18, [17]: fig. 5a, pl. 12 fig. 2).

Basioccipital. The basioccipital of the specimen from Lo Hueco is remarkable in having an occipital condyle that is much broader laterally than high in caudal view. Thus, the occipital condyle is wide (a little wider than the foramen magnum), but it is dorsoventrally low. The non-hemispherical form of the occipital condyle, which appears to be genuine given the lack of any indication of effective taphonomic compression, might have favored the dorsoventral and mediolateral motions of the head over those in the diagonal. The lowness of the condyle is responsible for a condylar neck whose lateral surfaces are very convex dorsoventrally, whereas the ventral side is much flatter. Therefore, the occipital condyle is not well separated from its neck ventrally. However, and despite the wideness of the neck, the occipital condyle stands out from it laterally (best seen on the better-preserved left size). The irregular surface of the occipital condyle is at least partly related to the loss of the original cartilaginous covering. No participation of the otoccipital in the occipital condyle is evident, but almost all sutures are obliterated. A part of the left basal tuber is visible at this level, which means that the complete braincase was especially low (the skull as a whole may have been high, though). Probably only the dorsal half or so of the preserved basal tuber was made up by the basioccipital. It is oriented laterally, and its lateral surface appears to have been rugose, which is common in titanosaurians. The occipital condyle is 28.6 mm wide and 15.8 mm deep. The tubera:condyle width ratio of MCCM-HUE-8741 is very high (at least 2.33). Like MCCM-HUE-8741, Ampelosaurus atacis has a transversely ovoid occipital condyle ([11]: fig. 4.2D). The juvenile titanosaurian braincase FGGUB 1007 ([15]: fig. 15) also has a somewhat laterally elongate occipital condyle, but it appears more rounded ventrally. The basioccipital of Lirainosaurus astibiae ([9]: figs. 2–3) is clearly different from that of the specimen from Lo Hueco. For instance, the occipital condyle is much taller (more hemispherical) in L. astibiae than in MCCM-HUE-8741. The ventral border of the articular surface of the occipital condyle stands out from the neck in a very marked way in L. astibiae, whereas this border is almost in continuity with the neck in the specimen from Lo Hueco. In addition, the basal tubera of L. astibiae are significantly more ventrally extended than those of the specimen from Lo Hueco, hinting at a deeper braincase in the former. They are also positioned much more caudally, more or less at the vertical level of the occipital condyle, whereas they are approximately at the vertical level of the supraoccipital in MCCM-HUE-8741. The basioccipital of the titanosaurian braincase reported by Allain [13] is also clearly different from that of MCCM-HUE-8741. For instance, in that specimen the occipital condyle is a bit deformed, but subtriangular in outline, not ovoid as in the Lo Hueco specimen. Its neck is also a little longer in proportion, the articular surface of the condyle extends ventrally beyond the ventral border of the neck and, above all, the ventrolateral sides of the neck are concave. The basal tubera have a more habitual (more ventral) position. In fact, the basal tubera emerge at a variable level in sauropods. Thus, the camarasaurid Camarasaurus lentus ([26]: fig. 23A), the dicraeosaurid Dicraeosaurus hansemanni ([27]: fig. 96), and other species (including most titanosaurians) have more ventral basal tubera than does MCCM-HUE-8741, which is similar in this respect to some species such as the brachiosaurid Giraffatitan brancai ([27]: figs. 5, 7), two other basal titanosauriforms ([18]: fig. 1B, D, [28]: fig. 3A, D), and the titanosaurians Mongolosaurus haplodon ([29]: fig. 2A) and Tapuiasaurus macedoi ([7]: fig. 1D). In MCCM-HUE-8741, the neck of the occipital condyle does not show the possibly autapomorphic ventral groove described by Díez Díaz et al. [14] in FAM 03.064. The basioccipital of MCCM-HUE-8741 differs unmistakably from that of Nemegtosaurus mongoliensis in the relative size and the outline of the occipital condyle. Thus, in N. mongoliensis, the occipital condyle is more than twice as broad as the foramen magnum and its dorsal surface is distinctly concave in the midline, whereas the ventral surface is convex in caudal view ([6]: fig. 9, [17]: fig. 5a, pl. 12 fig. 2). Whereas the difference in relative size of the occipital condyle between MCCM-HUE-8741 and Quaesitosaurus orientalis is less marked than it is between the former and N. mongoliensis, the difference in shape is more marked, as Q. orientalis has a fairly hemispheric occipital condyle ([2]: fig.2, [6]: fig. 18). The condyle width:height ratio of MCCM-HUE-8741 (1.81) are only approximated among sauropods by that of A. atacis (1.79) and that of an indeterminate titanosaurian braincase (1.86) described by Paulina-Carabajal and Salgado ([30]: fig. 2D), whereas the tubera:condyle width ratios (which cannot be ascertained in MDE C3–761 ) is close to that of the titanosaurian Bonatitan reigi (2.24) ([29]:tab. 1).

Basiphenoid-parasphenoid. Based on CT scan data, this bone complex (parabasisphenoid) would constitute the floor of the preserved braincase from the rostral edge of the basioccipital to the rostral wall of the pituitary fossa. The ventral half or so of the preserved basal tuber is probably made up by the basisphenoid. The pituitary fossa is ovoid in section, wider than long, and the dorsum sellae is caudoventrally inclined. Very close to the pituitary fossa, on both sides of it, the basisphenoid is pierced by a small foramen caudally. CT scan data substantiate that it was for the abducens nerve (VI), which therefore did not enter the pituitary fossa in contrast to what is generally observed in most sauropods [31]. In its middle part, the basisphenoid-parasphenoid is about 40.3 mm wide. The section of the pituitary fossa is 6.9 mm wide and 3.7 mm long. As in MCCM-HUE-8741 and most sauropods, the pituitary fossa seems to have been inclined caudoventrally in Ampelosaurus atacis ([11]: fig. 4.2) and Lirainosaurus astibiae ([9]: figs. 2–4). This was also the case in the specimen reported by Allain [13], to judge from the strongly caudoventrally oriented basisphenoid-parasphenoid.

Orbitosphenoid. The orbitosphenoid, which is rostral to the pituitary fossa and laterosphenoid, is poorly preserved and displaced dorsally. The left orbitosphenoid better preserves the border of a single median aperture for the optic nerve (II). The orbitosphenoid also ventrally borders the origin of the olfactory tracts (I), by flooring the wide rostral aperture of the braincase. The remaining border of the olfactory tract aperture (laterally and dorsally) was constituted by the frontals. As preserved, the orbitosphenoid extends rostrally from the pituitary fossa 17.5 mm along its median axis. The original configuration of the orbitosphenoid of MCCM-HUE-8741 was no doubt similar to that seen in Ampelosaurus atacis ([11]: fig. 4.2B–C), with a single conjoined optic foramen and a broad, but low, olfactory aperture. A single optic foramen seems to be homoplastically distributed in sauropods as it also appears to be present in the basal eusauropod Shunosaurus lii ([32]: fig. 7B) and Giraffatitan brancai ([27]: figs. 5–6), whereas Dicraeosaurus hansemanni ([27]: figs. 136–137), Camarasaurus lentus ([26]: fig. 24A), and Nemegtosaurus mongoliensis ([6]: fig. 8) have two distinct openings.

Prootic. The prootic is a dorsoventrally high but rostrocaudally short bone. It is bordered largely by the basisphenoid ventrally, the laterosphenoid rostrally, the parietal dorsally, and the otoccipital caudally. The prootic is marked by a robust and sharp otosphenoidal crest (crista otosphenoidalis = crista prootica), which runs rostroventrally from the basis of the paroccipital process to contact the basal tuber and most probably continued ventrally to reach the basipterygoid process. This crest borders caudally the large trigeminal foramen. The rostrocaudal distance between the otosphenoidal crest and the antotic crest (crista antotica) of the laterosphenoid (which corresponds approximately with the caudal and rostral limits of the prootic) is 10.6 mm at the level of the trigeminal foramen. The otosphenoidal crest of MCCM-HUE-8741 is sharper and more prominent than that of Ampelosaurus atacis ([11]: fig. 4.2B, E). This difference, however, is more marked with Lirainosaurus astibiae ([9]: figs. 3A, 4C), the specimen reported by Allain [13], and FAM 03.064 ([14]: figs. 4–5).