The sauropod braincase (MUCPv-334) describedhere is incomplete and shows only the occipitalregion. It consists of the following elements: supraoccipital, both exoccipital-opisthotics, part of the paroccipital processes, the basioccipital neck, basisphenoid, incomplete parasphenoid, both prootics, right laterosphenoid and the posterior part of the right orbitosphenoid (Figs. 1-4). The braincase measures 140 mm from the supraoccipial apex to the preserved distal end of the basipterygoid process, and has a width (between the paroccipital processes) of 120 mm. The preservation of the bone surface is good and except for the laterosphenoid-orbitosphenoid, which is displaced lateroventrally from its original anatomical position, there is almost no distortion. Most of the bones that form the braincase are fused or tightly connected, a common condition in sauropod braincases.
The foramen magnum is suboval with its longest axis directed dorsoventrally (30 mm high × 26 mm width). Based on the preserved portion of the basioccipital neck, it is likely smaller than the occipital condyle (not preserved). No evidence of the posttemporal fenestra is observed.
As in most titanosaurid braincases, the supraoccipital of MUCPv-334 is apparently fused with the exoccipital, and its shape is difficult to be determined. There are two symmetrical breaks directed dorsoventrally, but it is not clear if they actually indicate the limits of this bone. In any case, the supraoccipital is a robust element that forms the dorsal margin of the foramen magnum. At the midline it forms a transversely convex prominence that rises anterodorsally, reaching the dorsal plane of the skull and connecting with the parietals (not preserved). On either side of this prominence the supraoccipital levels out laterally and meets the exoccipital, forming an expanded and dorsally concave shelf. A shallow depression is present on the lateral side of the prominence. This prominence in MUCPV-334 is broader than in Antarctosaurus septentrionalis that also has deeper lateral depressions (Chatterjee and Rudra 1996, Fig. 11A). The latter has two distinct lateral depressions that are shallower in MUCPv-334. In MUCPv-334, the anterodorsal surface that contacts the parietals is convex and has an elongated triangular outline (Fig. 2). The supraoccipital of MUCPV-334 also differs from Saltasaurus (Powell 2003), Rapetosaurus (Rogers and Forster 2001), and Quaestitosaurus (Kurzanov and Bannikov 1983) by lacking a longitudinal groove (or midline depression) that coincides with the sagittal plane. MUCPv-334 differs with Rapetosaurus (Rogers and Forster 2001), Antarctosaurus wichmannianus (Huene 1929), Antarctosaurus septentrionalis (Huene and Matley 1933), Nemegtosaurus (Nowinsky 1971) and TMM 40435 (Tidwell and Carpenter 2003) in the absence of a clearly delimited nuchal crest.
The exoccipital participates in the dorsolateral rim and probably also in the lateral margin of the occipital condyle as in Antarctosaurus wichmannianus (Huene 1929). Therefore, this cranial element probably bounds completely the lateral walls of the foramen magnum and the floor of the neural channel in the condylar neck. Two protuberances occur on both sides of the foramen magnum as in the titanosaurid Saltasaurus and the diplodocid Amargasaurus (Salgado and Bonaparte 1991), but they are smoother and less developed in MUCPv-334. The surface of those protuberances is striated, suggesting that they were used for muscle attachment likely related with the neck musculature. The exoccipital expands dorsally and forms a wide contact for the parietal (18 mm). Laterally it expands and fuses with the opisthotic and, in posterior view, those elements cannot be individualized. The exoccipital-opisthotic further fuse with the paroccipital process. Near the lower rim of the foramen magnum, each exoccipital is pierced by a foramen for cranial nerve XII (N. Hypoglossus), as observed in "Antarctosaurus'' septentrionalis (Chatterjee and Rudra 1996, Fig. 11A).
The paroccipital processes are incomplete and lack the distal ends. Nevertheless the preserved section indicates that they were deflected as in Saltasaurus (Powel 1992) and Antarctosaurus. Although the dorsal limits are not clear due to fusion with the exoccipital-opisthotic, the top of the paroccipital process is level with the foramen magnum. From the preserved portion of those elements in MUCPv-334, the paroccipital processes lack a posterodorsal notch present in Rapetosaurus, Quaesitosaurus, and Jainosaurus reported by Rogers and Forster (2004).
The occipital condyle is missing and only the condylar neck is preserved (Figs. 1, 3). It is clear from the preserved portion that the occipital condyle is posteroventrally directed, as in most Titanosauromorpha (Salgado et al. 1997) and largely formed by the basioccipital. The sutures of the basioccipital and the surrounding elements are not visible.
The basisphenoid is completely fused with the parasphenoid. On each side it forms a small and poorly developed basal tuber and the basipterygoid process. The basal tuber and basipterygoid process are fused and participate in the anterior half of the crista prootica. Furthermore, the basal tubera are fused in the midline as in Saltasaurus, but can be differentiated in two distinct areas on the ventral side. The area closest to the occipital condyle shows a depression surrounded by a sharp ridge as in Antarctosaurus wichmannianus (Fig. 1). The distal parts of the tubera form a bony mass from which the basipterygoid processes arise (as in Saltasaurus). The basal tuber is similar to that of Saltasurus in its massiveness but each tuber is more elaborated in MUCPv-334. Between them there is a shallow basisphenoidal depression (Fig. 1).
The basipterygoid processes are not complete. They diverge from each other at an angle of less than 30 degrees relative to the midline. The preserved portion of the left basipterygoid process indicates that this element was stout and short. On the anterolateral part, close to the contact with the laterosphenoid, there is a foramen for the exit of cranial nerve VI (Fig. 2).
In anterior view, the parasphenoid (not well preserved dorsally) is long, robust, and directed anteroposteriorly. It expands anteriorly and curves ventro-posteriorly reaching the basal tubera as a crest. So far a parasphenoidal crest between the two basipterygoid processes has not been reported in any other titanosaurid skull.
The opisthotic is fused with the exoccipital and can only be distinguished in ventrolateral view (Fig. 3). This bone is hidden by the prootic. The prootic-opisthotic suture is partially preserved as a faint and almost straight line. The opisthotic forms most of the dorsal margin of the fenestra ovalis, which is bordered anteriorly by the prootic and posteriorly by paroccipital process. Ventrolaterally the opisthotic is pierced by some foramina that constitute the exits of the cranial nerves IX, X, and XI.
The prootic is well developed. Although the distal ends are lacking, the preserved portion ofthe crista prootica indicates that it is expanded laterally, more than in any other titanosaurid braincase described so far. The lateral border is directed downward and almost parallel to the skull axis. It forms the exit of cranial nerve V together with the laterosphenoid. Two strong sulci are situated on the anterior face of the crista prootica and constitutethe exits for the mandibular branch (placed laterally) and for the maxillary branch (placed anteriorly) of cranial nerve V. Posteriorly, the prootic is pierced by a foramen, possible the exit of the facial nerve (VII).
The laterosphenoid lies between the orbitosphenoid and the prootic. This portion of MUCPv-334 is only preserved on the right side, where it is ventrolaterally displaced from its original position. Dorsally the laterosphenoid is expanded, turning into a splint-like bone ventrally (Fig. 1). The contact surface with the frontal (not preserved) is flat and elongated lateromedially. The limit between the laterosphenoid and the orbitosphenoid passes through the foramina for cranial nerves III and IV. A third foramen, whose function is unknown, is situated close to the dorsal margin. Laterally the laterosphenoid is expanded, forming the crista antotica that is directed posteroventrally, forming the anterodorsal margin of a large passage for cranial nerve V. Based on this region of MUCPv-334, the area of Rapetosaurus krausei labeled as the laterosphenoid by Rogers and Forster (2001, 2004) most likely includes, besides the laterosphenoid, the orbitosphenoid and possible part of the prootic.
Only the right orbitosphenoid is partially preserved, clearly indicating that it met anteriorly with its counterpart, a common feature observed in other sauropod braincases. The suture with the laterosphenoid passes through the exit of cranial nerves III and IV. The portion with the exit of cranial nerve II is not preserved.
Answers to all your Biology Questions
