Data from: A basal lithostrotian titanosaur (Dinosauria: Sauropoda) with a complete skull: implications for the evolution and paleobiology of Titanosauria

We describe Sarmientosaurus musacchioi gen. et sp. nov., a titanosaurian
sauropod dinosaur from the Upper Cretaceous (Cenomanian–Turonian) Lower
Member of the Bajo Barreal Formation of southern Chubut Province in
central Patagonia, Argentina. The holotypic and only known specimen
consists of an articulated, virtually complete skull and part of the
cranial and middle cervical series. Sarmientosaurus exhibits the following
distinctive features that we interpret as autapomorphies: (1) maximum
diameter of orbit nearly 40% rostrocaudal length of cranium; (2) complex
maxilla–lacrimal articulation, in which the lacrimal clasps the ascending
ramus of the maxilla; (3) medial edge of caudal sector of maxillary
ascending ramus bordering bony nasal aperture with low but distinct 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) subvertical premaxillary, procumbent
maxillary, and recumbent dentary teeth; (8) cervical vertebrae with
‘strut-like’ centroprezygapophyseal laminae; (9) extremely elongate and
slender ossified tendon positioned ventrolateral to cervical vertebrae and
ribs. The cranial endocast of Sarmientosaurus preserves some of the most
complete information obtained to date regarding the brain and sensory
systems of sauropods. Phylogenetic analysis recovers the new taxon as a
basal member of Lithostrotia, as the most plesiomorphic titanosaurian to
be preserved with a complete skull. Sarmientosaurus provides a wealth of
new cranial evidence that reaffirms the close relationship of titanosaurs
to Brachiosauridae. Moreover, the presence of the relatively derived
lithostrotian Tapuiasaurus in Aptian deposits indicates that the new
Patagonian genus represents a ‘ghost lineage’ with a comparatively
plesiomorphic craniodental form, the evolutionary history of which is
missing for at least 13 million years of the Cretaceous. The skull anatomy
of Sarmientosaurus suggests that multiple titanosaurian species with
dissimilar cranial structures coexisted in the early Late Cretaceous of
southern South America. Furthermore, the new taxon possesses a number of
distinctive morphologies—such as the ossified cervical tendon, extremely
pneumatized cervical vertebrae, and a habitually downward-facing
snout—that have rarely, if ever, been documented in other titanosaurs,
thus broadening our understanding of the anatomical diversity of this
remarkable sauropod clade. The latter two features were convergently
acquired by at least one penecontemporaneous diplodocoid, and may
represent mutual specializations for consuming low-growing vegetation.