Abstract
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.
Highlights
Titanosaurian sauropod dinosaurs were extremely diverse and abundant in Upper Cretaceous continental paleoenvironments in the Gondwanan landmasses, and have been discovered throughout the world [1,2,3,4,5,6]
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
Phylogenetic analyses demonstrate that the new taxon is an archaic member of the titanosaurian subclade Lithostrotia, occupying a position more derived than Malawisaurus but more basal than taxa frequently regarded as nemegtosaurids (Nemegtosaurus, Rapetosaurus, and Tapuiasaurus) and saltasaurid titanosaurs such as Alamosaurus, Neuquensaurus, and Saltasaurus
Summary
Titanosaurian sauropod dinosaurs were extremely diverse and abundant in Upper Cretaceous continental paleoenvironments in the Gondwanan landmasses, and have been discovered throughout the world [1,2,3,4,5,6]. Complete or nearly complete skulls are known only for the following taxa: Nemegtosaurus, from the Upper Cretaceous Nemegt Formation of Mongolia [10,11]; Rapetosaurus, from the Upper Cretaceous Maevarano Formation of Madagascar [12,13]; and Tapuiasaurus, from the Lower Cretaceous Quiricó Formation of Brazil [14]. Several complete or nearly complete but distorted skulls of generically unidentified embryonic titanosaurs are known from the Campanian Anacleto Formation at the Auca Mahuevo locality in northern Patagonia, Argentina [15,16,17]. Complete or even reasonably complete titanosaur skulls remain unknown from the Albian— Santonian (a roughly 30 million year span of the mid- and Late Cretaceous; see Walker et al [65]), which represents a significant impediment to understanding of titanosaur cranial anatomy and evolution
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