Skull anatomy of a new basal eusauropod from the Cañadón Asfalto formation (Middle Jurassic) of central Patagonia

POL, DIEGO; RAUHUT, OLIVER W.; CARBALLIDO, JOSÉ L.

Bristol

Congreso; 69th Annual Meeting of the Society of Vertebrate Paleontology; 2009

Society of Vertebrate Paleontology

The Cañadon Asfalto Formation has provided a diverse Middle Jurassic vertebrate fauna, including two eusauropod taxa: Patagosaurus fariasi and Volkheimeria chubutensis. Recent work in this unit led to the discovery of abundant material of a new eusauropod taxon, including one of the most complete skulls and lower jaw from the Middle Jurassic of the Southern Hemisphere. The ascending process of the premaxilla is slender, posterodorsally oriented, and bears a distinct step at its anterior margin. The maxilla is low and elongated, has a rounded subnarial foramen on its lateral surface and ten neurovascular foramina that are not dorsoventrally aligned (the most posterior of which is moderately enlarged). The maxilla lacks an antorbital fossa and bears a lateral plate that partially covers its 13 spatulate teeth that are slightly procumbent and extend up to the posterior margin of the antorbital fossa. The teeth have wrinkled enamel, marginal denticles, buccal and lingual grooves, and many of them show v-shaped wear facets. The frontals are unfused and broader than long and the postorbital has a long and anteriorly curved cylindrical process for the jugal and a reduced squamosal process. The squamosal forms part of the supratemporal fossa and has an anteroposteriorly broad descending process. The quadrate bears a large, deep posterior fossa. The occipital surface is flat and the paroccipital processes project ventrolaterally, and lack a distinct expansion on their lateral ends. The basipterygoid processes are subtriangular in cross-section, slightly more than twice as long as wide, and directed ventrolaterally. Several features of the skull suggest that this taxon is more advanced than Shunosaurus (e.g., unaligned maxillary neurovascular foramina, supratemporal fenestra transversely elongated, elongation of basipterygoid process, depth of quadrate fossa, tooth count) but the presence of numerous plesiomorphic features indicates that the new taxon is basal to Neosauropoda (e.g., absence of preantorbital fenestra, cylindrical descending process of postorbital, laterally facing subnarial foramen, presence of external mandibular fenestra, teeth with marginal denticles).