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.

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RESUMO -É descrito aqui Aeolosaurus colhuehuapensis sp. nov. com base em uma série caudal articulada proveniente de uma ilha localizada no lago Colhué Huapi, Província de Chubut. Esta série é caracterizada por possuir arcos neurais localizados muito anteriormente em relação ao corpo vertebral e inclinados para frente; pré-zigapófises bem desenvolvidas projetadas antero-dorsalmente com a presença de facetas articulares amplas em relação as caudais anteriores. Aeolosaurus colhuehuapensis sp. nov. é o único que possui uma profunda e marcada fossa entre o processo transverso e a base da espinha neural nas vértebras caudais mais anteriores; além de possuir uma lâmina delgada que une a base da lâmina pré-espinhal com a parte inferior da faceta articular na parte medial da pré-zigapófises, que por sua vez define uma fossa em ambos os lados da lâmina pré-espinhal; e côndilo posterior em posição central em toda a série caudal. O gênero Aeolosaurus é registrado no CampanianoMaastrichtiano na Argentina e no Brasil, sendo considerado como integrante de uma associação faunística junto com hadrossaurídeos, Madtsoiinae, Chelidae, Sudamericidae e Theria não tribosfénicos encontrados em ambientes de planicies de inundação ou litorais. Sua presença no membro superior da Formacão Bajo Barreal, junto com outros registros paleontológicos da mesma localidade, além de observações geológicas, permitem atribuir preliminarmente uma idade Campaniano-Maastrichtiano? para os níveis superiores desta unidade estratigráfica, aflorantes no sudeste do lago Colhué Huapi, Província de Chubut.Palavras-chave: Titanosauria, Aeolosaurus, Cretáceo Superior, Patagonia, Argentina. ABSTRACT -AEOLOSAURUS COLHUEHUAPENSIS SP. NOV. (SAUROPODA, TITANOSAURIA), FROM THE BAJO BARREAL FORMATION, UPPER CRETACEOUS OF ARGENTINA. A new sauropod,Aeolosaurus colhuehuapensis. sp. nov., based on an articulated caudal series recovered from an emergent island in Lago Colhué Huapi, Chubut Province is described. This series is characterized by neural arches located very anteriorly above the centrum and inclined forward; well-developed prezygapophyses projecting forward and upward, and wide articular facets in the anterior caudals. Aeolosaurus colhuehuapensis sp. nov. is unique by the presence of a marked and deep fossa between the transverse process and the base of the neural spine in the most anterior caudal vertebrae; a slender lamina that connects the base of the prespinal lamina with the lower part of the articular facet in the medial part of the prezygapophysis, defining a fossa on both sides of the prespinal lamina; posterior condyle centrally placed in the entire caudal series. The genus Aeolosaurus is recorded in the Campanian-Maastrichtian in Argentina and Brazil, and is considered part of a faunal association with hadrosaurids, Madtsoiinae, Chelidae, Sudamericidae, and non-tribosphenic Theria in floodplain or littoral environments. Its presence in the Upper Member of the Bajo Barreal Formation, along with other paleontological records from the same locality, and geo...

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-FAUNISTIC ARRANGEMENT OF THE UPPER CRETACEOUS OF CHUBUT GROUP, GULF SAN JORGE BASIN, ARGENTINA. The Chubut Group, central Patagonia, Argentina, is characterized by a lacustrine and fl uvial-lacustrine system with variable participation of volcanic ash. This group includes the Bajo Barreal Formation (Cenomanian-Turonian) and a recently nested new lithostratigraphic unit, the Lago Colhué Huapi Formation (Coniacian-Maastrichtian). The Lago Colhué Huapi Formation overlies the Bajo Barreal Formation. These sedimentary units preserve a rich and diverse vertebrate fossil record including, among others, representatives of Crocodylomorpha, Testudines, Pterosauria and abundant Dinosauria. Nevertheless, the stratigraphic position of several of its taxa has been historically controversial. The unclear stratigraphic provenance of these taxa diffi cults the correct interpretation of the relationships with other Patagonian and South American basins. In this context, we present a detailed stratigraphic study to clarify the position of the vertebrate fossils of both Late Cretaceous formations. We also discuss the implications of this faunistic arrangement in terms of vertebrate evolution and paleobiogeography. Finally, this study broadens our knowledge on the fossil fauna of these units and therefore the vertebrate assemblages of central Patagonia.Key words: Chubut Group, Bajo Barreal and Lago Colhué Huapi formations, Late Cretaceous, vertebrate assemblages.RESUMO -O Grupo Chubut, Patagônia central, Argentina, é caracterizado por um sistema lacustre e fl úvio-lacustre, com uma participação variável de cinza vulcânica. Este grupo inclui a Formação Bajo Barrel (Cenomaniano-Turoniano) e uma nova unidade litoestratigráfi ca recentemente depositada, a Formação Lago Colhué Huapi (Coniaciano-Maastrichtiano). A Formação Lago Colhué Huapi se sobrepõe à Bajo Barreal. Estas unidades sedimentares contêm um rico e diversifi cado registro fóssil de vertebrados que inclui, entre outros, representantes de Crocodylomorpha, Testudines, Pterosauria e abundantes, Dinosauria. Entretanto, as posições estratigráfi cas de muitos destes táxons são historicamente controversas. A origem estratigráfi ca pouco clara desses táxons, difi culta a interpretação correta das relações de seus fósseis com a fauna de outras bacias patagônicas e sul-americanas. Sendo assim, dentro deste contexto, nós apresentamos um posicionamento estratigráfi co detalhado dos vertebrados fósseis das duas formações do Cretácio Superior e discutimos as implicações destes arranjos faunísticos em termos de evolução dos vertebrados e paleobiogeografi a. Por fi m, o presente estudo amplia nosso conhecimento sobre a fauna fóssil destas unidades e também das assembleias de vertebrados da Patagônia central. Palavras-chave:Grupo Chubut, formações Bajo Barreal e Lago Colhué Huapi, Cretáceo Superior, assembleias de vertebrados.

This paper reports on the biostratigraphic, paleobiogeographic and paleoecologic aspects of palynoflora recovered from sedimentary rocks preliminarily assigned to the Lago Colhué Huapi Formation, in the Golfo San Jorge Basin. The present palynological assemblage includes Maastrichtian marker species of the austral Proteacidites/Nothofagidites Province such as Quadraplanus brossus and Tubulifloridites lilliei. Typical species of the Maastrichtian paleotropical Palmae Province, Buttinia andreevi and Gabonisporis vigourouxii, are also recognized in the palynoflora. These facts indicate the mixed character of the paleoflora and the prevalence of transitional climatic conditions at the paleolatitude of the Chubut Province in the uppermost Cretaceous. The palynoflora composition suggests the local existence of a warm and low energy freshwater body.