New data on the morphology of Sphenothallus Hall: implications for its affinities

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73676/1/j.1502-3931.1992.tb01378.x.pd

A New Basal Sauropodomorph (Dinosauria: Saurischia) from Quebrada del Barro Formation (Marayes-El Carrizal Basin), Northwestern Argentina

BACKGROUND: Argentinean basal sauropodomorphs are known by several specimens from different basins; Ischigualasto, El Tranquilo, and Mogna. The Argentinean record is diverse and includes some of the most primitive known sauropodomorphs such as Panphagia and Chromogisaurus, as well as more derived forms, including several massospondylids. Until now, the Massospondylidae were the group of basal sauropodomorphs most widely spread around Pangea with a record in almost all continents, mostly from the southern hemisphere, including the only record from Antarctica. METHODOLOGY/PRINCIPAL FINDING: We describe here a new basal sauropodomorph, Leyesaurus marayensis gen. et sp. nov., from the Quebrada del Barro Formation, an Upper Triassic-Lower Jurassic unit that crops out in northwestern Argentina. The new taxon is represented by a partial articulated skeleton that includes the skull, vertebral column, scapular and pelvic girdles, and hindlimb. Leyesaurus is diagnosed by a set of unique features, such as a sharply acute angle (50 degrees) formed by the ascending process of the maxilla and the alveolar margin, a straight ascending process of the maxilla with a longitudinal ridge on its lateral surface, noticeably bulging labial side of the maxillary teeth, greatly elongated cervical vertebrae, and proximal articular surface of metatarsal III that is shelf-like and medially deflected. Phylogenetic analysis recovers Leyesaurus as a basal sauropodomorph, sister taxon of Adeopapposaurus within the Massospondylidae. Moreover, the results suggest that massospondylids achieved a higher diversity than previously thought. CONCLUSIONS/SIGNIFICANCE: Our phylogenetic results differ with respect to previous analyses by rejecting the massospondylid affinities of some taxa from the northern hemisphere (e.g., Seitaad, Sarahsaurus). As a result, the new taxon Leyesaurus, coupled with other recent discoveries, suggests that the diversity of massospondylids in the southern hemisphere was higher than in other regions of Pangea. Finally, the close affinities of Leyesaurus with the Lower Jurassic Massospondylus suggest a younger age for the Quebrada del Barro Formation than previously postulated

A General Overview of Gondwana Landscapes in Argentina

Gondwana Landscapes in Argentina were already identified by Juan Keidel and Walther Penck at the beginnings of the twentieth century, as well as by other geologists and naturalists of the different European schools that worked in this country. These studies were continued at a very good level in Brazil, thanks to the work of Lester C. King, later on intensively followed by João José Bigarella. However, these concepts gradually disappeared from the Argentine geological scene, dominated by the influence of American geomorphologists, and particularly William Thornbury, who doubted the existence of such ancient landforms, when one of the paradigms of the time was that “practically there is no landscape older than the Pleistocene.” These landforms are the result of the process of both deep chemical weathering, developed in very stable tectonic and climatic environments, under hyper-tropical climates, and pediment processes in semiarid to humid environments. The Gondwana Landscapes or their fragmented remnants have been recognized in Argentina, from north to south, in the basaltic hills of the province of Misiones; the Sierras Pampeanas of the provinces of Catamarca, La Rioja, and San Juan; the Sierras Chicas, Sierras Grandes, and Sierra Norte of Córdoba province; the Sierras de San Luis, the Sierra Pintada, or San Rafael Block of Mendoza province; the Sierras de Tandil, Sierra de la Ventana, and the Pampa Interserrana of Buenos Aires province; the Sierras de Lihuel Calel of the province of La Pampa; the Somuncurá or Northern Patagonian Massif in the provinces of Río Negro and Chubut; the Deseado Massif of Santa Cruz province; and the Malvinas-Falklands archipelago. In other regions of Argentina, these surfaces have been downwarped in tectonic basins and are covered by sedimentary and/or volcanic units of various ages. The ages for the development of the Gondwana Landscapes have been estimated in between the Middle Jurassic and the Paleogene. The Argentine Gondwana Landscapes were uplifted, fragmented, and eroded during the Middle to Late Tertiary. They have remained as mute testimony of the past above extensive pediplains and piedmont deposits, as climates and environments became more arid and cooler, approaching the present conditions.Fil: Rabassa, Jorge Oscar. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Austral de Investigaciones Científicas; Argentina. Universidad Nacional de Tierra del Fuego; ArgentinaFil: Carignano, Claudio Alejandro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; ArgentinaFil: Cioccale, Marcela Alejandra. Universidad Nacional de Córdoba; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin