Some scenarios of the Mesozoic continental and coastal biota in Argentina

APESTEGUÍA, S.; ARMELLA, C.; BONAPARTE, J.; CABALERI, N.; CAGNONI, A.M.; GARRIDO, A.; GASPARINI, Z.; GONZÁLEZ RIGA, B.; LEANZA, H. A.; LÓPE-ARBARELLO, A.; MANCUSO, A.; MARQUILLAS, R.; MARSICANO, C.; QUATTROCCHIO, M.; RAMOS, A.; RAUHUT, O.; SALFITY, J.; SILVA NIETO, D.; URIEN, C.; VALENCIO, S.; VOLKHEIMER, W.

Mendoza

Congreso; Gondwana 12, “Geological and Biological Heritage of Gondwana”; 2005

Gondwana 12 Conference, Organising Committe

The principal sedimentary basins fo the Argentinian Mesozoic are plotted on four paleogeographic maps of southern South America: a. Mid Triassic; b. Mid Jurassic, c. earliest Upper Cretaceous and d. latest Cretaceous. On these basic maps are indicated 20 scenarios of continental and coastal biota, which represent 20 time slices along the Mesozoic Era. Each of the scenarios is shown as a blockdiagram. An example is the Mid Triassic Chañares Formation of the Ischigualasto-Villa Union basin, whose important tetrapod assemblage includes dinosauromorphs (“dinosaur precursors”), not recorded, until now, elsewhere. The Mid Triassic Los Rastros Fm. of the same basin represents a cyclic lacustrine-deltaic environment. The lake margins are dominated by Ginkgoales, small Corystopermales and Sphenophyta; the river margin by Sphenophyta and the woodlands upstream in the floodplain with Corystospermales, and insects (Blattoptera, Homeoptera and Coleoptera) associated with shore-lake vegetation and woodlands of the fluvial system. Fish and temnospondyl amphibians inhabitate the affluent fluvial system. Therapsids, crurotarsal archosaurs and putative dinosaurs are represented by ichnites. The Ischigualasto succession (Late Triassic) is particularly important worldwide, as it includes the most complete and, among the oldest, dinosaur remains. In a silar way is treated the Cañadon Asfalto basin (Extraandean Patagonia, Chubut River valley), which yielded a diverse Jurassic tetrapod fauna, unique in South America, and remarkable for the Southern Hemisphere. Middle Jurassic (Callovian) dinosaurs from the mainly lacustrine Cañadon Asfalto Fm. are represented by basal Tetanura and Eurosauropoda, with no evidence of provincialisms. Conversely, the Late Jurassic dinosaurs from the fluvio-lacustrine Cañadón Calcáreo Fm. (Kimmeridgian-Tithonian), including one tetanuran theropod and three neosauropods (two macronarians and one dicraeosaurid), show incipient evolutionary differences from northern faunas. The nearly 800 km long Neuquén basin exhibits rich fossiliferous Jurassic and Cretaceous strata. From Early Callovian age, coastal marine reptiles (plesiosauroids: cf. Muraenosaurus sp. and cf. Clyptoclidus sp.) are presented in high energy environments of Chacaico Sur, whereas tidal flat in southern Mendoza (Calabozo Fm.) provided in- and seminfaunal bivalves, gastropods, echinoderms, brachiopods, bryozoans, forams, dasycladacean algae and halimedaceans covered by filaments of cyanobacteria. During the Late Cretaceous the Neuquén basin shows the most prolific gondwanan tetrapod record, which flourished in two palaeoecological scenarios. The former, the Neuquén Group (Cenomanian-Early Campanian) is a thick succession of alluvial fans, fluvial systems, dunes and playa-lake environments. In the northern region (Mendoza Province) the fluvial environments preserved huge saurópodos (Mendozasaurus), theropods, crocodiles, chelid turtles, coniferous wood and Scoyenia ichnofacies. In the southern region (Neuquén and Río Negro Provinces), diverse scenarios are summarized in the Candeleros, Portezuelo and Bajo de la Carpa Fms. Particularly, the fluvial braided system, dunes and playa-lake of the Candeleros Fm., yielded the Limayan assemblages (Cenomanian to Early Turonian), with large basal titanosaurs (Andesaurus), rebbachisaurids, tetanurans (Giganotosaurus) and iguanodontians, as well as small abelisaurs, dromaeosaurids, araripesuchids, snakes, sphenodontids, and chelids. The meanderous and sandy fluvial system of the humid Portezuelo Fm. include large Neuquenian titanosaurs and tetanurans, and smaller abelisaurs, alvaresaurids, dromaeosaurids, and ornithischians. The meanderous sandy fluvial system of the arid Bajo de la Carpa Fm. includes the last endemic Cretaceous South American faunas (Coloradoan), with diverse titanosaurs, birds (Neuquenornis), alvarezsaurids, abelisaurs, ziphosuchian crocodiles, snakes, and turtles. The second scenario, the Malargue Group (Late Campanian-Paleocene) includes fluvial, lacustrine, littoral and marine facies related to a first Atlantic ingression over northern Patagonia, yielding diverse vertebrates (fiches, ´leptodactylid´ frogs, turtles, snakes, dinosaurs and plesiosarus) including freshwather and littoral taxa. I is represented in the northern region by the Loncoche Fm. and, in the southern region, by the Allen Fm. and equivalents. It also yielded saltasaurines, carnotarines (Carnotaurus), dromaeosaurids, birds, sphenodontids, and mammals, as those from the Alamitos Fm., depicting a different evolutionary history from that of the Northern Hemisphere. Probable northern immigrants (ornithischians) preceded the mammalian faunal invasion that would be part of the Cenozoic terrestrial ecosystems.