Diatom and sedimentological analyses of two cores from San Blas Bay and the analysis of historical charts provide information about the environmental evolution of this estuarine complex during the last 5000 yr BP. The Holocene sea-level regression is recorded in San Blas Bay by infilling of a tidal channel. Two sediment cores were drilled at Arroyo Jabalí (San Blas Bridge) and at the former inlet of Arroyo Walker and Arroyo Jabalí (Paso Seco). The diatom record of the San Blas Bridge sequence is dominated by marine and brackish/freshwater plankton and tychoplankton and composed by fine to very fine sand and mud characterizing the infilling of a tidal channel. The Paso Seco sequence is more abundant in sand. Textural and diatom facies are interpreted as the infilling of a tidal channel very similar to the one at San Blas Bridge, that evolved towards the top into a sandflat characterized by the dominance of marine and marine-brackish epipsammon accompanied by marine tychoplankton. Arroyo Walker and Arroyo Jabalí were flowing eastward across a tidal channel during the Holocene. Because of obstruction of the channel, the tidal channel assemblages were replaced by sandflat assemblages. According to historical charts, as late as 1833 a tidal inlet still existed in this sector, which was later obstructed completely according to Twentieth Century charts. At this timescale, eustasy is considered to be a minor environmental control on the dynamics of the system. Instead, control is forced by other factors, such as inlet instability due to morphological changes of the sand barrier whether emergent or submerged.

REGISTRO DIATÓMICO Y SEDIMENTARIO DE LA EVOLUCIÓN DEL COMPLEJO ESTUARINO SAN BLAS DURANTE EL HOLOCENO MEDIO, PATAGONIA NORTE, ARGENTINA. El análisis de las diatomeas y la sedimentología de dos testigos obtenidos en la Bahía San Blas, y la comparación de cartas náuticas, han aportado información sobre la evolución ambiental de este complejo estuarino durante los últimos 5000 años AP. El relleno de canales de mareas representa la regresión marina del Holoceno en la Bahía San Blas. Se obtuvieron dos testigos, en el arroyo Jabalí (Puente San Blas) y en la antigua desembocadura de los arroyos Walker y Jabalí (Paso Seco). El registro diatómico de la secuencia Puente San Blas está dominado por plancton y ticoplancton marino y salobre/dulceacuícola y compuesto por arena fina a muy fina y fango, caracterizando el relleno de un canal de mareas. Los registros diatómicos y sedimentarios de la secuencia Paso Seco se interpretan como la colmatación de un canal de mareas muy similar a la secuencia de Puente San Blas, que evolucionó hacia una planicie arenosa en el tope, caracterizada por epipsammon marino y marino-salobre acompañado por ticoplancton marino. Durante el Holoceno, los arroyos Walker y Jabalí desaguaban hacia el este, a través de un canal de mareas. El proceso de obstrucción del canal (en Paso Seco) hizo que las asociaciones características de canales de mareas fueran reemplazadas por las típicas de planicies arenosas. De acuerdo a cartas antiguas, en 1833 aún existía una desembocadura de mareas en este sector, que se obstruyó completamente según cartas del Siglo XX. En ese momento, la eustasia habría sido un factor de control ambiental menor en este sistema. Un factor importante es la inestabilidad de la desembocadura debida a cambios morfológicos de la barrera medanosa emergida o sumergida.

POSTCRANIAL MATERIAL OF PROTEROCHAMPSA BARRIONUEVOI REIG, 1959 (DIAPSIDA: ARCHOSAURIFORMES) FROM THE UPPER TRIASSIC OF CENTRAL-WESTERN ARGENTINA. Proterochampsids are members of the clade Archosauriformes, a group distinguished from others because of its depressed skulls transversely expanded at the posterior end, narrow and longitudinally long snout, nares located close to the midline, absence of postfrontals, and presence of an assymmetric pes. The family Proterochampsidae includes five genera recorded in Triassic formations of Argentina and Brazil. In this family is included the Argentinean species Proterochampsa barrionuevoi Reig, from the Late Triassic Ischigualasto Formation. This taxon was known from skulls and cervical vertebrae, but the rest of the postcranium remained unknown until now. Herein, a new and almost complete specimen of Proterochampsa barrionuevoi is described in detail (PVSJ 606). The specimen includes skull, complete vertebral series, scapulae, coracoids, pelvic girdle, right humerus, right radius and ulna, both femora, and complete right hindlimb. An emended diagnosis considering neurocranial features —semilunar depression ventrolaterally exposed, basisphenoidal fossa surround by a rostrolateraly V-shaped ridge with convex branches— is provided. The features distinguishing P. barrionuevoi from the Brazilian species P. nodosa are: snout becoming narrow anteriorly in a less gradual manner than in P. nodosa, lower occiput, nares lanceolate with narrow anterior and posterior ends, and frontal less irregular that in P. nodosa.

The new insect species Platyperla marquati sp. nov. described herein is represented by a nymphal stage and belongs to the Order Perlida (= Plecoptera) Latreille (stoneflies). It comes from the uppermost section of the Potrerillos Formation, which crops out south of Cerro Cacheuta, at the southernmost end of the Precordillera (Mendoza Province, Argentina). The early Late Triassic Potrerillos/Cacheuta sedimentary succession in this area includes fluvial, deltaic and lacustrine facies that reflect deposition in the border of the Cacheuta depocenter of the Cuyo Basin. This is the second complete insect and the first autochthonous aquatic insect from Triassic beds in Argentina, and also the first record of the family Platyperlidae in Gondwana. This finding demonstrates the similarity of aquatic insect faunas in Mesozoic deposits all over the world, at least of the morpho-ecological types of the aquatic stages. The lacustrine insect fauna began evolving during the Triassic and became diverse during the Jurassic and Early Cretaceous. Detailed geological studies revealed that levels equivalent to the middle Triassic lower units of the Uspallata Group (Río Mendoza and Cerro de Las Cabras formations) lie exposed on the southeastern flank of Cerro Cacheuta. Therefore, a new geological map and interpretation for this area are also presented.

BODY-MASS ESTIMATION OF SPARASSODONTA SPECIES (MAMMALIA, METATHERIA) OF SANTACRUCIAN AGE (EARLY MIOCENE) BASED ON CENTROID SIZES OF POSTCRANEAL ELEMENTS: PALAEOECOLOGICAL INFERENCES. Sparassodonts have been described as a group with a large body-mass diversity, but few works had focused on body mass estimations. In this contribution, we calculated the centroid size of the anterior view of the distal humerus, lateral view of the proximal ulna, and proximal articular view of the tibia. This data together with the known body-mass for many current predators (marsupials and placentals) allowed us to derive new allometric equations to estimate body size in the Sparassodonta. We used comparative analyses to quantify the effect of the phylogenetic structure. The relationships between body mass, centroid size, and locomotory habits were explored. The phylogenetic effect was significant for all regressions. In the humerus analysis, we found an important relationship between body mass and locomotory habit, suggesting adaptive causes. We used the ulna and tibia regressions to predict body mass, since they were less influenced by locomotory habits. Our equations produced the following masses: Arctodictis sinclari Marshall, 39.9 kg; Borhyaena tuberata Ameghino, 36.4 kg; Cladosictis patagonica Ameghino, 6.6 kg; Lycopsis longirostrus Marshall, 29.77 kg; Prothylacynus patagonicus Ameghino, 31.79 kg; Sipalocyon gracilis Ameghino, 2.11 kg; and Thylacosmilus atrox Riggs, 117.36 kg. For Borhyaena and Thylacosmilus, we obtained values higher than previously published masses, while for Pseudonotictis pusillus Ameghino, we have made the first body mass estimation of 1.17 kg. The results suggest that the guild of the santacrucian predators was structured by the body mass and other variables, such as diet and locomotion.

The stratigraphy and palynology of borehole N°254 (DINAMIGE, Dirección Nacional de Geología y Minería) are described. This borehole is located in Tacuarembó Department, northeastern Uruguay. It cuts through —in ascending stratigraphic order— the San Gregorio, Melo and Yaguarí formations, late Carboniferous to Permian in age. Four palynological samples were studied and 84 species were identified. The two stratigraphically lowest samples are dominated by well-known sphenophyte, filicophyte and lycophyte species, such as Calamospora hartungiana Schopf in Schopf, Wilson and Bentall, Punctatisporites gretensis Balme and Hennelly, Leiotriletes virkkii Tiwari and Lundbladispora braziliensis Marques-Toigo and Picarelli, with less abundant gymnosperms, such as Caheniasaccites ovatus Bose and Kar emend. Gutiérrez 1993. This relative abundance relationship is gradually reversed up-section, where the gymnosperm species dominate, e.g., Lueckisporites virkkiae (Potonié and Klaus) emend. Clarke, Vittatina costabilis Wilson, Pakhapites ovatus (Bose and Kar) Playford and Dino, Vittatina vittifera (Luber) Samoilovich and Lunatisporites variesectus Archangelsky and Gamerro. This change in the palynoflora —together with shifts in sedimentary facies— is consistent with the climatic amelioration of Gondwana from peri-glacial conditions in the late Carboniferous to warm arid conditions by the middle Permian. The comparison of palynological assemblages with regional palynostratigraphic schemes suggests an age between the late Carboniferous/early Permian boundary for the San Gregorio Formation and the end of the early Permian for the Melo Formation.

SALINITY FLUCTUATIONS DURING THE HOLOCENE IN THE MAR CHIQUITA COASTAL LAGOON (BUENOS AIRES PROVINCE, ARGENTINA): A DIATOM-BASED QUANTITATIVE APPROXIMATION. Diatom assemblages recovered from a sediment core obtained from the north of Mar Chiquita coastal lagoon (Buenos Aires Province) were studied, in order to characterize the paleosalinity fluctuations during the Holocene. Data were analyzed quantitatively by means of the application of a paleosalinity transfer function available for estuaries of the southeastern Buenos Aires Province. Two main stages during the evolution of the lagoon were inferred. Between c. 8670 and 2500 yr ¹⁴C BP, marine planktonic and tychoplanktonic diatoms, such as Paralia sulcata (Ehr.) Cleve and Cymatosira belgica Grunow, indicate the presence of an environment subject to tidal action, with salinities ranging between 20 and 30‰. Towards c. 2500 years ¹⁴C BP the assemblages changed abruptly, being dominated by the tychoplankters Staurosira construens var. venter (Ehr.) Hamilton, Staurosirella pinnata (Ehr.) Williams & Round and Pseudostaurosira brevistriata (Grun.) Williams and Round, that also dominate in modern sediments of the sampling site. A significant decrease of the marine influence that culminated with the establishment of modern environmental conditions, with salinities lower than 7‰, was interpreted.

DERMAL ARMOUR ELEMENTS OF THE ANKYLOSAUR DINOSAUR POLACANTHUS OWEN, 1865, FROM THE LOWER CRETACEOUS OF MORELLA (CASTELLÓN, SPAIN). Isolated dermal armour elements of an ankylosaur from the lower Aptian Arcillas de Morella Formation at Morella (Castellón, Spain) are described herein. These elements are presacral and caudal spines, fragments of sacropelvic shield, keeled scutes and keeled ossicles. The possession of ungrooved presacral spines and a sa-cropelvic shield composed of irregularly arranged bosses and small tubercles are some of the features shared with Polacanthus. Thus we tentatively refer these elements to Polacanthus sp. This assemblage of Polacanthus dermal armour elements is currently the most important outside England. Furthermore, the new evidence of this ankylosaur in the Iberian record corroborates the great similarity between the Barremian—Aptian dinosaur faunas both the British and Iberian records.

THE TRIASSIC PALEOFLORA OF CERRO CACHEUTA, MENDOZA PROVINCE, ARGENTINA. PETRIELLALES, CYCADALES, GINKGOALES, VOLTZIALES, CONIFERALES, GNETALES INCERTAE SEDIS GYMNOSPERMS. Four stratigraphic logs were measured in detail at the Cacheuta hill (northwestern Mendoza Province, Argentina), and seven plantiferous levels were determined at Puesto Míguez and Agua de las Avispas. The main purpose of this paper is the systematic study of the paleoflora found in these secctions; the analized taxa belong to the Gymnospermopsida: Petriellales, Cycadales, Ginkgoales, Voltziales, Coniferales, Gnetales and Gymnospermophyta incertae sedis. The systematic study includes the following 25 identified taxa: Rochipteris truncata (Frenguelli) comb. nov., Nilssonia taeniopteroides Halle, Kurtziana brandmayri Frenguelli, K. cacheutensis (Kurtz) Frenguelli, Pseudoctenis falconeriana (Morris) Bonetti, P. spectabilis Harris, Baiera cuyana Frenguelli, B. rollerii Frenguelli, Ginkgoidium bifidum Frenguelli, Sphenobaiera argentinae (Kurtz) Frenguelli, Heidiphyllum elongatum (Morris) Retallack, Telemachus elongatus Anderson, T. lignosus Retallack, Rissikia media (Tenison-Woods) Townrow, Cordaicarpus sp., Gontriglossa sp., Yabeiella brackebuschiana (Kurtz) Ôishi, Y. mareyesiaca (Geinitz) Ôishi, Y. spathulata Ôishi, Y. wielandi Ôishi, Fraxinopsis andium (Frenguelli) Anderson y Anderson, F. major Wieland, F. minor Wieland, Linguifolium lilleanum Arber, L. steinmannii (Solms-Laubach) Frenguelli.

Non-marine calcareous microfossils (ostracods and charophytes) and primitive angiosperm seeds (Carpolithus sp.) from the Piedra Clavada Formation (early Albian to early late Albian) in its type area, at Quebrada Don Nielsen section, Santa Cruz Province, Argentina are described and illustrated for the first time. The microfauna suggests a marshy lagoon paleoenvironment. Affinities with Lower Cretaceous microfossil assemblages from the Golfo San Jorge Basin are included.

A new Cardiomyinae rodent (Hydrochoeridae) from Arroyo Seco de la Frazada, Mendoza Province, Argentina, is described. The new species, Caviodon cuyano sp. nov., is distinguished by its greater robustness, deeper fissures, and greater number of prisms in M3, which are derived characters within the context of Hydrochoeridae evolution. An assessment of the species of Caviodon Ameghino showed that tooth growth is allometric and occlusal morphology becomes more complex during ontogeny, as in hydrochoerines. The true diversity of this genus, when tooth postnatal ontogeny is considered, may be less than heretofore estimated. The fossil-bearing levels, the middle section of the Aisol Formation, have been considered no older than Huayquerian in age (late Miocene). The evolutionary stage of the new species suggests that the age of these sediments could be even younger, no older than early Pliocene and possibly up to late Pliocene.

PALEOFLORA FROM THE LOS RASTROS FORMATION (MIDDLE TRIASSIC), SAN JUAN PROVINCE, ARGENTINA. TAXONOMIC AND TAPHONOMIC CONSIDERATIONS. The taxonomy and taphonomy of the paleoflora from the Los Rastros Formation (Middle Triassic) San Juan Province, Argentina, is described. The taphofloristic analysis is made considering five fosiliferous levels of sections from Quebrada Agua de la Peña and Quebrada del León. The described taxa belong to the Orders Equisetales, Osmundales, Corystospermales, Peltaspermales, Ginkgoales, Coniferales, and to the Pteridophylla group (incertae sedis leaves). Thirty one taxa are described, 19 of them recorded for the first time for the formation. Generic and specific diversity as well as taphonomic attributes were observed at the different beds studied thus allowing to characterize the depositional environment and the paleofloristic composition. The Los Rastros Formation is composed of several successive cycles of black shales, mudstones and sandstones, corresponding to a lacustrine-deltaic environment, with autochtonous and allochtonous plant remains originated in marginal and upstream areas of this fluvial system.

Well-preserved palynological assemblages have been obtained from the Guandacol Formation at Río Francia Creek, Paganzo Basin, Argentina. The geological setting and stratigraphic provenance of the samples is provided. Forty-nine spore, twenty-three pollen and two algal species are identified. Some species are recognized for the first time from this unit, such as: Brevitriletes levis, B. leptoacaina, Lundbladispora areolata, Peppersites ellipticus, Psomospora detecta, Rattiganispora apiculata and Retusotriletes nigritelus. These assemblages are referred to Subzone A of the Raistrickia densa-Convolutispora muriornata Biozone, late Serpukhovian—early Bashkirian in age. Palynological samples from the Tupe Formation at the same locality were studied in order to compare the composition of both units, which present a palynological composition similar to the associations recovered from the Guandacol Formation. Spelaeotriletes ybertii is restricted to the palynological assemblages of the Tupe Formation studied herein. It is noteworthy that both units lack taeniate pollen. Some species restricted at this locality to the Guandacol Formation, have a wider stratigraphic distribution elsewhere. The only species from this unit that seems to have biostratigraphic significance is Tricidarisporites gutii. A new species of trilete spores is proposed, i.e., Cyclogranisporites cuerdae sp. nov. from the Tupe Formation.

MORPHOLOGY AND ANATOMY OF A FERN GROWING UNDER STRESS CONDITIONS FROM THE APTIAN OF SANTA CRUZ, ARGENTINA. The morphology and anatomy of pteridophyte fronds attached to rhizomes with adventitious roots from the Aptian Anfiteatro de Ticó Formation, Santa Cruz province, Argentina are described. The fossils consist of impressions and compressions preserved in lithofacies dominated mainly by volcanic ash-fall. Observations made with scanning electron microscope on rhizomes and rachides revealed the presence of primary xylem tissues; protoxylem, metaxylem fibers and parenchyma. According to the general morphology, this fern is included in the morphogenus Cladophlebis Brongniart. Comparisons made with known Cladophlebis species of Argentina show that the material here studied has different morphological features. Furthermore, rachides in organic connection to rhizomes having a well preserved anatomy was not reported for Cladophlebis species from Argentina. Therefore, a new name, Cladophlebis cinerium, is assigned to this fern. Finally, it is suggested that this fern probably grew under humid conditions given by the fluvio-lacustrine environment of the Anfiteatro de Ticó Formation, but also with continuous stress caused by the abundant ash fall produced by the recurrent explosive andean vulcanism that took place during the Lower Cretaceous in Patagonia.

SILURIAN MARINE ORGANIC-WALLED PHYTOPLAKTON AND MIOSPORES OF THE LOS ESPEJOS FORMATION, IN THE RIO DE LAS CHACRITAS SECTION, SAN JUAN PRECORDILLERA, ARGENTINA. Marine organic-walled phytoplankton (acritarchs and chlorophytes) and miospores were recorded from nine productive levels of the lower to middle section of the Los Espejos Formation, in the Río de Las Chacritas stratigraphic column, Central San Juan Precordillera. The marine phytoplankton is highly diversified and dominates the palynological assemblage through the section, with many species recorded for the first time in the Silurian of Argentina. Trilete spores and cryptospores assemblages are poorly diversified. In spite of the fact that most of the species display long stratigraphic ranges, palynological assemblages allow constraining the age of the Los Espejos Formation to the Ludlow. The presence of miospores, which first appeared in younger strata in other regions, could be interpreted as related to particular palaeoenvironmental conditions of this basin. The fitoplankton shows a strong correlation with coeval assemblages from Great Britain and important similarities with assemblages from Sweden, Spain and North Africa; thus demonstrating the lack of important biogeographic barriers and supporting a more cosmopolitan character of the late Silurian assemblages. The relationship between the acritarch and chlorophyte relative abundances shows important fluctuations throughout the studied section. Levels with greater abundance of clorophytes match the more prominent changes of sedimentary facies. Consequently, variations in the phytoplankton distribution could be revealing a strong dependence on local palaeoenvironmental factors.