Los Rastros Formation Research Articles

Plant reproductive structures of the Agua de la Zorra and Los Rastros formations, Triassic, Argentina

The Triassic paleofloras of Argentina are mainly known for their vegetative elements. During the last decade, several contributions have focused on the study of Triassic reproductive structures. Here, we provide new information on reproductive structures recovered from two Triassic units from the central-west of Argentina: the Agua de la Zorra Formation (Cuyana Basin) and the Los Rastros Formation (Ischigualasto-Villa Unión Basin). These specimens had been described in previous contributions, however, the taxonomic assignment of the materials remained undetermined. In this work, a redescription of the materials is provided, allowing the taxonomic assignment of the specimens. The Agua de la Zorra Formation female reproductive structures were assigned to Matatiella sp. (Matatiellaceae) and to Rissikiostrobus reductus (Podocarpaceae). The female strobilus specimen registered in the Los Rastros Formation was assigned to the genus Umkomasia (Umkomasiaceae). This contribution represents a step forward to a better understanding of the reproductive structures for the Triassic of Gondwana. Keywords: Matatiella, Rissikiostrobus, Umkomasia, Gondwana, Argentina.

Tectonically conditioned record of continental interior paleoclimate during the Carnian Pluvial Episode: The Upper Triassic Los Rastros Formation, Argentina

Abstract Discerning paleoclimate parameters in depositional systems of the continental interior is challenging because the system response and stratigraphic record of climate are controlled by tectonic processes and are mediated through landscape and hydrological evolution of fluvial lacustrine systems. Climate and tectonic signals cannot be deconvolved from stratigraphic patterns alone but require additional information or data sets that directly record climate or tectonic influence. The Carnian Los Rastros Formation in northwest Argentina provides an excellent case study that integrates an appropriate range of information in a system with strong climate and tectonic signals, being deposited in part during the Carnian Pluvial Episode and spanning the active rift phase of the Ischigualasto–Villa Unión Basin. We examined the stratigraphic and spatial patterns of carbon (C) and oxygen (O) stable isotopes in lacustrine carbonates from the Los Rastros Formation in multiple parts of the basin to constrain paleohydrological conditions and paleotemperatures. Practically all C and O isotope values are characterized by negative values: δ18Ocarb –11.6‰ and –15.7‰ (χ average –13.1‰; 1σ = 1.6) and δ13Ccarb –2.6‰ to –8.0‰ (χ average –5.1‰; 1σ = 2.1), reflecting the latitude, altitude, and continentality of the lake system and its vegetated and humid catchment area. Stratigraphic patterns of stable isotope data from two different localities (Cerro Bola North and Cerro Bola South) show a change from short water-residence time to long residence time and back to short residence time. This contrasts with sedimentologic, organic geochemical, and small-scale stratigraphic patterns that indicate an overfilled lake basin, which is expected to contain a completely open-hydrology isotopic signature. Paleotemperatures calculated from marginal lacustrine carbonates show a warm and quite variable paleothermal range consonant with their continental interior position and with Global Climate Models estimates for high paleolatitudes. Warmer paleotemperatures (linked to aridity, probably smaller lake size, and less thermal mass) precede the Carnian Pluvial Episode, whereas relatively cooler paleotemperatures coincide with the Carnian Pluvial Episode (linked to humidity, probably larger lake size, and more thermal mass). Carbon and oxygen stable isotope signatures integrated with sedimentologic and physiographic information allow us to propose that tectonics, specifically, half-graben tilting during the active synrift phase, dominated over climate effects as the cause of hydrological fluctuations of this system, even during the Carnian Pluvial Episode. Without appropriate stratigraphic-tectonic context, single-proxy reconstructions of continental-interior paleoclimate can be misleading. A robust interpretation of climate effects requires characterization of tectonic effects, geomorphology, paleohydrology, and sedimentary system responses.

The influence of volcanic activity and trophic state on plant taphonomic processes in Triassic lacustrine-deltaic systems of western Gondwana

The impact of volcanic activity and trophic state on the preservation of plant remains from two Triassic palaeolake deposits in western Gondwana is investigated. Six taphonomic modes are identified for the Potrerillos–Cacheuta sequence and the Los Rastros Formation, along with proposed taphonomic pathways for each mode. The taphonomic pathways were compared with those proposed for the Agua de la Zorra Formation, a coeval lacustrine-deltaic succession. In all three cases, plant preservation in off-shore lacustrine facies was enhanced by anoxic bottom conditions. In the more proximal near-shore facies, a high sedimentation rate would have been factor that enhanced the preservation. Variation in relative abundance and taxonomic diversity between the three successions are attributed to ecological differences rather than taphonomic differences among the lake systems. No evidence was recorded that the preserved plant material was produced by volcanic trauma. The preservation of the cuticle recorded in the Cacheuta Formation was likely enhanced by the apparently low thermal maturation of the rock and the presence of tuffaceous horizons. In the three successions, the abundance and distribution of remains throughout the sections suggest that the trophic state of the lake did not influence the preservation of the plant.

Evidence for the Carnian Pluvial Episode in Gondwana: New multiproxy climate records and their bearing on early dinosaur diversification

The Triassic hothouse world experienced dynamic changes in climate and ecosystems that set the Earth's climate and biotic states for much of the Mesozoic Era. A critical time interval was the Carnian Stage (~237–227 Ma), which not only saw the first appearance of dinosaurs in the fossil record but witnessed a large igneous province eruption (Wrangellia LIP) and a sudden climate shift, the Carnian Pluvial Episode (CPE). Compelling hypotheses suggest that the Wrangellia eruptions caused the CPE, which in turn set the stage for the origin and initial diversification of dinosaurs. Unfortunately, testing these hypotheses is difficult because most relevant Carnian sedimentary archives possess few if any absolute age constraints, and there are little detailed paleoclimatic data for the CPE outside of the Tethys region. There is little existing evidence for the CPE in Gondwana, even though this region contains by far the best fossil record of early dinosaurs and their close relatives. We present new high-resolution geochronologic and paleoclimatic data from Carnian-aged strata in the Ischigualasto-Villa Unión Basin of northwest Argentina, which also preserves fossils of the earliest-known dinosaurs. New CA-TIMS UPb zircon age of 234.47 ± 0.44 Ma from the lower Los Rastros Formation demonstrates that most of the overlying lacustrine strata within this basin was deposited during or after the CPE, including large dinosauriform footprints. Multiproxy paleoenvironmental data (sedimentology, clay mineralogy, C and O stable isotopes, and fossils) from the same strata are the first detailed paleoclimate data for the CPE in Gondwana, and provide evidence that the CPE interval in western Gondwana was indeed warmer and more humid than before or after. These data are consistent with the interpretation of the CPE as a global event, but direct linkages in Gondwana and the Tethys with the origin and initial diversification of dinosaurs are less clear.

Paleobotany of the Upper Triassic Los Rastros Formation, Ischigualasto–Villa Unión Basin, La Rioja, Argentina

The Los Rastros Formation was deposited in a deltaic-lacustrine environment during the Late Triassic. Plant remains from the Gualo area were systematically collected and compared with other localities from the Los Rastros Formation concerning the sedimentary evolution of the basin. Subsequently, the Los Rastros paleoflora was contrasted with other Carnian (Upper Triassic) units from Gondwana in a paleobiogeographic context using statistical methodologies: Raup-Crick similarity index and hierarchical cluster analysis. The systematic study revealed 14 taxa for the Los Rastros Formation at the Gualo area. Plant vegetative remains of Equisetales (Neocalamites sp.), Osmundaceae (Cladophlebis sp. cf. C. mesozoica), Umkomasiales (Dicroidium odontopteroides, Johnstonia stelzeriana, Xylopteris argentina, X. elongata, and Zuberia feistmantelii), Ginkgoales (Baiera pontifolia, Sphenobaiera sp. cf. S. argentinae), Voltziales (Heidiphyllum elongatum), Coniferales (Rissikia media), and gymnospermous reproductive structures (Cordaicarpus sp., Samaropsis sp. and strobilus sp. A). The stratigraphic distribution of taxa throughout the Los Rastros Formation in the Gualo section is variable. Diminution of taxa recorded observed across the section could be explained by the isolation of the paleolake product of the activity of a secondary fault. The plant taxa registered at the different localities of the Los Rastros Formation could be related to the location of each locality in the basin and the different depositional conditions during the infill history of the basin at each locality. Comparison among Los Rastros Formation plant records with other paleofloras from Gondwana by statistical analyses indicates differences with respect to a previous paleobiogeographic proposal.

Paleolimnology evolution in rift basins: the Ischigualasto–Villa Unión Basin (Central-Western Argentina) during the Triassic

Extensional basins offer a high preservation potential for sedimentary and fossil records. Nonmarine rift basins contain diverse environments, including fluvial and lacustrine. Lake deposits have long been recognized as significant archives of the paleoenvironmental record. The Ischigualasto–Villa Unión Basin represents the only nearly continuous continental Triassic record with up to 6000m of alluvial, fluvial, and lacustrine deposits. The deep lacustrine interval is known as the Los Rastros Formation and consists of several lacustrine–deltaic cycles. A detailed facies analysis allowed the characterization of five facies associations and two types of cycles: (1) a shoreline-dominated, progradational Type I cycle containing Facies association A (offshore lacustrine deposits) and Facies association B (marginal palustrine deposits along the shoreline of the lake) or Facies association C (lacustrine siliciclastic coastal deposits) and (2) a delta-dominated, progradational Type II cycle characterized by Facies association A (offshore lacustrine and prodelta deposits), Facies association D (delta front deposits), and Facies association E (delta plain deposits). The sedimentary evolution of the Chañares–Los Rastros sequence is defined within this extensional setting as one depositional sequence with three system tracts (lowstand, transgressive, and highstand system tract). Biotic, abiotic, and sedimentologic data allowed the interpretation of the limnologic dynamics of the Los Rastros paleolake as a deep freshwater lake within a half-graben, exhibiting meromixis, and showing a change from moderate eutrophic conditions with low oxygen content to relatively oxygenated oligotrophic conditions. The successive fluctuations recorded in lake level are considered in the sequence stratigraphic framework of Bohacs et al. to reflect the relative balance of sediment and water input versus accommodation in an overfilled lacustrine basin.

Taphonomic analysis of fish in rift lacustrine systems: Environmental indicators and implications for fish speciation

Two rift-basin lacustrine successions (Cacheuta and Los Rastros Formations) in the Middle Triassic of Argentina were studied to test two different hypotheses: 1) fish taphonomy is useful as an indicator of environmental conditions for these two paleolakes; and 2) fish taphonomy is useful for understanding environmental effects on speciation. The fish fossils are found, in two lacustrine deposits, as isolated and disarticulated scales and cranial bones, clumps and stringers of scales, articulated body segments, and complete specimens. Four taphofacies are defined for Cacheuta specimens (Ca-A to Ca-D), and three taphofacies are defined for Los Rastros fish assemblages (LR-A to LR-C). A Principal Components Analysis was performed, with the taphonomic features supporting the seven taphofacies defined.A taphonomic model is proposed that characterizes a lacustrine-deltaic environment developed in the rift basin and identifies three sub-environments, each one with its particular fossil assemblages. The fish assemblages in the Los Rastros Formation suggest that the fishes preserved in delta-front and turbidity-current deposits (LR-B and LR-C taphofacies) were allochthonous to the lake and probably inhabited the delta plains and affluent fluvial system. In contrast, the isolated fish remains, preserved in open-lake deposits (LR-A taphofacies), likely represent both autochthonous and/or allochthonous specimens. In the Cacheuta Formation, articulated and isolated fish remains preserved in open lake deposits (Ca-A and Ca-D tafofacies), might represent autochthonous lacustrine specimens, based on the good preservation of the material. The fish remains preserved in underflow and turbidity current deposits (Ca-B and Ca-C taphofacies) were allochthonous to the lake and probably inhabited the delta plain and affluent fluvial system.The fish taphonomic study of two different rift-basin lacustrine successions allows test of the two proposed hypotheses. Fish taphonomy was useful as an indicator of environmental conditions beacuse the differences in fish preservation allowed differentiation of the sub-environments and reconstruction of the environmental conditions and dynamics of the rift lake basins. Also, fish taphonomy was useful for understanding environmental effects on speciation by characterizing the environments and dynamics that promote the development of new fish species, recognizing fish-inhabited environments, and identifying the generalist and specialist species.

Paleoflora de la Formación los Rastros (Triásico Medio), Provincia de San Juan, Argentina. Consideraciones Taxonómicas y Tafonómicas

Abstract 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 Pena and Quebrada del Leon. 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 la...

Taphonomic analysis in lacustrine environments: Two different contexts for Triassic lake paleofloras from Western Gondwana (Argentina)

During the earliest Triassic several rift basins developed along the western margin of Gondwana associated with the pre-breakup of Pangea. They were filled by exclusively non-marine sediments including alluvial, fluvial, and lacustrine deposits. In the Ischigualasto–Villa Unión Basin, the lacustrine-deltaic succession is placed in the Los Rastros Formation and consists of several coarsening-upward cycles of black shale, siltstone, and sandstone. The paleontologic content of the succession includes abundant floral remains (related to the Dicroidium-type flora), invertebrates (conchostracans, insects), and vertebrates (fishes, a temnospondyl amphibian, ichnites). At the Cerro Puntudo area in the Cuyana Basin, the lacustrine succession forms the upper part of the Cerro Puntudo Formation and consists of limestone, stromatolitic limestone, mudstone, sandstone, and tuff. The paleontologic content includes scarce floral remains and rhizoliths; invertebrates are represented exclusively by traces (associated with ichnofacies of Skolithos and Scoyenia), and vertebrates by a fragment of the pelvic girdle of a basal arcosaur. The taphonomic analysis performed in the two Triassic lacustrine successions allows recognition of two different taphonomic histories for the plant remains. The Los Rastros lake preserved both autochthonous (originated in the littoral zone) and allochthonous (originated in the upstream fluvial system) elements. The offshore lacustrine area was dominated by autochthonous well-preserved elements and allochthonous plant debris and wood, which formed time-averaged accumulations. The delta deposits are characterized by allochthonous elements with varied preservational conditions, usually showing evidence of mechanical degradation and accumulation within a short time. Autochthonous and allochthonous material were preserved in the Los Rastros Lake by means of anoxic conditions in the offshore lacustrine area and high sedimentation rates in the delta. In contrast, the Cerro Puntudo Lake preserved only autochthonous elements (originated in the littoral zone), including rhizoliths and foliar material, which formed autochthonous and parautochthonous accumulations at the littoral zone in spite of aerobic conditions. This was the result of tuffaceous material that enhanced preservation. Thus, these very different lacustrine environmental contexts are showing different preservational modes. The fossil assemblages allowed the reconstruction of the original communities from this part of Gondwana. Thus, the Los Rastros lake margins were characterized by shrubs and small trees of Ginkgoales and Corystospermales, and herbaceous members of the Sphenophyta. The sphenophytes were also the dominant floral component along the river margins whereas the Corystospermales, Cycadales, Pteridophyta, and conifers formed the woodland upstream probably related to the floodplains of a trunk fluvial system. The littoral zone of the Cerro Puntudo Lake was dominated mainly by herbaceous sphenophytes and lycopsids. These fossil assemblages characterize the paleoflora associated with lacustrine systems. A fuller understanding of the processes that generate these assemblages is essential for comparisons with other continental paleobotanical records in the Middle Triassic of Gondwana (e.g., Australia, South Africa).

Late Triassic Environmental Evolution in Southwestern Pangea: Plant Taphonomy of the Ischigualasto Formation

The Ischigualasto Formation was deposited in a fluvial system and contains a very well preserved Triassic flora. This flora comprises seven taphofacies: (1) C(St), autochthonous silicified roots of low-statured, woody plants associated with low-sinuosity channels and crevasse-splay deposits; (2) A(Sm/Fm), autochthonous carbonized roots or root impressions of herbaceous plants in crevasse-splay and levee deposits; (3) B(Fsm/Fm), autochthonous root halos of herbaceous plants associated with levee deposits; (4) EI(St), allochthonous silicified tree trunks and charcoal associated with high- and low-sinuosity channel bars; (5) GI(St), leaf cuticles and charcoal associated with trough cross-bedded sandstone; (6) FH(Fl/C), leaf cuticles and impressions associated with palynomorphs in abandoned-channel deposits; and (7) D(Fl), autochthonous silicified stumps in abandoned-channel deposits. Taphonomically, the Ischigualasto Formation can be divided into four parts, and these partially correspond to changes in the environment. The basal part (∼0–45 m) includes the transition from the underlying Los Rastros Formation and is characterized by taphofacies 1 and 2. The fluvial sediments were deposited during tectonic subsidence of the basin, resulting in development of a fluvioaxial system. The next part (∼45–300 m) is characterized by taphofacies 1, 2, and 3 and associated with calcic paleosols that formed under a dry seasonal climate. The middle-upper portion (∼300–600 m) contains all the taphofacies associated with argillic paleosols, which were formed during a time of increasing humidity. The upper portion (∼600–700 m) is characterized by taphofacies 1, 2, and 3, associated with immature paleosols that formed under a dry seasonal climate. The changes in humidity during deposition of the Ischigualasto Formation may have resulted from an increase in rainfall generated on the western side of Pangea by maximal development of the megamonsoon during the middle Carnian Age. The climatic signal in the Ischigualasto Formation was probably modified by the tectonosedimentary development of the basin.

Paleoenvironments and taphonomy of a Triassic lacustrine system (Los Rastros Formation, central-western Argentina)

The Triassic Ischigualasto-Villa Union Basin is an extensional basin located in central-western Argentina. It includes the Los Rastros Formation, a lacustrine-deltaic sequence comprising several coarsening-upward cycles of black shale, siltstone, and sandstone. We performed a taphonomic analysis of the floral and faunal fossils of the Los Rastros succession and have defined five plant taphofacies, four invertebrate taphofacies, and four vertebrate taphofacies. Our taphonomic model characterizes four subenvironments within the lacustrine-deltaic environment of Los Rastros Formation. These include 1) offshore lacustrine, 2) prodelta, 3) deltaic mouth bar, and 4) deltaic plain subenvironment. Our analysis of fossil assemblages allows us to reconstruct the structure of the original ecosystem. The lake margins were vegetated with small ginkgophytes, corystosperms, and sphenophytes. River margins were characterized by riparian thickets of sphenophytes, while the proximal floodplains supported closed woodlands of corystosperms, cycadophytes, and pteridophytes. More distal floodplains were covered with open conifer forests. The invertebrate fauna included insects (Blattoptera, Hemiptera, Coleoptera) associated with lakeshore vegetation and conchostracans that inhabited both the lake shoreline and smaller ponds in the floodplains. Fish and temnospondyl amphibians probably inhabited the delta plain and incoming fluvial systems. The activity of nonmammalian therapsids, crurotarsal archosaurs, and putative dinosaurs is recorded by trackway surfaces in the lake shoreline subenvironment.

Un nuevo tallo permineralizado de Equisetales de la Formación Los Rastros (Triásico Medio - Superior), provincia de San Juan, Argentina

A new permineralized Equisetalean stem from Los Rastros Formation (Middle-Upper Triassic) from San Juan province, Argentina. Several stems remains of a new genus and species belonging to the order Equisetales are described. They are represented by ribbed stems with distinct nodes and internodes, a pith with some sclerenchymatic cells and carinal canals. Wedge-shaped primary xylem and fascicular wedges of secondary xylem alternating with uniseriate rays are preserved. Comparison of these stems with other Equisetalean fossils from Gondwana is difficult because of their different kinds of preservation. There are only few records of permineralized Equisetalean stems from the Gondwana Triassic because the majority of data are derived from compression-impression fossils; however, one of them, Spaciinodum Osborn & Taylor, from Antarctica, is also based on a permineralization. The anatomical structure of the studied material suggests that it is a new taxon, as their features differ from all previously known species.

Dinosaur origins: evidence from the footprint record

Triassic tracks and trackways assigned to dinosaur trackmakers or closest relatives have been mentioned from several Middle to the latest Triassic successions from both northern and southern Pangea. At present, the earliest gondwanan records are those from the Middle Triassic Los Rastros Formation in west-central Argentina. A reanalysis of Los Rastros ichnites at the Ischichuca area, including new material, has revealed the presence of a more diverse ichnofauna than previously suspected. The ichnocoenosis includes several tracks and trackways of bipeds with functionally tridactyl digitigrade pes, well developed claws, and a parasagittal posture of the hindlimbs. Previously, some large tridactyl footprints from the Ischichuca area were allied to theropod dinosaurs, although no synapomophies are preserved in the three-toed footprints that might discriminate among theropods, basal saurischians and basal ornithischian groups as their possible trackmakers. If the Ischichuca trackmakers are referred to a dinosaur taxon and/or to a close dinosaur sister-taxon, their presence in the Los Rastros levels suggests that derived dinosauriforms (including dinosaurs) had diverged and acquired their characteristic functionally tridactyl pes by at least the Middle Triassic, something that the body-fossil record has failed to document to date.

Freshwater actinopterygians of the Los Rastros Formation (Triassic), Bermejo Basin, Argentina

The present paper includes detailed descriptions of all fossil fishes known from the Los Rastros Formation (Ladinian; Bermejo Basin, Argentina). Four taxa are identified: <i>Gualolepis carinaesquamosa</i> n. gen. n. sp., <i>Rastrolepis</i> n. gen. with two species <i>R . riojaensis</i> n. sp. and <i>R. latipinnata</i> n. sp., and <i>Challaiaelongata</i> n. comb. <i>Gualolepis</i> n. gen. and <i>Rastrolepis</i> n. gen. are incertae sedis actinopterygians. The opercular bones of Gualolepis resemble those in peipiaosteid acipenseriforms and the fish might be related with the Chondrostei. The most distinctive features of Rastrolepis are the narrow opercular region and the presence of a very large plate-like branchiostegal bone resembling the condition in the Redfieldiiformes. <i>Challaiaelongata</i>, originally described in the Australian genus <i>Myriolepis</i>, is here refered to the genus <i>Challaia</i>, previously known from the nearby Cuyo Basin and representing the first fish taxon common to both continental sequences. Furthermore, Challaia is shown to be a member of the Acrolepidae and represents the youngest record of the family in Gondwana. <br><br> Alle fossilen Fische, die bisher von der Los Rastros Formation (Ladin; Bermejo-Becken, Argentinien) bekannt sind, werden im Detail beschrieben. Vier Taxa können identifiziert werden: <i>Gualolepis carinaesquamosa</i> n. gen. n. sp., <i>Rastrolepis</i> n. gen., mit zwei Arten, <i>R . riojaensis</i> n. sp. und <i>R . latipinnata</i> n. sp., und <i>Challaiaelongata</i> comb. nov. <i>Gualolepis</i> n. gen. und <i>Rastrolepis</i> n. gen. Actynopterygier unsicherer systematischer Stellung. Die Operkular-Knochen von <i>Gualolepis</i> ähneln jenen der peipiaosteiden Acipenseriformen und das Taxon mag mit den Chondrostei verwandt sein. Die auffälligsten Merkmale von <i>Rastrolepis</i> sind die sehr schmale Operkularregion und das Vorhandensein eines sehr großen, plattigen Branchiostegale, das dem Zustand in Redfieldiiformen ähnelt. <i>Challaiaelongata</i>, ursprünglich zu der australischen Gattung <i>Myriolepis</i> gestellt, wird hier der Gattung <i>Challaia</i> zugeordnet, die bisher nur aus dem nahegelegenen Cuyo-Becken bekannt war und repräsentiert somit das erste Fisch-Taxon, das in beiden Sequenzen vorkommt. Zudem kann gezeigt werden, dass Challaia zu den Acrolepidae gehört und somit den jüngsten Nachweis dieser Gruppe in Gondwana darstellt. <br><br> doi:<a href="http://dx.doi.org/10.1002/mmng.200600011" target="_blank">10.1002/mmng.200600011</a>

Freshwater actinopterygians of the Los Rastros Formation (Triassic), Bermejo Basin, Argentina

Abstract. The present paper includes detailed descriptions of all fossil fishes known from the Los Rastros Formation (Ladinian; Bermejo Basin, Argentina). Four taxa are identified: Gualolepis carinaesquamosa n. gen. n. sp., Rastrolepis n. gen. with two species R . riojaensis n. sp. and R. latipinnata n. sp., and Challaiaelongata n. comb. Gualolepis n. gen. and Rastrolepis n. gen. are incertae sedis actinopterygians. The opercular bones of Gualolepis resemble those in peipiaosteid acipenseriforms and the fish might be related with the Chondrostei. The most distinctive features of Rastrolepis are the narrow opercular region and the presence of a very large plate-like branchiostegal bone resembling the condition in the Redfieldiiformes. Challaiaelongata, originally described in the Australian genus Myriolepis, is here refered to the genus Challaia, previously known from the nearby Cuyo Basin and representing the first fish taxon common to both continental sequences. Furthermore, Challaia is shown to be a member of the Acrolepidae and represents the youngest record of the family in Gondwana. Alle fossilen Fische, die bisher von der Los Rastros Formation (Ladin; Bermejo-Becken, Argentinien) bekannt sind, werden im Detail beschrieben. Vier Taxa können identifiziert werden: Gualolepis carinaesquamosa n. gen. n. sp., Rastrolepis n. gen., mit zwei Arten, R . riojaensis n. sp. und R . latipinnata n. sp., und Challaiaelongata comb. nov. Gualolepis n. gen. und Rastrolepis n. gen. Actynopterygier unsicherer systematischer Stellung. Die Operkular-Knochen von Gualolepis ähneln jenen der peipiaosteiden Acipenseriformen und das Taxon mag mit den Chondrostei verwandt sein. Die auffälligsten Merkmale von Rastrolepis sind die sehr schmale Operkularregion und das Vorhandensein eines sehr großen, plattigen Branchiostegale, das dem Zustand in Redfieldiiformen ähnelt. Challaiaelongata, ursprünglich zu der australischen Gattung Myriolepis gestellt, wird hier der Gattung Challaia zugeordnet, die bisher nur aus dem nahegelegenen Cuyo-Becken bekannt war und repräsentiert somit das erste Fisch-Taxon, das in beiden Sequenzen vorkommt. Zudem kann gezeigt werden, dass Challaia zu den Acrolepidae gehört und somit den jüngsten Nachweis dieser Gruppe in Gondwana darstellt. doi:10.1002/mmng.200600011

Algas Chlorococcales como indicadores paleoambientales: nuevos datos de la Formación Los Rastros, Triásico del centro-oeste de Argentina

Chlorococcalean algae as palaeoenvironmental indicators: new data from the Los Rastros Formation, Triassic of central-western Argentina. Lacustrine strata of the Los Rastros Formation (Triassic) at the Ischigualasto Provincial Park section, San Juan province, yield a distinctive palynological assemblage of miospores and Chlorococcalean algae. The miospore association is characterized by a relative abundance of corystosperm pollen grains with subordinate spores and inaperturates, diploxylonoid disaccates, monocolpates, monosaccates and striate pollen grains. The phytoplankton is represented by the Chlorococcalean Botryococcus and Plaesiodictyon. Geological data and variations in phytoplankton content indicate that the lacustrine system should be moderately eutrophic towards the lower third of the section, evolving into a body with oligotrophic conditions through the middle and upper part of the unit. Comparisons with the Rio Gualo section suggest palaeoenvironmental similarities between the two areas. One new disaccate pollen grain species, Platysaccus rhombicus, is proposed.

Millerocaulis (Erasmus) ex Tidwell (Osmundales, Filices) de la Formación Carrizal (Triásico Superior) de Marayes, provincia de San Juan, Argentina

Millerocaulis (Erasmus) ex Tidwell (Osmundales, Filices) from the Carrrizal Formation (Upper Triassic) from Marayes, San Juan province, Argentina. Based on newly found specimens, «Marayea lutzi» Herbst 1994, of the Upper Triassic Carrizal Formation, from Marayes (San Juan province , Argentina) is analyzed. Anatomical characters, specially those from the central cylinder, now defines the species as an ectophloicdictyoxylic siphonostele (sensu Miller) and shows that it must be changed to Millerocaulis. Thus, Marayea defined as a siphonostelic genus becomes invalid and an emended diagnosis for the type species is given. Also, from the same locality and age other, rather big specimens, are described. They are not well preserved but the available characters allow to incorporate them as Millerocaulis sp. These two species and others formerly described of this genus, M. herbsti (Archangelsky & de la Sota) Tidwell and M. stipabonettii Herbst, from the Laguna Colorada Formation (Santa Cruz province) and Cepeda Formation (San Juan province) respectively from Argentina along with M. beardmorensis Schopf and Ashicaulis woolfei Rothwell, Taylor & Taylor, from the Fremouw Formation (Antarctica) and the frond Osmunda claytoniites Phipps et. al.,as well as some petioles lacking cellular structure but undoubtely belonging to Millerocaulis, from the Los Rastros Formation (San Juan province), all from the upper Middle to Upper Triassic, show the diversification of the Osmundales as early as this time span, at least in this part of Gondwana

Continental fish taphonomy: a case study in the Triassic of Argentina

The Triassic Los Rastros Formation, Ischigualasto-Villa Unión Basin, consists of alternating cycles of lacustrine black shales and deltaic sandstones. In each cycle, two facies assemblages have been defined: the lake (Ffl and SFm) and the deltaic (Fm, Sr, Sh, Sp/St, and FSC). The fossil content includes plants (macroflora and microflora), invertebrates (conchostracans, insects, and bivalves), and vertebrates (actinopterigian fish, temnospondyl amphibians, and archosaur footprints). Fossil fish occur in three facies (Ffl, SFm, Sh), and their remains have different taphonomic features in each. Three taphofacies were recognized, namely, A, B, and C. Taphofacies A is characterized by disarticulated fish material that tends to be dispersed or in loosely packed clumps; the remains have a low density of accumulation and a high degree of sorting. This taphofacies is related to distal lake deposits. Taphofacies B is characterized by densely packed, partially articulated remains, has a moderate density of accumulation and poor sorting, and is related to distal turbidity current deposits. Taphofacies C is characterized by partially to totally articulated remains that are densely to loosely packed with a moderate density of accumulation and poor sorting; this taphofacies is related to the mouth-bar lateral margin deposit. The fish assemblages studied document different taphonomic histories, and it is proposed that they were allochthonous to the lake, probably inhabiting instead the affluent fluvial system.

Paleoenvironment and Taphonomy of the Chanares Formation Tetrapod Assemblage (Middle Triassic), Northwestern Argentina: Spectacular Preservation in Volcanogenic Concretions

Other| October 01, 2001 Paleoenvironment and Taphonomy of the Chañares Formation Tetrapod Assemblage (Middle Triassic), Northwestern Argentina: Spectacular Preservation in Volcanogenic Concretions RAYMOND R. ROGERS; RAYMOND R. ROGERS 1Department of Geology, Macalester College, St. Paul, MN 55105 Search for other works by this author on: GSW Google Scholar ANDREA B. ARCUCCI; ANDREA B. ARCUCCI 2Museo de Paleontologia, Instituto de Anthropologia, Universidad Nacional de La Rioja, Avenida Ortiz de Ocampo 1700, 5300 La Rioja, Argentina Search for other works by this author on: GSW Google Scholar FERNANDO ABDALA; FERNANDO ABDALA 3Laboratorio de Paleontologia, Museu de Ciências e Tecnologia, Pontifícia Universidade Católica do Rio Grande do Sul, Avenida Ipiranga 6681, Porto Alegre (90619-900), Brazil Search for other works by this author on: GSW Google Scholar PAUL C. SERENO; PAUL C. SERENO 4Department of Organismal Biology and Anatomy, University of Chicago, Chicago, IL 60637 Search for other works by this author on: GSW Google Scholar CATHERINE A. FORSTER; CATHERINE A. FORSTER 5Department of Anatomical Sciences, State University of New York, Stony Brook, NY 11794 Search for other works by this author on: GSW Google Scholar CATHLEEN L. MAY CATHLEEN L. MAY 6Institute for Environmental Education, Geological Society of America, 3300 Penrose Place, Boulder, CO 80301 Search for other works by this author on: GSW Google Scholar PALAIOS (2001) 16 (5): 461–481. https://doi.org/10.1669/0883-1351(2001)016<0461:PATOTC>2.0.CO;2 Article history accepted: 30 Mar 2001 first online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation RAYMOND R. ROGERS, ANDREA B. ARCUCCI, FERNANDO ABDALA, PAUL C. SERENO, CATHERINE A. FORSTER, CATHLEEN L. MAY; Paleoenvironment and Taphonomy of the Chañares Formation Tetrapod Assemblage (Middle Triassic), Northwestern Argentina: Spectacular Preservation in Volcanogenic Concretions. PALAIOS 2001;; 16 (5): 461–481. doi: https://doi.org/10.1669/0883-1351(2001)016<0461:PATOTC>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyPALAIOS Search Advanced Search Abstract The enigmatic concretionary exposures that typify the Chañares Formation (Ladinian, northwestern Argentina) long have defied precise paleoenvironmental characterization. Recent work indicates that the formation accumulated in an alluvial-to-lacustrine setting within an active rift basin that received sedimentary detritus from surrounding highlands, as well as copious amounts of volcanic ash. Ash- flow sheets were emplaced presumably as secondary mass flows on alluvial surfaces characterized by small fluvial channels and shallow lakes. Thin bentonite beds intercalated in the Chañares Formation indicate that ash also accumulated via direct airfall, although this mode of emplacement accounts for a very small fraction of the overall section. A shift to widespread lacustrine deposition is recorded by the superjacent Los Rastros Formation, which preserves at least six shallowing-upward hemicycles, five of which commenced amidst explosive volcanic activity as evidenced by intercalated bentonite beds.Volcanism played an important role in the generation and preservation of the Chañares Formation's exceptional tetrapod fossil record. This is especially true of the classic Los Chañares locality, where more than 100 individuals representing a diverse array of taxa (archosaurs, cynodonts, dicynodonts) are entombed in volcanogenic concretions with matrices of relic glass shards diagenetically replaced by calcite. Taphonomic attributes of the Los Chañares locality are consistent with the scenario of mass mortality, and several clues hint at the nature of the event. The killing agent was lethal to a variety of taxa, killed both adults and juveniles, and led to the concentration of taxa that under normal circumstances would tend to dissociate, such as carnivores and their potential prey. It also produced a counterintuitive bias against the preservation of large-bodied taxa, which may have been largely unsusceptible to the death event, or perhaps were excluded from the Los Chañares death assemblage via post-mortem sorting. The spatial arrangement of skeletal material in a small sample of concretions is consistent with the stranding of tetrapod carcasses along a strandline, and it is feasible that volcanism led to catastrophic flooding of the landscape via damming and/or diversion of local drainages. Uncompacted skeletal elements and relic outlines of glass shards indicate that carbonate concretions formed shortly after skeletal material was buried in reworked volcanic ash. The microbial decay of organic matter presumably catalyzed concretion diagenesis. There is no indication that bone hydroxyapatite diffused into the entombing glassy matrix and contributed to concretion formation. Bones entombed within early diagenetic concretions were safeguarded from subsequent destructive pedogenic and/or diagenetic processes, and were incorporated in exquisite quality into the fossil record. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Paleoenvironment and Taphonomy of the Chanares Formation Tetrapod Assemblage (Middle Triassic), Northwestern Argentina: Spectacular Preservation in Volcanogenic Concretions

Other| October 01, 2001 Paleoenvironment and Taphonomy of the Chañares Formation Tetrapod Assemblage (Middle Triassic), Northwestern Argentina: Spectacular Preservation in Volcanogenic Concretions RAYMOND R. ROGERS; RAYMOND R. ROGERS 1Department of Geology, Macalester College, St. Paul, MN 55105 Search for other works by this author on: GSW Google Scholar ANDREA B. ARCUCCI; ANDREA B. ARCUCCI 2Museo de Paleontologia, Instituto de Anthropologia, Universidad Nacional de La Rioja, Avenida Ortiz de Ocampo 1700, 5300 La Rioja, Argentina Search for other works by this author on: GSW Google Scholar FERNANDO ABDALA; FERNANDO ABDALA 3Laboratorio de Paleontologia, Museu de Ciências e Tecnologia, Pontifícia Universidade Católica do Rio Grande do Sul, Avenida Ipiranga 6681, Porto Alegre (90619-900), Brazil Search for other works by this author on: GSW Google Scholar PAUL C. SERENO; PAUL C. SERENO 4Department of Organismal Biology and Anatomy, University of Chicago, Chicago, IL 60637 Search for other works by this author on: GSW Google Scholar CATHERINE A. FORSTER; CATHERINE A. FORSTER 5Department of Anatomical Sciences, State University of New York, Stony Brook, NY 11794 Search for other works by this author on: GSW Google Scholar CATHLEEN L. MAY CATHLEEN L. MAY 6Institute for Environmental Education, Geological Society of America, 3300 Penrose Place, Boulder, CO 80301 Search for other works by this author on: GSW Google Scholar PALAIOS (2001) 16 (5): 461–481. https://doi.org/10.1669/0883-1351(2001)016<0461:PATOTC>2.0.CO;2 Article history accepted: 30 Mar 2001 first online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation RAYMOND R. ROGERS, ANDREA B. ARCUCCI, FERNANDO ABDALA, PAUL C. SERENO, CATHERINE A. FORSTER, CATHLEEN L. MAY; Paleoenvironment and Taphonomy of the Chañares Formation Tetrapod Assemblage (Middle Triassic), Northwestern Argentina: Spectacular Preservation in Volcanogenic Concretions. PALAIOS 2001;; 16 (5): 461–481. doi: https://doi.org/10.1669/0883-1351(2001)016<0461:PATOTC>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyPALAIOS Search Advanced Search Abstract The enigmatic concretionary exposures that typify the Chañares Formation (Ladinian, northwestern Argentina) long have defied precise paleoenvironmental characterization. Recent work indicates that the formation accumulated in an alluvial-to-lacustrine setting within an active rift basin that received sedimentary detritus from surrounding highlands, as well as copious amounts of volcanic ash. Ash- flow sheets were emplaced presumably as secondary mass flows on alluvial surfaces characterized by small fluvial channels and shallow lakes. Thin bentonite beds intercalated in the Chañares Formation indicate that ash also accumulated via direct airfall, although this mode of emplacement accounts for a very small fraction of the overall section. A shift to widespread lacustrine deposition is recorded by the superjacent Los Rastros Formation, which preserves at least six shallowing-upward hemicycles, five of which commenced amidst explosive volcanic activity as evidenced by intercalated bentonite beds.Volcanism played an important role in the generation and preservation of the Chañares Formation's exceptional tetrapod fossil record. This is especially true of the classic Los Chañares locality, where more than 100 individuals representing a diverse array of taxa (archosaurs, cynodonts, dicynodonts) are entombed in volcanogenic concretions with matrices of relic glass shards diagenetically replaced by calcite. Taphonomic attributes of the Los Chañares locality are consistent with the scenario of mass mortality, and several clues hint at the nature of the event. The killing agent was lethal to a variety of taxa, killed both adults and juveniles, and led to the concentration of taxa that under normal circumstances would tend to dissociate, such as carnivores and their potential prey. It also produced a counterintuitive bias against the preservation of large-bodied taxa, which may have been largely unsusceptible to the death event, or perhaps were excluded from the Los Chañares death assemblage via post-mortem sorting. The spatial arrangement of skeletal material in a small sample of concretions is consistent with the stranding of tetrapod carcasses along a strandline, and it is feasible that volcanism led to catastrophic flooding of the landscape via damming and/or diversion of local drainages. Uncompacted skeletal elements and relic outlines of glass shards indicate that carbonate concretions formed shortly after skeletal material was buried in reworked volcanic ash. The microbial decay of organic matter presumably catalyzed concretion diagenesis. There is no indication that bone hydroxyapatite diffused into the entombing glassy matrix and contributed to concretion formation. Bones entombed within early diagenetic concretions were safeguarded from subsequent destructive pedogenic and/or diagenetic processes, and were incorporated in exquisite quality into the fossil record. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.