Depositional environments and stratigraphy of the Upper Cambrian-lower Ordovician Santa Rosita formation at the Alfarcito area, Cordillera Oriental, Argentina: integration of biostratigraphic data within a sequence stratigraphic framework

The Upper Cambrian-Tremadocian rocks exposed in the Alfarcito area of Cordillera Oriental, northwest Argentina reveal a series of transgressive-regressive cycles punctuated by incision of fluvio-estuarine valleys. An integrated sedimentologic, biostratigraphic and sequence stratigraphic study permits characterization of the sedimentary facies and stratal stacking patterns, and a revision of the stratigraphic framework of this succession. The Upper Cambrian-Tremadocian succession is included within the existing Santa Rosita Formation, which is formally subdivided here into six units, the Tilcara, Casa Colorada, Pico de Halcón, Alfarcito, Rupasca and Humacha members. This scheme recognizes the internal complexities of the formation, in that the Tilcara and Pico de Halcón members record sedimentation in areally-restricted incised valleys and contain structures indicative of tidal dominance, whereas the Casa Colorada, Alfarcito and Rupasca members typify wave-dominated open-marine environments, where background suspension-fallout was punctuated by oscillatory flows during storms. The Tilcara Member is incised into the underlying Mesón Group. The Casa Colorada Member reflects a basinwide Late Cambrian transgression and subsequent progradation and consists of lower to upper offshore deposits. The Pico de Halcón Member is incised into the Casa Colorada Member. The Alfarcito Member is divided into three distinctly upward-coarsening and-thickening intervals. The sandstone-dominated intervals are composed of lower to middle shoreface deposits with minor amounts of offshore transition and upper shoreface deposits. The heterolithic, fine-grained intervals record more distal deposition, ranging from the lower offshore to the offshore transition. The Rupasca Member is a dominantly fine-grained unit that records a major late early to early late Tremadocian transgression. The lower interval of this member consists of lower offshore to offshore-transition deposits, while the upper part is made up of shelf to lower offshore deposits. The Humacha Member is not exposed in the study area. The importance of integrating biostratigraphy within a sedimentologic and sequence-stratigraphic framework is underscored here, in that it enables us to construct a more accurate stratigraphic framework which permits a clear understanding of the sedimentary dynamics of this system

Trilobite expansion into estuarine environments during the Ordovician radiation

Trilobites have traditionally been considered fully marine. Through the integration of ichnological, palaeobiological and sedimentological datasets within a sequence-stratigraphic and strati­graphic palaeobiology framework, we challenge this assumption. This analysis is based on the study of incised fluvio-estuarine valley deposits from the Furongian Tilcara Member (TM) and the latest Furongian Pico de Halcón Member (PHM) of the Santa Rosita Formation, the early late Tremadocian Cardonal Formation (CF), and the DapingianâDarriwilian Alto del Cóndor Formation (ACF), from Cordillera Oriental of northwest Argentina. These valleys were incised into wave-dominated shallow-marine strata and filled with transgressive deposits that accumulated in tide-dominated estuaries. Whereas the TM lacks any body or trace fossil evidence of the presence of trilobites in estuarine settings, the other three units reveal that trilobites were able to inhabit these settings. The PHM and CF are host to trilobite trace fossils in outer estuarine facies, both containing various ichnospecies of Cruziana (e.g., C. omanica and C. semiplicata in the TM) and Rusophycus (e.g., R. latus in both units). In addition, the PHM also contains body fossils of the olenid trilobite Neoparabolina frequens argentina in the same deposits in which the trace fossils are preserved, as well as from middle estuarine facies. The ACF displays trilobite trace fossils of the C. rugosa group in inner, middle, and outer estuarine deposits, illustrating further landward incursions. This unit also contains body fossils of the asaphid trilobite Ogyginus sp. Accordingly, our data indicate two attempts of landward exploration via brackish water: phase 1 in which the outer to middle portion of estuaries were colonized by olenids (Furongianâearly late Tremadocian) and phase 2 involving exploration of the inner, middle, and outer estuarine zones by asaphids (DapingianâDarriwilian). Our study indicates that these trilobites were tolerant to salinity stress and able to make use of the ecological advantages offered by marginal-marine environments migrating up-estuary, following salt wedges either reflecting amphidromy or as euryhaline marine wanderers. It is suggested that tolerance to salinity stress arose independently among different trilobite groups as a result of the broad array of behaviors and adaptations of trilobites during the Ordovician radiation. We speculate that the assumption that all trilobites were stenohaline may have resulted in the misinterpretation of some tide-dominated estuarine deposits as fully marine

Assessing the carcinogenic potential of low-dose exposures to chemical mixtures in the environment: the challenge ahead.

Lifestyle factors are responsible for a considerable portion of cancer incidence worldwide, but credible estimates from the World Health Organization and the International Agency for Research on Cancer (IARC) suggest that the fraction of cancers attributable to toxic environmental exposures is between 7% and 19%. To explore the hypothesis that low-dose exposures to mixtures of chemicals in the environment may be combining to contribute to environmental carcinogenesis, we reviewed 11 hallmark phenotypes of cancer, multiple priority target sites for disruption in each area and prototypical chemical disruptors for all targets, this included dose-response characterizations, evidence of low-dose effects and cross-hallmark effects for all targets and chemicals. In total, 85 examples of chemicals were reviewed for actions on key pathways/mechanisms related to carcinogenesis. Only 15% (13/85) were found to have evidence of a dose-response threshold, whereas 59% (50/85) exerted low-dose effects. No dose-response information was found for the remaining 26% (22/85). Our analysis suggests that the cumulative effects of individual (non-carcinogenic) chemicals acting on different pathways, and a variety of related systems, organs, tissues and cells could plausibly conspire to produce carcinogenic synergies. Additional basic research on carcinogenesis and research focused on low-dose effects of chemical mixtures needs to be rigorously pursued before the merits of this hypothesis can be further advanced. However, the structure of the World Health Organization International Programme on Chemical Safety 'Mode of Action' framework should be revisited as it has inherent weaknesses that are not fully aligned with our current understanding of cancer biology

Assessing the carcinogenic potential of low-dose exposures to chemical mixtures in the environment: the challenge ahead

Lifestyle factors are responsible for a considerable portion of cancer incidence worldwide, but credible estimates from the World Health Organization and the International Agency for Research on Cancer (IARC) suggest that the fraction of cancers attributable to toxic environmental exposures is between 7% and 19%. To explore the hypothesis that low-dose exposures to mixtures of chemicals in the environment may be combining to contribute to environmental carcinogenesis, we reviewed 11 hallmark phenotypes of cancer, multiple priority target sites for disruption in each area and prototypical chemical disruptors for all targets, this included dose-response characterizations, evidence of low-dose effects and cross-hallmark effects for all targets and chemicals. In total, 85 examples of chemicals were reviewed for actions on key pathways/mechanisms related to carcinogenesis. Only 15% (13/85) were found to have evidence of a dose-response threshold, whereas 59% (50/85) exerted low-dose effects. No dose-response information was found for the remaining 26% (22/85). Our analysis suggests that the cumulative effects of individual (non-carcinogenic) chemicals acting on different pathways, and a variety of related systems, organs, tissues and cells could plausibly conspire to produce carcinogenic synergies. Additional basic research on carcinogenesis and research focused on low-dose effects of chemical mixtures needs to be rigorously pursued before the merits of this hypothesis can be further advanced. However, the structure of the World Health Organization International Programme on Chemical Safety ‘Mode of Action’ framework should be revisited as it has inherent weaknesses that are not fully aligned with our current understanding of cancer biology

Assessing the carcinogenic potential of low-dose exposures to chemical mixtures in the environment: the challenge ahead

Lifestyle factors are responsible for a considerable portion of cancer incidence worldwide, but credible estimates from the World Health Organization and the International Agency for Research on Cancer (IARC) suggest that the fraction of cancers attributable to toxic environmental exposures is between 7% and 19%. To explore the hypothesis that low-dose exposures to mixtures of chemicals in the environment may be combining to contribute to environmental carcinogenesis, we reviewed 11 hallmark phenotypes of cancer, multiple priority target sites for disruption in each area and prototypical chemical disruptors for all targets, this included dose-response characterizations, evidence of low-dose effects and cross-hallmark effects for all targets and chemicals. In total, 85 examples of chemicals were reviewed for actions on key pathways/ mechanisms related to carcinogenesis. Only 15% (13/85) were found to have evidence of a dose-response threshold, whereas 59% (50/85) exerted low-dose effects. No dose-response information was found for the remaining 26% (22/85). Our analysis suggests that the cumulative effects of individual (non-carcinogenic) chemicals acting on different pathways, and a variety of related systems, organs, tissues and cells could plausibly conspire to produce carcinogenic synergies. Additional basic research on carcinogenesis and research focused on low-dose effects of chemical mixtures needs to be rigorously pursued before the merits of this hypothesis can be further advanced. However, the structure of the World Health Organization International Programme on Chemical Safety ‘Mode of Action’ framework should be revisited as it has inherent weaknesses that are not fully aligned with our current understanding of cancer biology

The trilobite Maurotarion megacephalum sp. nov. (Aulacopleuridae) in the Lower Devonian of Argentina: phylogenetic and paleobiogeographic remarks

Maurotarion megacephalum sp. nov., from the Pragian (Lower Devonian) of the Talacasto Formation, Precordillera basin, central-west Argentina, is described and interpreted as the first Malvinokaffric species of the genus outside the endemic subgenus M. (Malvinotarion). Based mainly on librigenal characters, the coeval M. periergum, from the Wenban Limestone, Nevada, U. S. A., is considered the closest allied taxon. Both species are interpreted as those most closely related to M. (Malvinotarion). Evidence suggests faunal exchange between low and high paleolatitudes for the Pragian, a hypothesis previously discarded in light of low sea level conditions interpreted during this time, and linked to a pattern of vicariance and radiation previously recognized from Malvinokaffric Maurotarion species.Maurotarion megacephalum sp. nov., del Pragiano (Devónico Inferior) de la Formación Talacasto, cuenca de Precordillera, centro-oeste de Argentina, es descrito e interpretado como la primera especie malvinocáfrica del género, no perteneciente al subgénero endémico M. (Malvinotarion). Con base en caracteres librigenales, principalmente, M. periergum, de la Caliza Wenban, Nevada, E.U.A., de edad similar, es considerado el taxón más cercanamente relacionado. Ambas especies son interpretadas como las más cercanas al subgénero M. (Malvinotarion). Las evidencias sugieren intercambio faunístico entre paleolatitudes bajas y altas para el Pragiano, una hipótesis previamente descartada en atención a las condiciones de nivel bajo del mar interpretadas para este intervalo, y ligadas a patrones de vicarianza y radiación con base en datos de Maurotarion malvinocáfricos

Porongodendron minitensis gen. nov. sp. nov. a new lycopsid from the Mississippian of Argentina with adaptations to tundra-like conditions

peer reviewedThe Mississippian is characterized worldwide by a general cooling of the environment and a climatic stratification of Earth. Argentina at that time was part of the megacontinent Gondwana. It occupied a position close to the South Pole around 60°S. Whereas paleoequatorial and paleotropical floras are relatively well known, higher latitude environments are still poorly understood. In this framework, a revision of Mississippian deposits is ongoing in Argentina. An extensive and detailed survey of the Sierra de las Minitas (La Rioja Province, western Argentina) involved drawing detailed sections and the systematic verification of all rock layers. Fossils were sampled and studied in the laboratory. We describe a new isoetalean lycopsid with a rare combination of characters. This plant presents an estimated low stature (approximately 30 cm), a branched rhizomorph, an inflated intermediary zone, a monocaulous growth form and evidence for cyclic fertility. These features are interpreted as representing specific adaptations to the harsher tundra conditions prevailing in the Lower Carboniferous Southern Polar environments

Latest Ordovician-earliest Silurian chitinozoans from the Puna region, north-western Argentina (Western Gondwana)

International audienceLate Ordovician-early Silurian chitinozoans from the Upper Member of the Salar del Rincon Fonnation of northwestern Argentina are described. The study area belongs to the Central Andean Basin situated on the western Gondwana margin during the Early Palaeozoic. Chitinozoan assemblages correlate with those from Northern Gondwana, where the effects of the glacial and postglacial events that occurred around the Ordovician-Silurian boundary, are quite well-documented. The recovered chitinozoan associations from the upper part of the Salar del Rincon Fonnation record the uppermost Ordovician-lowest Silurian deposits representing the postglacial stage of the late Himantian glaciation. Similar strata are usually absent in other parts of the Central Andean Basin. Tasmanites tzadiaensis is recovered for the first time outside of the northeast of Africa, supporting the northern Gondwana affinities. Associated land-derived components indicate a nearshore environment with terrestrial input. The other associated palynomorphs support the latest Hirnantian-earliest Rhuddanian postglacial stage correlation, as well. The ranges of several Late Ordovician-early Silurian Spinachitina and Cyathochitina species arc discussed in tams of the Ordovician-Silurian boundary. Five new species are formally described: Spinachitina titae sp. nov., Cyathochitina brussai sp. nov., Cyathochitina lariensis sp. nov., Cyathochitina punaensis sp. nov. and Ramochitina deynouxi sp. nov

A new Mississippian hexactinellid sponge from the western Gondwana: Taxonomic and paleobiogeographic implications

A Mississippian hexactinellid sponge from the western Argentina improves the extremely poor late Paleozoic sponge records from Gondwana. The sponge is included in the subfamily Thysanodictyinae of family Dictyospongiidae. The new genus and species Minitaspongia parvis is erected, and its well-preserved spicular structure is described in detail representing the first approximation of the spicule assemblage in Thysanodictyinae. The skeleton is clathrate, three-dimensional with at least two ranks of rectangular openings. This first report of this subfamily outside North America represents the best-known hexactinellid and the first dictyosponge record from the Carboniferous of Gondwana. Unlike the occurrences of Thysanodictyinae in North America, with thick skeletons linked to high-energy shallow water settings, Minitaspongia occurs in low-energy water siliciclastic settings related to a cold climate and glacimarine deposits. Accordingly, the complex wall structure of this sponge should not be invoked as a necessary adaptation to high energy and shallow water settings