Teresa M. Sánchez

Early Ordovician (Arenig) faunal assemblages from western Argentina: biodiversification trends in different geodynamic and palaeogeographic settings

Abstract A survey of early Ordovician faunal assemblages from different geodynamic and palaeogeographic settings from the west of Argentina has been carried out. The distribution and dominance of four fossil groups (rhynchonelliform brachiopods, trilobites, sponges, and bivalves) are analysed and compared in three distinct basins: a passive-margin carbonate platform (Precordillera), a volcanic-arc island platform located at intermediate latitude (Famatina) and a siliciclastic pericratonic epeiric platform placed at intermediate–high latitude (Cordillera Oriental). Scales of analysis range from the level of the patterns exhibited by individual clades to the level of the assemblages along the onshore–offshore gradient. Overall taxonomic diversity (at genus and family levels), alpha or within habitat diversity, and ecospace utilisation were evaluated and contrasted among bathymetric zones and among different basins. On this basis a mosaic of five distinctive type assemblages is characterised: (1) a demosponge–brachiopod type assemblage (Precordillera), (2) a brachiopod–bivalve type assemblage (Famatina), (3) a brachiopod–trilobite type assemblage (Famatina), (4) a trilobite–brachiopod type assemblage (Cordillera Oriental), and (5) a trilobite type assemblage (Cordillera Oriental). Possible large-scale controls in the configuration of these type assemblages are assessed. The structure and distribution of the type assemblages from the west of Argentina are interpreted to be largely controlled by environmental dynamics at each geodynamic setting coupled with the latitudinal position of the basins. Both large-scale factors regulate a number of regional parameters such as sedimentary regime, volcanic activity, oceanic circulation, temperature, etc. In particular temperature appears as a critical control over the food supply and primary productivity as well as seasonality of the resources. According to the evidence analysed in the Argentine basins, the latter variables might have played a key role in the diversification of suspension-feeding organisms that, in turn, promoted the development of the Palaeozoic evolutionary faunas and the Ordovician radiation. Our case study from the west of Argentina further represents a small-scale model of the broad spectrum of the possible regional conditions that promoted the differential worldwide expressions of the Ordovician radiation and expansion of the three evolutionary faunas.

NEW BIVALVIA AND ROSTROCONCHIA FROM THE EARLY ORDOVICIAN (LATE TREMADOC–MIDDLE ARENIG) OF NORTHWESTERN ARGENTINA

Abstract New species of Early Ordovician bivalves and rostroconchs from the northwestern Argentina basin are described. Bivalves include the following species: Cienagomya bidentata new genus and species, Goniophorina tenuicostata, Lipanella purmamarcensis new genus and species, Ucumaropsis lermaensis new genus and species, and Coxiconchia babini new species. Among rostroconchs, Ribeiria pojetai new species is recognized. Cienagomya bidentata (late Tremadoc–early Arenig) may represent a phylogenetic link between the families Intihuarellidae and Cycloconchidae. The new family Lipanellidae is erected to include Lipanella n. gen. Recognition of Ucumaropsis n. gen. in the early Arenig attests to the important radiation of the Anomalodesmata in northwestern Argentina.

Lithofacies, taphonomy, and brachiopod assemblages in the Silurian of western Argentina: A review of Malvinokaffric Realm communities

Abstract Distribution of faunal associations in the Silurian sequences of the Precordilleran and Northwestern Basins of Argentina is integrated with sedimentologic and taphonomic data, demonstrating that marked temporal and spatial changes in the Malvinokaffric brachiopod assemblages are related mainly to the type and intensity of the hydraulic regime of the shelf environment. The vertical arrangement of associations within each basin is controlled by local fluctuations in water depth, whereas the spatial variations are linked to minor environmental changes (substrate type and oxygen content). The basal strata of the Middle-Upper Silurian Los Espejos Formation of the Precordilleran Basin are mudstones of an open-shelf environment. These pass upward into facies of a storm-dominated shelf and shoreface. Four brachiopod assemblages have been recognized in the muddy facies: the Harringtonina Low-Diversity and Australina -Dominated associations characterize distal storm beds; the Harringtonina High-Diversity and Coelospira -Dominated communities occur in the fine-grained rocks of proximal tempestites. Storm shell beds, which dominate the middle-upper part of the sequence, are represented by allochthonous and parautochthonous coquinas. The Clarkeia Association flourished in shallow-water, high-energy, nearshore environments. Silurian rocks in the Northwestern Basin were deposited in an offshore, low-energy, restricted environment, and two main faunal assemblages have been recognized: a more distal Heterorthella Association and a relatively more proximal Ancillotoechia Association.

Fauna cámbrica versus fauna paleozoica en el Ordovícico temprano del Oeste de Argentina : Interacciøn entre provincialismo y ambiente

Resumen Las secuencias carbonaticas de la Formacion San Juan (Tremadociano superior-Llanvirniano inferior) dela cuenca de Precordillera contienen un grupo de comunidades dominadas por organismos suspensivoros, especialmente braquiopodos y poriferos. La Formacion Acoite (Arenigiano inferior-medio) de la cuenca del Noroeste Argentino, en parte contemporanea con la Formacio San Juan, muestra una asociacion de comunidades dominadas por trilobites. De acuerdo al modelo global de faunas evolutivas, la primera asociacion se encuadra en la “Fauna Paleozoica” mientras que la segunda puede ser referida a la “Fauna Cambrica”. El analisis comparado de la estructura de los dos tipos de comunidades, teniendo en cuenta sus estrategias de estratificacion (“tiering”) y de los diferentes “guilds”, muestra que los valores de diversidad y abundancia, asi vomo del numero de guilds y de niveles ocupados, son semejantes. Es necesario destacar tambien que las comunidades de trilobites ocuparon diferentes medios de la plataforma desde off-shore hasta shoreface, asi como el medio estuarino, sin mostrar cambios significativos en las proporciones de los taxones mas importantes, sino una estasis ecologica a traves del tiempo. Las explicaciones de esta desviacion en relacion con el modelo deben buscarse en la posicion paleogeografica de las dos regiones durante el Arenigiano que definieron las condiciones ecologicas dominantes durante ele desarrollo de ambas asociaciones de comunidades. Se puede concluir que el modelo de las tres faunas evolutivas es solo aplicable (al menos durante del Ordovicico) a las faunas ecuatoriales y subecuatoriales, mientras que las faunas gondwanicas y perigondwanicas deben ser incluidas en un modelo evolutivo diferente. Para esto, sera necesario contar con informaciones complementarias sobre otras asociaciones del Ordovicico inferior de Gondwana.

Additional Mollusca (Bivalvia and Rostroconchia) from the Suri Formation, Early Ordovician (Arenig), western Argentina

The bivalve molluscs Colpomya ? sp. and Goniophorina ( Cosmogoniophorina )? tenuicostata Harrington are recorded for the first time in the Arenigian Suri Formation (Famatina Range, western Argentina), and the new taxa Suria ovalis new genus and new species, and Redonia riojana new species are proposed. Additionally, the rostroconchid Ribeiria spinosa Babin and Branisa is also described. Some taxa shared with the Acoite Formation (Cordillera Oriental, Northwestern Argentina) and the Sella Formation (Cordillera Oriental, Bolivia) suggest that the Suri Formation was geographically related to these basins, and consequently, it could likewise be considered as a peri-Gondwanan area.

The early bivalve radiation in the Ordovician Gondwanan basins of Argentina

The Ordovician diversification is marked by an increase in both marine diversity and ecospace occupation. Bivalves, like other groups, underwent a remarkable diversification in the Early Ordovician. The early phases of such a bivalve diversification took place in the Gondwanan basins of western Argentina. In the Northwestern Argentina (NWA) Basin, three clades originated during late Tremadocian–Floian times. In the Floian successions of the Famatina Basin, a probable basal arcoid is recorded. Genera from these two basins belong to 13 families. Phylogenetic analysis of the NWA heteroconchian bivalves indicates that redoniids and coxiconchinids may have originated during this radiation event. This taxonomic radiation also implies an ecological diversification. Ten guilds are recognized on the basis of bauplan, mode of life, and feeding types. Lifestyles included free endofaunal, free semi-endofaunal, semi-endobyssate, and epibyssate; feeding types included suspensivorous and detritivorous habits. Physiological changes imposed by colonization of low-salinity environments also account for guild definitions. Recent discoveries of Tremadoc to early Darriwilian bivalves from the NWA and Famatina basins indicate that the dominance of higher groups (e.g. Heteroconchia, Pteriomorphia) deviates from the patterns evident in other Gondwanan basins. This agrees with previous ideas supporting the importance of local radiations during the Ordovician diversification. Two new taxa are described, Eoredonia orientalis gen. et sp. nov. and Babinka notica sp. nov., and Coxiconchia sellaensis Sánchez & Babin is first reported from the NWA Basin.

New Late Ordovician (early Caradoc) bivalves from the Sierra de Villicum (Argentine Precordillera)

A new and unusually well preserved bivalve fauna of Late Ordovician age was recovered from pebbles within the Late Ashgill (Hirnantian) glacigenic diamictite of the Don Braulio Formation, in the Argentine Precordillera. According to the associated brachiopods and the presence of graptolites of the bicornis Zone in the matrix of the diamictite, an early Caradoc age is accepted for the bivalve fauna. The clasts contain bivalve molluscs representing nine genera, of which seven are new. Identified taxa are the following: Praenucula sp., Cuyopsis symmetricus new genus and species. Villicumia canteraensis new genus and species. Trigonoconcha acuta new genus and species. Concavodonta ovalis new species. Hemiconcavodonta minuta new genus and species. Hemiconcavodonta sp., Emiliania cuerdae new genus and species, and Concavoleda braulense new genus and species. Two new subfamilies are proposed: Praenuculinae and Concavodontinae. The genera Praenucula Pfab, Cuyopsis new genus. Villicumia new genus and Trigonoconcha new genus are included in the new subfamily Praenuculinae. The genera Concavodonta Babin and Melou, Hemiconcavodonta new genus and Emiliania new genus are included in the new subfamily Concavodontinae.

Silurian and Devonian molluscan bivalves from precordillera region, western Argentina

Silurian and Devonian bivalves recovered from the Los Espejos (Silurian to Lower Devonian) and Talacasto (Devonian) Formations of the Northern Precordillera Mountain belt are described and illustrated. The fauna comprises some new nuculoids Deceptrix (Devonodeceptrix) jachalensis n. subgen. and n. sp., Nuculites argentinum n. sp., and Anthracoleda (Pseudoleda) minuta n. subgen. and n. sp., as well as the new pterioid Actinopteria modesta n. sp. Additionally, many poorly known South American bivalves are redescribed and figured. Silurian–Devonian pelecypod faunas from some South American localities show differences both in diversity and in faunal composition. Causes for these differences are briefly discussed. Controls other than latitude are suggested in order to explain biogeographical affinities of the studied fauna.

Natasiinae, a new subfamilyof Tironuculidae (Bivalvia, Palaeotaxodonta) from the Acoite Formation (Early-Middle Arenig), Northwestern Argentine

Abstract The abundant collection of specimens of Natasia boliviensis ( Babin & Branisa ) from the Acoite Formation (Early to Middle Arenig, Northwestern Argentine) allows to analyze the pattern of dental growth and to propose the new subfamily Natasiinae, included in the Tironuculidae (Palaeotaxodonta). In the Acoite Formation two distinct populations of N. boliviensis from the late Early Arenig and Middle Arenig levels respectively, could be recognized. Minor differences between them allows to differentiate two subspecies: N. boliviensis boliviensis and N. boliviensis fragilis new ssp.

Un nuevo genero de Tironuculidae (Bivalvia, Palaeotaxodonta) en el arenigiano del noroeste argentino

Resumen En la Formacion Acoite, de edad Arenigiana inferior media, aflorante en el Noroeste de Argentina se ha detectado un nivel coquinoide que contiene un elevado numero de ejemplares de bivalvos pertenecientes a la Familia Tironuculidae. Sobre la base de la peculiar morfologia dentaria, estos ejemplares han sido asignados a nurvo genero, Natasia. El analisis de numerosos ejemplares ha permitido conocer las variaciones ontogeneticas en la poblacion. En este estudio se muestra que en los estadios juveniles la charnela esta constituida por una serie continua de dientes taxodontes, de los cuales los anteriores y posteriores son en chevron y los subumbonales, ortomorfos, mientras que en los adultos la charnela es de tipo actinodonto. Simultaneamente, en el transcurso de la ontogenia, el contorno de la conchilla pasa de trigonal a subrectangular.