Osvaldo L. Bordonaro

Global Cambrian trilobite palaeobiogeography assessed using parsimony analysis of endemicity

Abstract Palaeobiogeographical data on Cambrian trilobites obtained during the twentieth century are combined in this paper to evaluate palaeoceanographic links through c. 30 myr, once these arthropods biomineralized. Worldwide major tectonostratigraphic units are characterized at series intervals of Cambrian time and datasets of trilobite genera (629 for Cambrian Series 2, 965 for Cambrian Series 3, and 866 for the Furongian Series) are analysed using parsimony analysis of endemicity. Special attention is given to the biogeographical observations made in microcontinents and exotic terranes. The same is done for platform-basinal transects of well-known continental margins. The parsimony analysis of endemicity analysis resulted in distinct palaeogeographical area groupings among the tectonostratigraphic units. With these groupings, several palaeobiogeographical units are distinguished, which do not necessarily fit the previously proposed biogeographical realms and provinces. Their development and spatial distributions are broadly controlled by Cambrian palaeoclimates, palaeogeographical conditions (e.g. carbonate productivity and anoxic conditions) and ocean current circulation. Supplementary material: Global dataset of Cambrian Epoch 2 (A), Cambrian Epoch 3 (B) and the Furongian Epoch (C) trilobite genera are provided at: http://www.geolsoc.org.uk/SUP18669

Cambrian agnostoid trilobites from Mendoza, Argentina: A systematic revision and biostratigraphic implications

Agnostoid trilobites from the Carlos Rusconi collections in the Museum of Natural Sciences of Mendoza, Argentina, are revised. The agnostoids were collected in the southern Precordillera of Mendoza Province, western Argentina. Revised taxa include Agnostus microcephalus (Rusconi), Lotagnostus ( Lotagnostus ) peladensis ? (Rusconi), Lotagnostus ( Lotagnostus ) lasherensis (Rusconi), Homagnostus pehuenchensis Rusconi, Kormagnostus seclusus (Walcott), Kormagnostus ? beltensis (Lochman in Lochman and Duncan), Hypagnostus parvifrons (Linnarsson), Ptychagnostus cuyanus (Rusconi), Ptychagnostus aculeatus (Angelin), Lejopyge laevigata (Dalman), Tomagnostella exsculpta (Angelin), Diplagnostus planicauda (Angelin), Oedorhachis typicalis Resser, Clavagnostus calensis Rusconi and Clavagnostus repandus (Westergard in Holm and Westergard). Some taxa of uncertain affinities are recognized and new specimens assignated to Lotagnostus and Pseudagnostinae are illustrated. Hypagnostus parvifrons, L. laevigata and C. repandus are reported for the first time in South America. Global correlation with previously described agnostoid faunas indicates that the outcrops range from middle Middle Cambrian to latest Late Cambrian in age.

Trilobite-based biostratigraphic model (biofacies and biozonation) for the Middle Cambrian carbonate platform of the Argentine Precordillera

A preliminary biostratigraphic model that combines trilobite biofacies and biozonation is proposed for the early Marjumian (Middle Cambrian) of the Precordillera of western Argentina. This interval involves a variety of paleogeographic–paleoenvironmental settings across the carbonate platform, ranging from its interior to exterior areas beyond the platform margin and including both open-marine and near-shore environments. This model is based on three stratigraphic sections of the basal part of the Alojamiento Formation, which crops out in the Cordon del Alojamiento (Provincia de Mendoza) and Cordon de Santa Clara (Provincia de San Juan); and three sections of the Soldano Member of La Laja Formation in the Sierra Chica de Zonda (Provincia de San Juan). Three biofacies were recognized on the basis of trilobite associations that show a marked environmental distribution: 1) The Ptychagnostidae Biofacies is represented by an assemblage exclusively dominated by agnostoids, most of them belonging to Ptychagnostus; this assemblage occurs in mudstones and calcareous shales deposited below storm wave-base seaward of the platform margin; 2) the Peronopsidae Biofacies also developed in the external areas but it occurs in limestones deposited between fair-weather and storm wave-base; this assemblage is likewise dominated by agnostoids, but instead belonging to Peronopsis, with the rest of the fauna comprising polymeroids (Dolichometopidae, Oryctocephalidae, Zacanthoididae and Alokistocaridae); 3) the Alokistocaridae Biofacies occurs in grainstones, mudstones and calcareous shales deposited between fair-weather and storm wave-base under changeable conditions located near the platform margin and in the interior. It is dominated by alokistocarids whose relative abundance increases landward from 52% to 58% and to 95%. The remaining components belong to Dorypygidae, Zacanthoididae and Oryctocephalidae; Eodiscidae and Peronopsidae comprise minor proportions in the most seaward location. Due to the contrasting, environmentally mediated associations, these sections belong to three essentially synchronous biozones erected previously for the Middle Cambrian of western Laurentia, the Ptychagnostus gibbus, Oryctocephalus and Ehmaniella Biozones.