Guillermo L. Albanesi

New late Tremadocian (Early Ordovician) conodont and graptolite records from the southern South American Gondwana margin (Eastern Cordillera, Argentina)

New late Tremadocian (Early Ordovician) conodont and graptolite faunas from the eastern and central belts of the Eastern Cordillera (Jujuy Province, northwestern Argentina) are reported. The conodont fauna includes the guide species Paltodus deltifer pristinus, Paltodus deltifer deltifer, and Acodus deltatus (sensu lato), in association with other taxa, and the graptolites Aorograptus victoriae, Ancoragraptus cf. bulmani, and Adelograptus cf. altus. Overlapping ranges of the recorded species allow for a partial correlation between the Acodus deltatus- Paroistodus proteus and Aorograptus victoriae zones, and the Notopeltis orthometopa trilobite Zone. The conodont fauna includes a mixture of forms typical of the Baltoscandian and Laurentian provinces, respectively. The Early Ordovician basin of northwestern Argentina may correspond to the Shallow-Sea Realm and Cold Domain and probably records the development of a differentiated conodont province in the southern South American margin of Gondwana.

Tremadocian Graptolite-Conodont Biostratigraphy of the South American Gondwana margin (Eastern Cordillera, NW Argentina)

The Tremadocian graptolite and conodont faunas of the Gondwana Margin recorded at the Eastern Cordillera (EC) of Salta and Jujuy provinces (northwestern Argentina) are studied. The previous data and the new provided information indicate that a “no nominated interval” and several graptolite zones (Anisograptus matanensis, Rhabdinopora flabelliformis anglica, Bryograptus, Kiaerograptus, Kiaerograptus supremus, Araneograptus murrayi, and Hunnegraptus copiosus zones), as well as several conodont zones (Iapetognathus, Cordylodus angulatus, Paltodus deltifer, and Acodus deltatus - Paroistodus proteus zones) occur in the Ordovician successions studied. New or reappraised data from sections such as Angosto del Moreno, Angosto de Lampazar, Parcha-Incamayo-Incahuasi area, and Cajas range (EC western margin), and Alfarcito area and Mojotoro range (EC eastern margin), provide new information on the composition of the fossil faunas. A composite graptoliteconodont biostratigraphic framework, including records of key trilobites as external control group, is proposed for the Tremadocian of the South American margin of Gondwana. Both fossil groups are generally recorded from outer platform to ocean basin environments, showing significant similarities with the Baltoscandian faunas. Nevertheless, particular shallow-water facies include either specific forms or species associations that are common to the epeiric shallow-water seas of Laurentia. This fact demonstrates an important interplay between faunas of different regions of the Iapetus Ocean during the Tremadocian.

Biostratigraphy and palaeoecology of Middle–Late Ordovician conodont and graptolite faunas of the Las Chacritas River section, Precordillera of San Juan, Argentina

A conodont-graptolite biostratigraphic study was carried out on the top strata of the Sann Juan, Las Chacritas and Las Aguaditas formations in the La Trampa Range, Precordillera ofn San Juan in western Argentina. Significant conodont records in the San Juan and Lasn Chacritas formations allow for the recognition of the Yangtzeplacognathus crassus,n Eoplacognathus pseudoplanus ( Microzarkodina hagetiana andn M. ozarkodella subzones) and Eoplacognathus suecicus zones of Darriwilian age. Index species and co-occurrences of graptolites and conodontsn were recorded in the Las Aguaditas Formation allowing the identification of then Nemagraptus gracilis and the Pygodus anserinus zones,n which represent the Sandbian Stage. These data indicate a hiatus between the Las Chacritasn and the Las Aguaditas formations, corresponding to the Pygodus serra Zonen and the Pterograptus elegans and Hustedograptusn teretiusculus zones (upper Darriwilian). A total of 7287 identifiable conodontn elements were recorded from the study section. The species frequency registered for eachn zone shows that Periodon and Paroistodus are the mostn abundant taxa, which are indicative of open marine environments. The records of particularn conodont taxa, such as Histiodella, Periodon, Microzarkodina,n Eoplacognathus and Baltoniodus , allow a precise globaln correlation with other regions such as south-central China, Baltoscandia, North America,n Great Britain, Southern Australia and New Zealand. The graptolite fauna identified heren are recognized worldwide in equivalent strata in the Baltic region, Great Britain, Northn America, China, southern Australia and New Zealand. The presence of graptolites in then ribbon limestones of the Las Chacritas Formation is documented for the first time.

High resolution conodont-graptolite biostratigraphy in the Middle-Upper Ordovician of the Sierra de La Invernada Formation (Central Precordillera, Argentina)

Outcrops on the western slope of the La Invernada Range, San Juan Precordillera (W Argentina), were surveyed for lithofacial analysis and graptolite-conodont high resolution biostratigraphy. The study profile for the upper Sierra de La Invernada Formation is approximately 500 m thick and is located in the northern part of the La Invernada Range. Successive shallowing-deepening sedimentary cycles crop out throughout the upper part of the formation. The dominantly lower turbiditic sandy deposits alternate with shaly pelagic sedimentation, and occasional coarser intervals that record gravitational flows. Hummocky structures occur in the uppermost part of the formation, indicating sedimentation under storm weather conditions. Rich graptolite faunas representing six upper Middle to lower Upper Ordovician biozones have been recorded from this section: Pterograptus elegans (tentative assignment), Hustedograptus teretiusculus, Nemagraptus gracilis, Climacograptus bicornis, Diplacanthograptus caudatus, and Climacograptus tubuliferus biozones. This graptolite biostratigraphy is complemented by the conodont records from successive productive levels, which yielded conodonts that represent the Pygodus serra, P. anserinus, Amorphognathus tvaerensis, and A. superbus biozones. The recorded conodont-graptolite assemblages allow for a correlation with the upper Darriwilian, Sandbian and lower Katian global stages, and provide preliminary biofacies records for the upper Sierra de La Invernada Formation.

Lower Paleozoic biostratigraphy of South America: New contributions and advances

The early Paleozoic was the most critical time interval for the diversification of the biota on planet Earth. This phenomenon was driven by important paleocontinental and paleoceanographic changes, caused by intensive plate tectonic and climatic processes. The Cambrian explosion of life providing bauplans, the great Ordovician biodiversification event, the second most important mass extinction of life in the Phanerozoic that occurred in the Late Ordovician, and the appearance of land plants represent benchmarks in this evolutionary process (e.g., Albanesi et al., 2003; Webby et al., 2004; Munnecke and Servais, 2007; Li et al., 2007)