destacan las facies de lutitas oscuras del intervalo superior del Miembro Alfarcito. De estos niveles se describe una fauna de trilobites compuesta por Leptoplastides marianus (Hoek), Parabolinella sp., Hapalopleura acantha Malanca y Brandán, y Asaphellus sp. Una calcarenita interestratificada en las limolitas contiene una asociación poco diversa de conodontes asignable a la Zona de Cordylodus angulatus (Tremadociano inferior). La edad ordovícica temprana está confirmada, además, por el registro del graptolito Rhabdinopora flabelliformis ssp. en una arenisca que subyace a los niveles estudiados. Las lutitas oscuras se acumularon por decantación durante períodos de máxima inundación, en un ambiente reductor de offshore distal. La fauna de trilobites está ampliamente dominada por L. marianus. Resulta interesante destacar que la parte superior del Miembro Alfarcito no

The middle part of the Santa Rosita Formation (Tremadocian) is well exposed in the Iruya area, northwestern Argentina. At the Pantipampa and Rodeo Colorado localities, it is composed of shale and sandstone representing a wave-dominated shelf with influence of storm activity. Twenty-two trilobite species are described from these localities. Low-diversity assemblages from the lower part of the succession (Kainella meridionalis Kobayashi, Asaphellus catamarcensis Kobayashi, Leptoplastides marianus [Hoek]) are diagnostic of the early Tremadocian Kainella meridionalis Zone. Trilobites from the middle part of the sequence are much more diverse. Kainella teiichii Vaccari and Waisfeld, Gymnagnostus kobayashii n. sp., Conophrys sp. indet., Asaphellus clarksoni n. sp., A. stenorhachis (Harrington), A. isabelae Meroi Arcerito, Waisfeld and Balseiro, Ogygiocaris? iruyensis n. sp., Niobe (Niobella) inflecta (Harrington and Leanza) n. comb., Metayuepingia riccardii n. sp., Pseudokainella keideli Harrington, Apatokephalus rugosus n. sp., Onychopyge acenolazai n. sp., O. gonzalezae n. sp., Nileus cingolanii n. sp., N. erici n. sp., Leptoplastides marianus, Parabolinella sp. indet., Hapalopleura sp. indet., and Ceratopygidae gen. et sp. indet., occur at different levels of the Kainella teiichii Zone. This biostratigraphic unit includes the oldest records of Nileus Dalman and Ogygiocaris? Angelin; Metayuepingia Liu, Niobe (Niobella) Reed, and Onychopyge Harrington are here first reported from the Tremadocian of southwest Gondwana. Finally, the uppermost part of the succession is characterized by the absence of the genus Kainella Walcott and the occurrence of Bienvillia tetragonalis (Harrington), Asaphellus stenorhachis, Pseudokainella keideli and Leptoplastides sp. indet., which are indicative of the middle Tremadocian Bienvillia tetragonalis Zone. The trilobites described in this paper provide a basis for the refinement of correlations with other Lower Ordovician sections of the Cordillera Oriental. The genera recognized have their closest affinities with faunas from Scandinavia, Great Britain, and China.

The upper part of the Santa Rosita Formation (Ordovician, Tremadocian) in the Nazareno area, Cordillera Oriental, northwestern Argentina, records the vertical passage of high-energy, shallow water platform environments to offshore settings. Eighteen trilobite species are described from this locality for the first time. Although the taxa from the lower part of the succession (Leptoplastides sp., Asaphellus sp.) are scarce and non-age diagnostic, those from the upper part include diverse assemblages partially assigned to the late Tremadocian Notopeltis orthometopa Zone. Systematic descriptions of several species (Geragnostus nesossii Harrington and Leanza, G. callaveiformis Harrington and Leanza, Asaphellus jujuanus Harrington, Notopeltis orthometopa [Harrington], Mekynophrys nanna Harrington, Ceratopyge forficuloides Harrington and Leanza, Apatokephalus tibicen Přibyl and Vanĕk) are improved, the genus Nileus Dalman (including N. australis n. sp.) is first reported from the Tremadocian of western Gondwana, and new species of Asaphellus Callaway (A. nazarenensis n. sp.), Conophrys Callaway, and Apatokephalus Brøgger are described. The trilobites have their closest affinities with faunas from Norway and Sweden. Notopeltis orthometopa and Mekynophrys nanna are restricted to the uppermost part of the succession, well above the first records of most other trilobites recognized.

The upper part of the Santa Rosita Formation (Ordovician, Tremadocian) in the Nazareno area, Cordillera Oriental, northwestern Argentina, records the vertical passage of high-energy, shallow water platform environments to offshore settings. Eighteen trilobite species are described from this locality for the first time. Although the taxa from the lower part of the succession (Leptoplastidessp.,Asaphellussp.) are scarce and non-age diagnostic, those from the upper part include diverse assemblages partially assigned to the late TremadocianNotopeltis orthometopaZone. Systematic descriptions of several species (Geragnostus nesossiiHarrington and Leanza,G. callaveiformisHarrington and Leanza,Asaphellus jujuanusHarrington,Notopeltis orthometopa[Harrington],Mekynophrys nannaHarrington,Ceratopyge forficuloidesHarrington and Leanza,Apatokephalus tibicenPřibyl and Vanĕk) are improved, the genusNileusDalman (includingN. australisn. sp.) is first reported from the Tremadocian of western Gondwana, and new species ofAsaphellusCallaway (A. nazarenensisn. sp.),ConophrysCallaway, andApatokephalusBrøgger are described. The trilobites have their closest affinities with faunas from Norway and Sweden.Notopeltis orthometopaandMekynophrys nannaare restricted to the uppermost part of the succession, well above the first records of most other trilobites recognized.

El Miembro Peña Negra de la Formación Volcancito (Tremadociense) del Sistema de Famatina (noroeste argentino) consta de pizarras negras graptolíticas, que se investigaron paleontológicamente, capa a capa, a lo largo de 400 m de sucesión en las secciones complementarias de Peña Negra y río Volcancito, la segunda de las cuales incluye los niveles más modernos. Los resultados bioestratigráficos permiten una correlación precisa con las asociaciones evolutivas y cronozonas globales de graptolitos del Tremadociense, y la reevaluación de todas las dataciones e identificaciones previas de la unidad. Los graptolitos más antiguos se localizan en el Miembro Filo Azul de la formación (Cámbrico Superior-Tremadociense) y se adscriben muy probablemente a la cronozona de Rhabdinopora flabelliformis parabola de la base del Ordovícico, lo que concuerda con los datos suministrados por los trilobites y conodontos. Desde el punto de vista paleontológico, se identifican y describen Rhabdinopora fl abelliformis flabelliformis, R. f. anglica, R. f. socialis, R. f. bryograptoides?, R. cf. enigma, Anisograptus matanensis y Bryograptus? sp. La forma Rhabdinopora flabelliformis famatinensis se propone restringirla al holotipo para obviar su mala caracterización taxonómica. Las nuevas formas Rhabdinopora turneri sp. nov. y R. flabelliformis acenolazai subsp. nov., guardan ciertas similitudes con los graptolitos planctónicos R. praeparabola y R. flabelliformis parabola, respectivamente. Sin embargo, los nuevos taxones del Famatina coexisten en horizontes más modernos de las cronozonas de A. matanensis y R. f. anglica, que claramente postdatan la aparición del desarrollo proximal trirradiado.