Biostratigraphic implications of the phacopid Reedops and other co-ocurrig trilobites in the Pragian (Lower Devonian) of the Argentine Precordillera

RUSTÁN, JUAN JOSÉ; PERALTA, SILVIO

Congreso; XII Congreso de la Asociación Paleontológica Argentina; 2021

The phacopid tri lo bite Reedops is considered a Oevonian indicator not earlier than Pragian based on records from Euro pe, North America and northern Africa. Three poorly known specimens were reported from Argentina by Baldis and Peralta in 1991, putatively coming from the Silurian and Oevonian of the San Juan Province. However, due to the lack of illustrations and repository numbers, these fossils were neglected in subsequent contributions. After that first mention, Reedops cf. bronni was certainly record ed in Argentina from i solated outcrops of the lo wer strati graphi c interval of the Lower Oevon ian Talacasto Formation at Sierra de las Minitas in the La Rioja Province, to the north by Holloway and Rustán. We recently revised the original specimens of Baldis and Peralta housed at the Universidad Nacional de San Juan. The eroded specimen reported as Reedopssp. b from the Talacasto Formation at Cerro del Molle section, most probably corresponds to Echidnops tophomimus, on the basis of a very well-expressed vincular furrow medially, wide and strongly tubercled glabella and a ridge-like posterior border. In turn, the specimen originally reported as Reedops sp. b from the upper Silurian of the Mogotes Negros Formation at the Quebrada La Poi a secti on, accordin g to nomen clatural priority of th e stratigraphi c units comes from the La Rinconada Formation at north of the Quebrada Don Braulio, whose late Silurian to putative Oevonian age is under discussion. This specimen represents a different indeterminate phacopid genus based on the well-expressed vincular furrow, wide tubercled glabella and thoracic axial rings with large tubercles or transverse ridges. The specimen reported as Reedops sp. a, from the Talacasto Formation at the Río de las Chacritas section, indeed belongs to this genus based on the medially effaced vincular furrow, forwardly projected glabella and antero-laterally located eyes. Our revision restricts the biozone of Reedops to the lower (but not the lowermost) dark fine-grained part of the Talacasto Formation in theArgentine Precordillera This stratigraphic position is in accordance with a Pragian age based on palynological information about the Lochkovian/Pragian boundary in the Argentine Precordillera. otherwise virtually devoid of marker macrofossils. However, in spite of its biostratigraphic significance, Reedopsis ver y scarce and the bearing interval exhibits no lithological variations useful for correlation. In this context. the more abundant co-occurring trilobites Echidnops tophomimus and the calmoniid To/ocostops zore/oe, although endemic, define a typical trilobite assemblage easily recognizable. Hence, these taxa beco me valuable for identification of the Pragian in the Argentine Precordillera.