Machaeridians from the middle and upper ordovician of the Argentine precordillera

Abstract

The machaeridians are marine Palaeozoic annelids, which developed a dorsal skeleton composed of calcite sclerites, achieving a world-wide distribution from the Early Ordovician to the Middle Permian. The absence of records from this group is notable in Argentina in spite of intense studies of Palaeozoic rocks. This fact can be explained by biases in sampling or because of preservation problems. Machaeridian sclerites of the families Plumulitidae and Turrilepadidae collected in Middle and Upper Ordovician rocks from the San Juan Precordillera, Argentina, are described here for the first time. Plumulites sp. is recorded in early Darriwilian strata from the Gualcamayo Formation exposed in the western flank of the Cerro La Chilca locality, and from the Las Aguaditas Formation in the Quebrada de Las Aguaditas section. Plumulitid and turrilepadid sclerites are described from the upper member of the Los Azules Formation (late Sandbian) in the Cerro Viejo de Huaco area. The material of the Los Azules Formation is the first record of turrilepadids in Sandbian rocks, which are scarce in Ordovician successions. The referred strata bear pelagic, benthic and nektobenthic faunal assemblages, constituting a similar taphocoenosis preserved in shales and calcareous silty shales related to transgressive events. The record of machaeridians expands our knowledge of the Ordovician biotas from Argentina and provides new information on the palaeobiogeographical distribution of this Palaeozoic annelid group.