The Permian brachiopod genus Trigonotreta Koenig and its occurrence in Argentina

Abstract

A review of the Permian spiriferid brachiopod genus Trigonotreta and its occurrence in the Early Permian of Argentina is provided herein. Several species are known from the Late Palaeozoic sequences of the Argentinean Precordillera. These include Trigonotreta sp. and Trigonotreta riojanensis (Lech and Aceñolaza), from the Río del Peñón Formation (Río Blanco Basin). Trigonotreta pericoensis (Leanza), from the Tupe Formation at the La Herradura creek locality (Paganzo Basin) and Trigonotreta saniuaneneis (Lech and Aceñolaza), from Del Salto Formation (Calingasta-Uspallata Basin). These species are characterised by being small to medium sized, relatively transverse, with cardinal extremities often strongly angular. Costa e are weakly bifurcated and superimposed on weakly developed lateral flank plications adjacent to the fastigium and sulcus. The Argentinean species are close to the oldest known Indian species of the genus, Trigonotreta hesdoensis (Sahni and Dutt), Particularly with respect to the nature of its weakly fasciculated costae. Further study will refine the details of the relationship of the South American species with those from elsewhere in Gondwana and may permit the recognition of a distinctive lineage. The presence of the genus in Argentina in the earliest Permian is an important palaeobiogeographical observation that raises questions about the probable migration routes of the genus from the western Gondwanan South American margin to eastern Australia and India. The Precordilleran region appears to be the likely site of the first appearance of Trigonotreta. Species with relatively simple costae appeared first. These gave rise to more complex species with a more elaborate costal pattern indicating an evolutionary progression through time.