Early craniides (Brachiopoda, Craniiformea) from the Tremadocian of NW Argentina (Central Andean basin)

Abstract

Only a few representatives of the Order Craniida have been described from the lower Paleozoic so that its early evolutionary history remains poorly understood. Sofar, the oldest known craniids are from the upper Tremadocian of the Bohemian Massif and Bavaria. Here, we describe new craniid material from siliciclastic rocks ofthe Coquena Formation exposed in the Cordillera Oriental of northwestern Argentina. The material consists of some well-preserved dorsal valves belonging to twodifferent forms whose morphological features do not fit into any known genus, but available specimens are insufficient to formally erect new taxa. The fossil-bearinghorizon has been confidently dated as early late Tremadocian (Tr2) based on conodonts of the Paltodus deltifer Zone and associated trilobites, making this the oldestrecord of the Craniida worldwide. This discovery supports previous hypothesis that the origin and early diversification of Craniida took place in the cold-watershelves of western Gondwana, then spreading to temperate waters (Baltica, Kazakhstan, Iran, Cuyania terrane) by the Mid Ordovician.