Caradoc bivalves from the Argentine precordillera: A local radiation-extinction event

Abstract

Pebbles with a diverse shelly fauna, including bivalves, brachiopods, gastropods, and bryozoans havebeen found in the basal diamictite of the Don Braulio Formation, cropping out in the Sierra de Villicum, Argentine Precordillera. The Don Braulio Formation is of Ashgill age, but according to the brachiopods the fauna of the pebbles is of early Caradoc age. On the basis of this age and some elements in common, pebbles probably come from the underlying La Cantera Formation. Bivalves include nine genera, seven of which are new. The relatively high diversity and endemism of this fauna contrasts with the low diverse and widespread Caradocian associations from other regions of South America. The life span of this association is short. The Ashgillian beds of the Don Braulio Formation have yielded a different, low diverse association of bivalves. Causes of this brief radiation-extinction event affecting the bivalves are not clear. According to the recently proposed palaeobiogeographic reconstructions, during the early Caradoc the Precordillera attained the “pre-accretion stage”, and was placed near to the Gondwana continent. Consequently, a geographic isolation cannot be advocated to account for the high endemism of the bivalves. The principal cause of the endemism could be linked to climatic conditions. According to the global and local temperature curves, during the Caradoc an increase in water temperature occurred. Caradoc units of the Precordillera (Las Plantas and Las Aguaditas formations) include some carbonate deposits suggesting temperate waters. This could be the reason for the rapid diversification of the bivalves. Conversely, the dominantly clastic sedimentation of other Gondwanan areas such as the Central Andean basin of NW Argentina and Bolivia suggests that water temperature could have been cooler than in the Precordillera basin and that cold waters could have been a climatic barrier precluding expansion. The rapid extinction of the fauna could be linked to the progressive cooling during the Ashgill which culminates with the latest Ordovician glacial event.