Abstract

The Ordovician diversification is marked by an increase in both marine diversity and ecospace occupation. Bivalves, like other groups, underwent a remarkable diversification in the Early Ordovician. The early phases of such a bivalve diversification took place in the Gondwanan basins of western Argentina. In the Northwestern Argentina (NWA) Basin, three clades originated during late Tremadocian–Floian times. In the Floian successions of the Famatina Basin, a probable basal arcoid is recorded. Genera from these two basins belong to 13 families. Phylogenetic analysis of the NWA heteroconchian bivalves indicates that redoniids and coxiconchinids may have originated during this radiation event. This taxonomic radiation also implies an ecological diversification. Ten guilds are recognized on the basis of bauplan, mode of life, and feeding types. Lifestyles included free endofaunal, free semi-endofaunal, semi-endobyssate, and epibyssate; feeding types included suspensivorous and detritivorous habits. Physiological changes imposed by colonization of low-salinity environments also account for guild definitions. Recent discoveries of Tremadoc to early Darriwilian bivalves from the NWA and Famatina basins indicate that the dominance of higher groups (e.g. Heteroconchia, Pteriomorphia) deviates from the patterns evident in other Gondwanan basins. This agrees with previous ideas supporting the importance of local radiations during the Ordovician diversification. Two new taxa are described, Eoredonia orientalis gen. et sp. nov. and Babinka notica sp. nov., and Coxiconchia sellaensis Sánchez & Babin is first reported from the NWA Basin.