Radiation, diversity and environmental expansion of Early Ordovician ostracods: a view from the Southern Hemisphere

Tremadocian-Floian siliciclastic successions in the Argentine Cordillera Oriental are of great interest for the study of early ostracod radiation. Four genera and five species of ostracods have so far been documented in the basin from the Early-Late Tremadocian (Tr2). This contrasts with occurrences from other palaeocontinental areas, where typically only one species is described. Patterns of environmental distribution and ecological approaches of ostracod assemblages were analysed on the basis of richness, Fisher's alpha diversity, relative abundance and occupancy. Ecological patterns remained stable throughout the studied interval, mostly with one genus notably dominant over the others. At the regional scale, dominant taxa occur at a relatively large number of sites, which display high occupancy. The literature data on carbonate successions from the Baltoscandian basin show a similar pattern, with a single taxon dominant over the others in local assemblages and wide regional distribution of these dominants. This ecological pattern would therefore have persisted at least until the Dapingian, independently of the sedimentary regime. Ostracods were already present along the onshore-offshore gradient during the initial stages of radiation, showing a strong preference for deep subtidal settings during Tr2 and then spreading to shallower environments in Tr3 and to deeper ones in the Floian (Fl2-Fl3). A compilation of ostracod diversity data from several regions shows a remarkable increase during the Darriwilian. Based on the regions studied herein, this rise in diversity is decoupled from the environmental expansion of ostracods, as they were already recorded along the onshore-offshore profile early in their history on both siliciclastic and carbonate shelves. The Argentine Cordillera Oriental could be considered as a cradle of diversity, with an important role in the subsequent radiation of the group.