BALSEIRO, D., B.G. ; WAISFELD, B.G.; VACCARI, N.E.,
The Cambrian-Ordovician boundary interval is a crucial moment in the ecology of trilobite communities. Temporal and spatial changes in the structure of olenid dominated communities included in the Parabolina Fauna, that flourished in uppermost Cambrian, largely storm-dominated, successions of Northwestern Argentina are analyzed. Species abundance distributions (SADs), richness, and evenness are investigated at three different spatial scales. At the local scale, relatively flat SADs and high evenness in upper offshore-offshore transition settings of the early HST, and uneven SADs in species-poor communities of the lower offshore and shelf environments of the TST are documented. This pattern is likely consistent with predictions of the intermediate disturbance hypothesis and is interpreted to be related to a trend in intensity and frequency of storm disturbance along local shallowing-up gradients. At the regional scale diversity trend across the sampled west-east transect is difficult to interpret, it does not match either the general depth or oxygen-related gradients. At the biogeographic scale, the patterns of abundance of two key taxa (Parabolina and Asaphellus) from the Cordillera Oriental were compared with those of the Famatina Ranges (Argentina) and the Oaxaca block (Mexico). At the Cordillera Oriental Parabolina is the most abundant taxon, while Asaphellus is restricted to a few samples with low abundances. At both Famatina and Oaxaca the opposite pattern was found. This could be interpreted in light of mass effect metacommunity dynamics, assuming environmental heterogeneity and dispersal across neighboring areas as influential factors. Low sample evenness values for the Cordillera Oriental contrast with those of coeval Laurentian communities, accounting for a possible diachronic change in the ecological structure of benthic communities during the Cambrian-Ordovician boundary interval.