Decoupling of local and regional dominance in trilobite assemblages from northwestern Argentina: new insights into Cambro-Ordovician ecological changes

Abstract

Components of biodiversity are strongly scale dependent, but the relative importance of the patterns that operate at different scales and the links between them have been overlooked. To disentangle the ecological structure of Cambro-Ordovician trilobite assemblages from the Argentine Cordillera Oriental at different scales, we explore patterns of abundance, dominance and occupancy across the onshore–offshore profile, and through three time intervals: Furongian, earliest Late Tremadocian (Tr2), latest Middle Floian–earliest Late Floian (Fl2–Fl3). At the regional scale, single taxa are overwhelming dominant in the Furongian (Parabolina) and in the earliest Late Tremadocian (Leptoplastides). Several dominants occur in the Floian, but just one (Famatinolithus) attains high occupancy and, rarely, high dominance. In contrast, only the Furongian records highly dominated local assemblages, whereas dominance distinctly decreases among Tr2 and Fl2–Fl3 ones. Thus, when both scales of analysis are combined, an unexpected scenario becomes evident: Tr2 assemblages resemble those of the Furongian at the regional scale, but mirror those of the Floian at the local scale. These results highlight a decoupling in local versus regional structures triggered by an earlier switch in dominance in local communities and a delayed change at the regional scale. Interestingly, this decrease in local dominance matches previous analyses accounting for a coeval step-up in local evenness, suggesting that the Tr2 appears as a pivotal interval in the reorganization of communities in the Cordillera Oriental. This scenario emphasizes that biogeographical regions witnessed different regional-scale processes, and suggests that scaling local and regional patterns provides new insights to unravel the history of biodiversity among benthic communities.