Dominance trends in trilobite communities from Northwest Argentina. New insights into the cambrian-ordovician radiations.

WAISFELD, B. G.; BALSEIRO, D.

Mendoza

Congreso; 4th International Palaeonological Congress; 2014

International Palaeontological Association

Significant changes in the ecological structure of benthic marine communities took place in Cambrian and Ordovician times. These changes have been mostly addressed in the literature on the basis of local trends in richness and evenness, however, and despite that patterns of biodiversity are strongly scale-dependent, aspects beyond the local scale are still little investigated. Previous research in Cambro-Ordovician successions of the Cordillera Oriental envisaged largely intergrading, highly dominated trilobite communities with little spatial turnover among different environments. In order to disentangle this ecological structure at different spatial scales we explore patterns of abundance, evenness, and occupancy of dominant taxa across the onshore-offshore profile, through three time intervals (Furongian, middle Tremadocian, middle-upper Floian). The analysis of local dominance at the regional scale shows single taxa overwhelming dominant in the Furongian and in the middle Tremadocian (Parabolina and Leptoplastides respectively) across different environments, but six different dominants in the Floian. In that interval only one taxon (Famatinolithus) attains high occupancy but seldom reaches high dominance. In contrast, at the local scale only the Furongian records highly dominated assemblages, whereas local dominance decreases in the middle Tremadocian and the Floian. Thus, when both scales of analysis are combined, an unexpected scenario becomes evident. Strongly dominated assemblages at local and regional scales are just restricted to the Furongian, whereas middle Tremadocian assemblages are much less dominated at the local scale, and only mirror Furongian ones regionally, being most assemblages dominated by the same taxon along the whole gradient. Dominance decreases during the Floian, accounting for more even assemblages at both local and regional scales. Interestingly, the important middle Tremadocian decrease in local dominance matches previous analyses of local evenness trajectory in the Cordillera Oriental. Evenness progressively rose during the Cambro-Ordovician but exhibited a significant step up in the middle Tremadocian without an associated increase in richness. Overall, the middle Tremadocian appears as a pivotal interval in the organization of communities from the Cordillera Oriental when a dramatic change in ecological structure took place. The switch in dominance occurred at first in local communities but only later at the regional scale, implying a decoupling in local versus regional dominance structures. These results highlight the impact of different scaled perspectives in the way the ecological processes are understood, and suggest that the aggregation of patterns and processes developed at local and regional scales within the same region are also influential to unravel Early Paleozoic reorganization of marine communities.