Morphospace trends underlying a global turnover: ecological dynamics of trilobite assemblages at the onset of the Ordovician Radiation

Abstract

The early Palaeozoic was an interval of critical importance in the history of life as it records major events such as the Cambrian Explosion and the Ordovician Radiation. Analyses of macroevolutionary and macroecological dynamics explored from a morphospace perspective at different scales and from a palaeoecological viewpoint remain scarce. In this contribution we show the potential power that morphospace analysis can bring to the study of the ecological context of radiations. For this we focus on important trilobite assemblages ranging from the late Cambrian - Early Ordovician of the Argentine Cordillera Oriental. We calculated trilobite disparity through seven time bins that encompass the Furongian, Tremadocian and the Floian. Our results show a hierarchical structure in disparity patterns, where regional disparity is modulated by its beta component during the Furongian – lower Tremadocian, changing towards the Floian where alpha disparity becomes a stronger contributor. These trends reveal a shift in the assembly of regional disparity, modulated by the way local coexistence is controlled. Additionally, the temporal decoupling recorded in diversity and disparity trends can be related to the way these patterns are structured across the ecological hierarchy. This coincides with moments of high compositional turnover, congruent with the timing of transition between trilobite evolutionary faunas (TEFs) at a global scale. Peaks in regional disparity are related to the sum of these faunas contributing high values rather than to a growing disparity of the successive TEFs, suggesting that the rise in disparity is due to the coexistence of evolutionary faunas. Regional-scale dynamics described herein coupled with previous evidence regarding global patterns allow us to consider that the transition between trilobite evolutionary faunas was mainly related to increasing niche differentiation, followed by an increase in dispersal. These results suggest a context of higher ecological complexity than previously envisaged for the early stages of the Ordovician Radiation.