Extinction drives both expansion and contraction of morphological disparity in trilobites

Among the many contributions of palaeontology to evolutionary biology, understanding the evolution of morphological diversity is one of the most relevant. Disparity dynamics throughout the history of a clade have been widely explored in the fossil record, mostly by comparing taxonomic and morphological diversity. However, such comparisons reflect net diversification but fail to individualize extinction and origination rates, limiting our understanding of macroevolutionary processes. Here, we analyse Cambrian-Devonian trilobite disparity by integrating morphometric data with stratigraphic ranges from the Paleobiology Database. We generate time series for taxonomic and morphological diversity, extinction and origination rates and individualize the selectivity of extinction and origination relative to morphology using logistic regression. Our results show that net diversification alone does not explain disparity trends, as major increases in trilobite morphological diversity occur during intervals of stable taxonomic diversity. However, the selectivity of extinction and origination provides key insights into the underlying trends. A modified measure of the macroevolutionary influence of extinction and origination shows how these processes interact to shape disparity, producing increases and decreases individually. Notably, extinction emerges as the main driver of morphospace expansion and contraction in trilobites, challenging the classic view of extinction as a constraining factor for morphological evolution.