The aging of eco-genealogical units during the Phanerozoic

Abstract The chances that existing taxa are replaced by new taxa decreases over Phanerozoic time, and older taxa persist for longer and longer. This long-term trend has been interpreted as resulting from repeated culling and non-replacement of groups with high turnover or, in other words, from long-term selection for extinction resistance. It is therefore assumed that taxon-specific properties are critical for this trend. However, taxa are part of higher-level ecological genealogical units that represent shared genealogies and histories of ecological interactions. Structural aspects such as the composition of the evolving eco-genealogical units as a possible explanation have received little attention. Here we analyzed the temporal change within Phanerozoic benthic marine mega-assemblages (PMAs). PMAs are groups of marine animal genera delineated through hierarchical modular partitioning of a multilayer spatiotemporal network and interpreted as eco-genealogical units. For each higher-level PMA, we calculated how its median genus age, its richness, the evolutionary rates, and the spatial β-diversity changed over time. The resulting estimates support the assumption that the assemblages are meaningful eco-evolutionary units, because their temporal patterns differ from those of randomly chosen Phanerozoic time intervals. PMAs differ from randomly chosen intervals in that the increase of genus age is associated with a decline in origination proportions and with increasing β-diversities. This contradicts the hypothesis of a simple Phanerozoic sorting toward extinction resistance and generalization. The observed pattern can be explained as a process of eco-evolutionary entrenchment resulting from conditioned assembly and weak within-lineage directionality toward specialization and geographic restriction.