ORIGINATION, EXTINCTION, AND CHANGES IN THE PALEOECOLOGICAL COMPOSITION OF THE OCEANS DURING THE PHANEROZOIC

Although taxonomic and ecological diversity of marine Metazoa have both increased during the Phanerozoic, the processes driving diversification are debated. Here, we examine changes in the ecological composition of the global oceans by applying a recently developed method for simultaneously analyzing the magnitude, selectivity, and compositional change of extinction and origination. We used a large paleoecological dataset of 22,784 genera coded using the 3D ecospace framework of Bush and Bambach. On a per-interval basis, we calculate the per-capita origination and extinction rates of genera within each ecological mode. Following Bush et al. 2020, we define magnitudes of origination and extinction as the mean rates of the individual ecological modes, compositional change as the standard deviation of those rates, and selectivity as the coefficient of variation, which quantifies the compositional change relative to magnitude. Our analyses produced four main findings: 1) The per-stage compositional changes caused by origination and extinction have declined across the Phanerozoic. In addition, the pattern of decline appears to occur in two phases: a Paleozoic decline that ends with a sharp increase at the end-Permian mass extinction, followed by a second post-Paleozoic decline. 2) Overall, origination drove slightly more compositional change than extinction, particularly in the later Phanerozoic. 3) Compositional changes caused by mass extinctions were elevated above background levels while changes due to origination during recovery intervals were not. 4) Ecological selectivity of extinction and origination during mass extinction and recovery intervals was lower than during background times; intense ecological restructuring was driven by the high magnitude of mass extinctions, not unusually intense selectivity. Origination, on the other hand, restructured ecological composition with more measured pacing, by the increasingly even filling of genera across ecological modes in the aftermath of successive extinction events. Thus, extinction and origination drove ecological change in different important ways. The Phanerozoic-scale decline in both is associated with increased ecological stability from stage to stage—that is, a decline in the ecological volatility of the oceans.