Abstract
Mass extinctions can have dramatic effects on the trajectory of life, but in some cases the effects can be relatively small even when extinction rates are high. For example, the Late Ordovician mass extinction is the second most severe in terms of the proportion of genera eliminated, yet is noted for the lack of ecological consequences and shifts in clade dominance. By comparison, the end-Cretaceous mass extinction was less severe but eliminated several major clades while some rare surviving clades diversified in the Paleogene. This disconnect may be better understood by incorporating the phylogenetic relatedness of taxa into studies of mass extinctions, as the factors driving extinction and recovery are thought to be phylogenetically conserved and should therefore promote both origination and extinction of closely related taxa. Here, we test whether there was phylogenetic selectivity in extinction and origination using brachiopod genera from the Middle Ordovician through the Devonian. Using an index of taxonomic clustering (RCL) as a proxy for phylogenetic clustering, we find that A) both extinctions and originations shift from taxonomically random or weakly clustered within families in the Ordovician to strongly clustered in the Silurian and Devonian, beginning with the recovery following the Late Ordovician mass extinction, and B) the Late Ordovician mass extinction was itself only weakly clustered. Both results stand in stark contrast to Cretaceous-Cenozoic bivalves, which showed significant levels of taxonomic clustering of extinctions in the Cretaceous, including strong clustering in the mass extinction, but taxonomically random extinctions in the Cenozoic. The contrasting patterns between the Late Ordovician and end-Cretaceous events suggest a complex relationship between the phylogenetic selectivity of mass extinctions and the long-term phylogenetic signal in origination and extinction patterns.
Highlights
Mass extinctions can change the trajectory of life by eliminating established lineages and allowing other taxonomic groups to diversify, but in some cases the effects of the extinction can be relatively small even when extinction rates are high [1,2,3,4,5,6]
Evolutionary history refers to the length of the branches separating two taxa in a phylogenetic tree [9], and quantifies the amount of evolution that has occurred since the divergence of these taxa from their common ancestor
We find that family membership predicts extinction and origination in the Silurian and Devonian, but the signal is much weaker in the Ordovician, including the Late Ordovician mass extinction, suggesting that a major shift in the nature of diversification was established in the recovery interval and lasted for tens of millions of years
Summary
Mass extinctions can change the trajectory of life by eliminating established lineages and allowing other taxonomic groups to diversify, but in some cases the effects of the extinction can be relatively small even when extinction rates are high [1,2,3,4,5,6]. Phylogenetic Clustering across the Late Ordovician Mass Extinction end-Cretaceous ranks fifth. The end-Cretaceous, in contrast, ranks near the top of ecological severity, second only to the Late Permian mass extinction. This disconnect may arise because the intensity of a mass extinction is traditionally determined by counting taxa, typically the number of genera or families to go extinct [8], a metric that ignores the amount of evolutionary history removed by mass extinction. Random extinctions in a phylogenetic tree eliminate relatively little evolutionary history, even when extinction intensity is high, whereas the same level of extinction can remove much greater portions of evolutionary history when clustered (selective) [10,11]. A more complete understanding of these factors requires comparative studies between different events, time intervals, and taxa
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