Extreme environmental changes have pushed global biodiversity past its breaking point just a handful of times, now referred to as the “Big Five” mass extinctions. These events probably represent “perfect storms,” where individual pressures, often severe in themselves, combine to catastrophic effect, driving sweeping changes to the biota. Better constraints on the timing of biotic and environmental changes and on the spatial locations and biologies of victims and survivors have improved analyses aiming to identify the roles of traits and other factors in promoting survival. These new data also help to identify hitchhiking effects, where certain evolutionary lineages or biological traits were lost or survived not because of the direct action of the extinction drivers, but because they were carried along by other traits, such as geographic-range size. Adding other dimensions or currencies of biodiversity, such as biological form or function, gives further insights into the evolutionary roles of mass extinctions: modes of life are surprisingly extinction-resistant, even in the face of extensive species loss. However, the extinction filter is just one major factor in reshaping biodiversity at these events. Longer-term impacts also flow partly from their ensuing rebounds, and more work is needed to uncover the circumstances that spur some groups and modes of life to re-diversify while others are relegated to marginal roles in the post-extinction world. Analyses of past extinction events and their rebounds bring macroevolutionary insights to the present-day biodiversity crisis—approaching a “perfect storm” in the intensity and scale of its pressures—and help to pinpoint the lineages, modes of life, and organismal forms most vulnerable to extinction and failed rebounds.
Mass extinctions are natural experiments on the short- and long-term consequences of pushing biotas past breaking points, often with lasting effects on the structure and function of biodiversity. General properties of mass extinctions—exceptionally severe, taxonomically broad, global losses of taxa—are starting to come into focus through comparisons among dimensions of biodiversity, including morphological, functional, and phylogenetic diversity. Notably, functional diversity tends to persist despite severe losses of taxonomic diversity, whereas taxic and morphological losses may or may not be coupled. One of the biggest challenges in synthesizing and extracting general consequences of these events has been that they are often driven by multiple, interacting pressures, and the taxa and their traits vary among events, making it difficult to link single stressors to specific traits. Ongoing improvements in the taxonomic and stratigraphic resolution of these events for multiple clades will sharpen tests for selectivity and help to isolate hitchhiking effects, whereby organismal traits are carried by differential survival or extinction of taxa owing to other organismal or higher-level attributes, such as geographic-range size. Direct comparative analyses across multiple extinction events will also clarify the impacts of particular drivers on taxa, functional traits, and morphologies. It is not just the extinction filter that deserves attention, as the longer-term impact of extinctions derives in part from their ensuing rebounds. More work is needed to uncover the biotic and abiotic circumstances that spur some clades into re-diversification while relegating others to marginal shares of biodiversity. Combined insights from mass extinction filters and their rebounds bring a macroevolutionary view to approaching the biodiversity crisis in the Anthropocene, helping to pinpoint the clades, functional groups, and morphologies most vulnerable to extinction and failed rebounds.
