MASS EXTINCTIONS AND THE STRUCTURE AND FUNCTION OF ECOSYSTEMS PINCELLI M. HULL AND SIMON A. F. DARROCH Department of Geology & Geophysics, Yale University, PO Box 208109, New Haven, CT 06520-8109 USA <pincelli.hull@yale.edu> ABSTRACT.–Mass extinctions shape the history of life and can be used to inform understanding of the current biodiversity crisis. In this paper, a general introduction is provided to the methods used to investigate the ecosystem effects of mass extinctions (Part I) and to explore major patterns and outstanding research questions in the field (Part II). The five largest mass extinctions of the Phanerozoic had profoundly different effects on the structure and function of ecosystems, although the causes of these differences are currently unclear. Outstanding questions and knowledge gaps are identified that need to be addressed if the fossil record is to be used as a means of informing the dynamics of future biodiversity loss and ecosystem change. INTRODUCTION Mass extinctions profoundly influence the history of life. Although they are defined by their impacts on taxonomic diversity (e.g., Raup and Sepkoski, 1982; Sepkoski, 1986), their effects extend far beyond the loss of species richness. Mass extinctions have shaped the history of Phanerozoic biodiversity, sparked innovations in morphology, life history, and ecology, and led to the construction of entirely new ecosystems in their aftermath (e.g., Sepkoski, 1981; Bambach et al., 2002; Wagner et al., 2006; Erwin, 2008). For example, the Permo–Triassic extinction event permanently altered the composition of marine ecosystems from one dominated by brachiopods and crinoids to one dominated by mollusks and echinoderms (Gould and Calloway, 1980; Sepkoski, 1981; Erwin, 1990; Greene et al., 2011). Similarly, the Cretaceous–Paleogene mass extinction reset open-ocean food webs, with two groups of top marine predators in late Cretaceous food webs, non-acanthomorph fishes and marine reptiles, replaced in the Paleogene by acanthomorph fishes and marine mammals, respectively (Friedman, 2010; Uhen, 2010). More broadly, the ecological effects of mass extinctions include the flourishing of unusual ecosystems in their immediate aftermath, feedbacks within and between the geosphere and biosphere, and long- term changes in the structure and function of earth’s ecosystems. A paleontological perspective on the history of life, particularly regarding past biotic crises, is now in demand (Erwin, 2009; Barnosky et al., 2011; Harnik et al., 2012; Hönisch et al., 2012). The cumulative effects of humanity on the earth, ocean, atmosphere, and biosphere arguably are driving the sixth major mass extinction (Myers, 1990; Leaky and Lewin, 1992). While this classification is still debated (Barnosky et al., 2011), the potential for anthropogenic activity to change the biosphere is not. Anthropogenic influences detrimental to ecosystems include extensive habitat modification and fragmentation, the introduction of nonnative and invasive species, overexploitation of natural resources, pollution, global climate change, and ocean acidification, among many others (IUCN; Hassan et al., 2005; Fischlin et al., 2007; Halpern et al., 2008). Past ecosystem dynamics may help inform the present diversity crisis by revealing how the interaction between environmental perturbations, species diversity, and ecosystem structure influence species extinction and ecosystem change. This includes understanding the potential importance of ecosystems in mitigating or enhancing the effects of environmental disturbance on the loss of species, the rate of rediversification, or the change in ecosystem function across mass-extinction events (Erwin, 2001; Jackson and Erwin, 2006; Barnosky et al., 2011). In Ecosystem Paleobiology and Geobiology, The Paleontological Society Short Course, October 26, 2013. The Paleontological Society Papers, Volume 19, Andrew M. Bush, Sara B. Pruss, and Jonathan L. Payne (eds.). Copyright © 2013 The Paleontological Society.