۱٫ Introduction

The past three decades have seen major advances in our understanding of mass extinctions, and yet consensus has not been reached on the causes of each of Earth’s greatest biotic catastrophes, and much less so its numerous lesser calamities. The past decade in particular has seen research efforts directed toward understanding the context and nature of environmental changes associated with extinction events. This has resulted in significant new data and observations from the fields of geochronology, geochemistry, mineralogy, palaeontology, sedimentology, stratigraphy, palaeomagnetism, volcanology and geophysics, several of which are the focus case studies in this Special Issue. Mass extinction theories have developed from the simple death-bysea-level-change hypothesis first proposed almost fifty years ago (Newell, 1967) into ever more complex, multicausal scenarios. The body of evidence associated with mass extinctions lends much support to proximal kill mechanisms that include anoxia (e.g. House, 1985; Buggisch, 1991; Wignall and Hallam, 1992; Brenchley et al., 1994, 2001; Isozaki, 1994, 1997; Wignall and Twitchett, 1996; Bond et al., 2004; Grice et al., 2005; Bond and Wignall, 2010; Shen et al., 2016; Wang et al., 2016), global warming (e.g. McElwain et al., 1999, 2005; Beerling and Berner, 2002; Wilf et al., 2003; Joachimski et al., 2009, 2012; Gómez and Goy, 2011; Sun et al., 2012, 2015; Punekar et al., 2014; Petersen et al., 2016), and ocean acidification (e.g. Hautmann, 2004; Payne et al., 2007; Hautmann et al., 2008a, 2008b; Clapham and Payne, 2011; Montenegro et al., 2011; Beauchamp and Grasby, 2012; Greene et al., 2012; Hinojosa et al., 2012; Martindale et al., 2012; Heydari et al., 2013; Clarkson et al., 2015) coupled with changes in atmospheric greenhouse gases, notably CO2, to name just a few. Advances in analytical capabilities have led to the identification of new kill mechanisms, such as toxic metal poisoning (e.g. Sanei et al., 2012; Sial et al., 2013, 2014; Grasby et al., 2015, 2016; Percival et al., 2015; Font et al., 2016; Thibodeau et al., 2016). It is increasingly widely thought that large igneous province (LIP) eruptions might be the driver of many of the purported proximal kill mechanisms, and the temporal link between volcanism and extinction is now well-established (Courtillot, 1999; Wignall, 2001; Courtillot and Renne, 2003; Bond and Wignall, 2014). However, the link between the two phenomena is still not fully understood. Whilst the deleterious effects of LIP volcanism are implicated in many extinction scenarios, a role for extra-terrestrial drivers should not be ignored. Bolide impact is of course famously implicated in the end-Cretaceous crisis (Alvarez et al., 1980; Hildebrand et al., 1991), and a role for deadly bursts of cosmic gamma rays also has advocates (Piran and Jimenez, 2014), notably for the Late Ordovician extinction (Melott et al., 2004).