that although some studies have addressed phylogenetic patterns across mass extinction boundaries, many of these are at high taxonomic level and broad temporal scope. Several studies have addressed the issue of whether phylogenetic analyses to “correct’’ ranges using ghost lineages provide a better estimate of diversity than a purely taxic approach [compare Norell (1993) with Wagner (2000) and Lane et al. (2005)], but this is a different issue from using phylogenetic analysis to understand the structure of an extinction. No studies have explicitly addressed the impact of mass extinctions on phylogenetic diversity to my knowledge. Could one develop a metric of the severity of past extinction crises based on the extent of phylogenetic diversity lost? Identifying a reliable standard of comparison will be challenging, but is likely to provide a very different perspective from taxic studies.
Every paleontologist is familiar with lost, unique morphologies: the “weird wonders” of the Middle Cambrian Burgess Shale fauna, or Arthropleura, the immense, 20-cm-wide, several-meter-long millipede of the Carboniferous. Paleontologists have characterized such morphological distinctiveness as disparity [reviewed in Foote (1997) and Erwin (2007a)]. Although paleontologists have long used ranks of the Linnean hierarchy as a proxy for disparity, quantitative analyses of form have demonstrated that neither taxonomic rank nor taxic diversity is a reliable proxy for disparity (Foote, 1993). A host of quantitative methods has been proposed to analyze different aspects of morphology, and the occupation of morphospace by particular clades, with the appropriate techniques dependent on the question being addressed, and whether continuously variable characters or meristic characters are being used (Wagner, 1995). In almost every case examined, morphometric studies of disparity have demonstrated overwhelmingly that morphospace is rapidly constructed early in the evolutionary history of major clades, with taxonomic diversification often lagging behind (Erwin, 2007a).
Patterns of disparity have been analyzed across a number of mass extinctions, principally to understand the patterns and processes involved in the reestablishment of ecospace after these crises. Despite significant reductions in disparity in the immediate aftermath of a mass extinction, studies of brachiopods, crinoids, blastozoans, and ammonoids generally demonstrate rapid reexpansion of morphospace, although often in a different region than was occupied before the extinction [Foote (1999), Ciampaglio (2002, 2004), McGowan (2004), Villier and Korn (2004), and discussion in Erwin (2007a)]. Even in clades that experience almost catastrophic decreases in diversity and disparity, such as echinoids and ammo-