Morphologic Disparity

Every paleontologist is familiar with lost, unique morphologies: the "weird wonders'' of the Middle Cambrian Burgess Shale fauna, or Arthro-pleura, 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 morpho-space 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-

noids during the end-Permian mass extinction, disparity can rebound to even greater levels [e.g., McGowan (2004) for ammonoids]. Critically, these studies suggest that at least within broad body plans, the developmental process does not become so increasingly constrained with time as to limit the exploration of morphospace.

0 0

Post a comment