The taxonomic diversity of ammonoids, in terms or the number ot taxa
  preserved, provides an incomplete picture of the extinction pattern during the
  Permian because of a strongly biased fossil record. The analysis of
  morphological disparity (the variety of shell shapes) is a powerfut comple-
  mentary tool for testing hypotheses about the selectivity of extinction and
  permits the recognition of three distinct patterns.  First, a trend of decreasing
  disparity, ranging for about 30 million years, led to a minimum disparity
  immediatety before the Permian-Triassic boundary.  Second, the strongly
  selective Capitanian crisis fits a modet of background extinction driven by
  standard environmental changes.  Third, the end-Permian mass extinction
  operated as a random, nonselective sorting of morphologies, which is
  consistent with a catastrophic cause.