Taxonomic diversity is only one possible biodiversity metric. Studies of taxonomic and morphologic diversity indicate that the two need not be closely linked. Mass extinctions, and their associated recovery periods, can be viewed as natural experiments for testing the link. After the Permian–Triassic mass extinction (ca. 253 Ma), ammonoid taxonomic diversity rapidly recovered to preextinction levels. Morphologic diversity, measured as variance, was used to compare taxonomic and morphologic recovery patterns of ammonoids after the Permian–Triassic. Morphologic diversity decreased significantly between the first and second stages of the Triassic (Griesbachian and Dienerian, respectively), despite an increase in taxonomic diversity and a tripling of sample size. During the third stage (Smithian), morphologic diversity returned to Griesbachian levels. The loss of representatives from two morphologically distinctive lineages at the end of the Griesbachian, followed by the evolution of many morphologically convergent forms, explains this pattern. These findings strengthen the case that morphologic metrics are valuable complements to taxonomic metrics in characterizing evolutionary patterns.

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