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Science 27 July 2007:
Vol. 317. no. 5837, pp. 499 - 502
DOI: 10.1126/science.1142964
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Reports

A Cambrian Peak in Morphological Variation Within Trilobite Species

Mark Webster

Morphological variation within species is a raw material subject to natural selection. However, temporal change in morphological diversity has usually been studied in terms of variation among rather than within species. The distribution of polymorphic traits in cladistic character-taxon matrices reveals that the frequency and extent of morphological variation in 982 trilobite species are greatest early in the evolution of the group: Stratigraphically old and/or phylogenetically basal taxa are significantly more variable than younger and/or more derived taxa. Through its influence on evolutionary tempo, high intraspecific variation may have played a major role in the pronounced Cambrian diversification of trilobites.

Department of the Geophysical Sciences, University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637, USA. E-mail: mwebster{at}geosci.uchicago.edu

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Science. ISSN 0036-8075 (print), 1095-9203 (online)