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Wonderful strife: systematics, stem groups, and the phylogenetic signal of the Cambrian radiation

Published online by Cambridge University Press:  08 April 2016

Derek E. G. Briggs  and
Richard A. Fortey
Derek E. G. Briggs
Affiliation:
Department of Geology and Geophysics, Yale University, Post Office Box 208109, New Haven, Connecticut 06520-8109. E-mail: [email protected]
Richard A. Fortey
Affiliation:
Department of Palaeontology, The Natural History Museum, Cromwell Road, London SW7 5BD, United Kingdom. E-mail: [email protected]
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Abstract

Gould's Wonderful Life (1989) was a landmark in the investigation of the Cambrian radiation. Gould argued that a number of experimental body plans (“problematica”) had evolved only to become extinct, and that the Cambrian was a time of special fecundity in animal design. He focused attention on the meaning and significance of morphological disparity versus diversity, and provoked attempts to quantify disparity as an evolutionary metric. He used the Burgess Shale as a springboard to emphasize the important role of contingency in evolution, an idea that he reiterated for the next 13 years. These ideas set the agenda for much subsequent research. Since 1989 cladistic analyses have accommodated most of the problematic Cambrian taxa as stem groups of living taxa. Morphological disparity has been shown to be similar in Cambrian times as now. Konservat-Lagerstätten other than the Burgess Shale have yielded important new discoveries, particularly of arthropods and chordates, which have extended the range of recognized major clades still further back in time. The objective definition of a phylum remains controversial and may be impossible: it can be defined in terms of crown or total group, but the former reveals little about the Cambrian radiation. Divergence times of the major groups remain to be resolved, although molecular and fossil dates are coming closer. Although “superphyla” may have diverged deep in the Proterozoic, “explosive” evolution of these clades near the base of the Cambrian remains a possibility. The fossil record remains a critical source of data on the early evolution of multicellular organisms.

Type
Generating Diversity
Information
Paleobiology , Volume 31 , Issue S2 , 2005 , pp. 94 - 112
Copyright
Copyright © The Paleontological Society 

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