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Morphological complexity increase in metazoans

Published online by Cambridge University Press:  08 February 2016

James W. Valentine ,
Allen G. Collins  and
C. Porter Meyer
James W. Valentine
Affiliation:
Museum of Paleontology and Department of Integrative Biology, University of California, Berkeley, California 94720
Allen G. Collins
Affiliation:
Museum of Paleontology and Department of Integrative Biology, University of California, Berkeley, California 94720
C. Porter Meyer
Affiliation:
Museum of Paleontology and Department of Integrative Biology, University of California, Berkeley, California 94720
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Abstract

The number of cell types required for the construction of a metazoan body plan can serve as an index of morphological (or anatomical) complexity; living metazoans range from four (placozoans) to over 200 (hominids) somatic cell types. A plot of the times of origin of body plans against their cell type numbers suggests that the upper bound of complexity has increased more or less steadily from the earliest metazoans until today, at an average rate of about one cell type per 3 m.y. (when nerve cell types are lumped). Computer models in which increase or decrease in cell type number was random were used to investigate the behavior of the upper bound of cell type number in evolving clades. The models are Markovian; variance in cell type number increases linearly through time. Scaled to the fossil record of the upper bound of cell type numbers, the models suggest that early rates of increase in maximum complexity were relatively high. The models and the data are mutually consistent and suggest that the Metazoa originated near 600 Ma, that the metazoan “explosion” near the Precambrian/Cambrian transition was not associated with any important increase in complexity of body plans, and that important decreases in the upper bound of complexity are unlikely to have occurred.

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Articles
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Paleobiology , Volume 20 , Issue 2 , Spring 1994 , pp. 131 - 142
Copyright
Copyright © The Paleontological Society 

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