McShea, Daniel W. “Perspective metazoan complexity and evolution: is there a trend?.” Evolution 50, no. 2 (1996): 477-492.
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The notion that complexity increases in evolution is widely accepted, but the best-known evidence is highly impressionistic. Here I propose a scheme for understanding complexity that provides a conceptual basis for objective measurement. The scheme also shows complexity to be a broad term covering four independent types. For each type, I describe some of the measures that have been devised and review the evidence for trends in the maximum and mean. In metazoans as a whole, there is good evidence only for an early-Phanerozoic trend, and only in one type of complexity. For each of the other types, some trends have been documented, but only in a small number of metazoan subgroups.