Cope's rule concerns only the radiation phase of a clade, overlooking the phase of the clade decline; thus it is incomplete. Changes of body size during the entire evolutionary history of a clade are exemplified here by three trilobite groups - Ptychopariina, Asaphina and Phacopida. Increasing diversity of the clade is associated with increase in maximum body size during the radiation phase, and decreasing diversity is generally associated with a decrease in maximum body size. Two basic patterns of the maximum body size changes are observed during the decline of the clade. The first one is characterized by a high correlation between diversity and the maximum body size, and indicative of species attrition that is nonselective with respect to the body size. The second one is characterized by a weak correlation between diversity and maximum body size, and typical of selective species attrition in relation to size.

Key words: body size, evolution, trends, trilobites

Jerzy Trammer, Instytut Geologii Podstawowej, Uniwersytet Warszawski, al. Żwirki i Wigury 93, PL-02-089 Warszawa, Poland; Andrzej Kaim, Instytut Paleobiologii PAN, ul. Twarda 51/53, PL-00-114 Warszawa, Poland.