A test for Cope's rule.

Cope's Rule refers to the tendency of body size to increase along an evolutionary lineage. This rule is commonly tested by comparing size differences in pairs of taxa, one of which is assumed to be ancestral to the other. It has recently been pointed out that this approach fails to account for the unknown number of speciation events separating each pair. Here, a test that does account for this degree of separation is described and applied to some published data for dinosaurs. A by-product of the analysis is an estimate of the origination rate of dinosaur species.