Macroevolutionary trends in the Dinosauria: Cope's rule

Cope's rule is the tendency for body size to increase over time along a lineage. A set of 65 phylogenetically independent comparisons, between earlier and later genera, show that Cope's rule applied in dinosaurs: later genera were on average about 25% longer than the related earlier genera to which they were compared. The tendency for size to increase was not restricted to a particular clade within the group, nor to a particular time within its history. Small lineages were more likely to increase in size, and large lineages more likely to decrease: this pattern may indicate an intermediate optimum body size, but can also be explained as an artefact of data error. The rate of size increase estimated from the phylogenetic comparisons is significantly higher than the rate seen across the fauna as a whole. This difference could indicate that within-lineage selection for larger size was opposed by clade selection favouring smaller size, but data limitations mean that alternative explanations (which we discuss) cannot be excluded. We discuss ways of unlocking the full potential usefulness of phylogenies for studying the dynamics of evolutionary trends.