Cope's Rule in Equidae: a quantitative view

Estimated body masses of 40 species of horses plotted against the geological time at which they occur. The upward swing of the plot suggests an increasing size trend moving towards the present day. In fact, the plot reflects mostly an increase in diversity: small-bodied horse species have co-existed with larger forms throughout most of their geological history. It is only in the last few million years that the lineage has been dominated by large-bodied forms.