Ruminant artiodactyls are one of the most diverse clades of large mammals. Although the earliest reported fossils date back to the Eocene, the group became ecologically dominant and widespread in the Late Oligocene and Early Miocene. This time also marks the origin of most families of horned ruminants, many of which are still extant. The phylogenetic relationships among these families have been difficult to discern, despite several analyses of molecular data and detailed studies of fossil taxa. It has been suggested that this difficulty is due to a radiation in which the lineages diverged in rapid succession leaving little time for evolutionary divergence to accumulate between them. This study explores the effect of this radiation on the morphological evolution of the Ruminantia. Specifically, I ask whether rates of morphological evolution during the initial radiation differ significantly from corresponding rates during “background” periods.

Data for this study consist of nearly 200 discrete osteological and dental characters for 135 genera spanning 13 ruminant families. Evolutionary changes were reconstructed on a genus-level phylogeny of the clade, and per branch rates of change were estimated by dividing the number of changes along individual branches by their temporal durations.

The branches of the phylogeny were divided into two a priori categories: those that diverged during the window of time thought to include the radiation (regardless of phylogenetic position), and those that diverged at other times in the clade’s history. In general, there was no significant difference in the number of changes or rate of evolution between these two sets of branches. In a subsequent analysis, I compared branches that immediately led to the families involved in the radiation to branches from elsewhere in the phylogeny. The data suggest that branches marking the origin of the component families show significantly higher frequencies of character change than do other branches, and thus reject the idea of homogenous rates of character evolution.