[6] Molecular dating studies estimate that Ruminantia split into the two sister clades Pecora and Tragulina around 45 million years ago, during the Eocene.

[2] The appearance of many Pecoran fossils during the Miocene suggests that its rapid diversification may correspond to the climate change events of that epoch,[6][7] as this time period was marked by much of Earth's forest habitats being replaced by grasslands due to widespread cooling and drying.

[8] The white-tail and mule-deer have been observed to primarily use the extended suspension, since in this phase of their gallop they leap over bushes and logs that are present in their brush environment.

However, a 2003 phylogenetic study by Alexandre Hassanin (of National Museum of Natural History, France) and colleagues, based on mitochondrial and nuclear analyses, revealed that Moschidae and Bovidae form a clade sister to Cervidae.

Several characteristics distinguish Pecora from its sister taxon, Tragulina: an astragalus with parallel sides, a loss of the trapezium, and differences in parts of the skull such as the petrosal bone.