Whitneyan

Aetiocetus is a genus of extinct basal mysticete, or baleen whale that lived , in the Oligocene in the North Pacific ocean, around Japan, Mexico, and Oregon. It was first described by Douglas Emlong in 1966 and currently contains known four species, A. cotylalveus, A. polydentatus, A. tomitai, and A. weltoni. These whales are remarkable for their retention of teeth and presence of nutrient foramina, indicating that they possessed baleen. Thus, Aetiocetus represents the transition from teeth to baleen in Oligocene mysticetes. Baleen is a highly derived character, or synapomorphy, of mysticetes,

The genus Leptocyon (Greek: leptos slender + cyon dog) includes 11 species and is the oldest known canine. They were small and weighed around . They first appeared in North America around 34 million years ago in the Oligocene, at the same time as the Borophaginae, with whom they share features, indicating that these were two sister groups. Borophaginae skull and dentition were designed for a powerful killing bite compared with the Leptocyon which were designed for snatching small, fast-moving prey. The species L. delicatus is the smallest canid to have existed. At the close of their genus 9 mi

Simocetus (from simus, "pug-nosed", and cetus, "whale") is an extinct genus of toothed whale that lived during the Oligocene period, approximately , making it the oldest named toothed whale, although older unnamed toothed whales exist.