High Quality Content by WIKIPEDIA articles! Pantolestidae is an extinct family of semi-aquatic, placental mammals that took part in the first placental evolutionary radiation together with other early mammals such as the leptictids. Forming the core of the equally extinct order Pantolesta, the pantolestids evolved as a series of increasingly otter-like forms, ranging from the Middle Paleocene (60 mya) Bessoecetor to the Middle Eocene (50-40 mya) Buxolestes. They first appear in North America from where they spread to Europe.