The Chadronian is named after the Chadron Formation, a widespread component of the White River Group in Nebraska and South Dakota.
Other mammals which first appear at the start of the stage include Merycoidodon dunagani (a merycoidodontid), Brachyrhynchocyon dodgei, (a daphoenine amphicyonid, or "bear dog"), and Archaeotherium (an entelodont).
[1] Type assemblages (diagnostic fossil faunas used to define the subdivision) are bolded and underlined.
From youngest to oldest: Part of the Cypress Hills Formation (Saskatchewan, Canada) is also considered to be Chadronian in age.
[6] The Iniyoo local fauna (Oaxaca, Mexico) was initially reported to be Chadronian,[7] though revised dating suggests that it is Arikareean (late Oligocene) instead.