From the chronology of the sites (probably outdated table), it appears that the Group A is very old: Thomson, Echo Quarry, Trinity River. Even the brachyodont Mesohippus, Miohippus (Meso-Mio), Hypohippus, Megahippus and Parahippus (Hypo-Mega-Para) have the same shortness of the naso-incisival notch.

The first Cormohipparion A seems to be Early Clarendonian (Big Spring 15871); it is present at MacAdams (73912) together with the first Group B representant (MacAdams 74400).

Cormohipparion B is the last to appear: Hans Johnson, Xmas Quarry.