 For example, the Chadronian Land Mammal Age in the late Eocene was defined by the boundaries of the Chadron Formation in Nebraska as well as the co-occurrence of Mesohippus, an early horse, and titanotheres, a family of rhinoceros-like animals.Titanotheres have since been found above the Chadron Formation, leaving the definition of the age uncertain.Since NAMLs are not tied to stratigraphic sections, they are not true chronostratigraphic stages, so some authors place quotes around "Ages".