That was the fate of the earliest-known horse, Sifrhippus sandrae (or its jaw bone, at least). It started out at around 5.4 kilograms (12 pounds) but during the Paleocene-Eocene Thermal Maximum ...

Dawn Horse lived in a much hotter world than ours: the tropical Eocene. Giant fern trees grew in Canada’s Arctic and grass had yet to evolve. These ancestor horses clambered through dense ...