*The hotter it gets, the smaller the animal?*
When Sifrhippus sandae, the earliest known horse, first appeared in the forests of North America more than 50 million years ago, it would not have been mistaken for a Clydesdale.
It weighed in at around 12 pounds–and it was destined to get much smaller over the ensuing millennia.
Sifrhippus lived during the Paleocene-Eocene Thermal Maximum (PETM), a 175,000-year interval of time some 56 million years ago in which average global temperatures rose by about 10 degrees Fahrenheit. (more…)