Evolution of the Earliest Horses Driven by Climate Change in the Paleocene-Eocene Thermal Maximum. 2012. R. Secord, et al. Science 335: 959-962.
Sifrhippus sandae, first appeared in the forests of North America more than 50 million years ago. 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, a 175,000-year interval of time some 56 million years ago in which average global temperatures rose by about 10 degrees Fahrenheit. About a third of mammal species responded with a significant reduction in size during this time, some by as much as one-half.
Sifrhippus shrank by about 30 percent, to the size of a small house cat--about 8.5 pounds--in the first 130,000 years, then rebounded to about 15 pounds in the final 45,000 years. Secord says that the finding raises important questions about how plants and animals will respond to rapid change in the not-too-distant future.
Ornithologists, Secord says, have already started to notice that there may be a decrease in body size among birds. link