National Geographic : 2011 Oct
INTERLUDE WITH DWARFS Plentiful holes in fossil leaves such as this one (right) indicate that insects in the Bighorn Basin got more abundant and voracious as CO and temperatures rose during the PETM. Some mammals adapted by temporarily shrinking. Horses had shinbones the size of chicken drumsticks. The lesson, says Philip Gingerich: When it needs to be, "evolution is fast." One of the earliest complete horse skeletons (above), from a few million years later in the Eocene, is similar but 50 percent larger.