Fossils are a bit tricky for me to identify families/genera/species so I'm going to have to leave this identification for a later date (did you get the pun in there?). Anyway this is a small but well preserved leaf fossil from Early Eocene Epoch some 50 million years ago. It was dug up as part of the Salmon River Flora in Lemhi County, Idaho. During this time period the Earth was actually warming up and keep in mind that despite being 50 million years old this is relatively recent period as the continents were very close to their present location. You can check out an Eocene Map here. And to give you a reference point the Cretaceous-Tertiary Extinction event (that's what killed off the dinosaurs) was 65. 5 million years ago.