A fossil leaf of Populus wyomingiana, possibly a relative of living poplar trees.
P. wyomingiana expanded its range north into Wyoming some 55 million years ago, after the Earth became warmer, during the period known as the Paleocene-Eocene Thermal Maximum. After the climate cooled again, P. wyomingiana disappeared from this region. Excavated in Wyoming’s Big Horn Basin by Scott Wing, a paleobotanist at the Smithsonian’s National Museum of Natural History, fossils such as this are helping scientists predict how modern-day plants will respond to global warming.