Willwood Formation

The Willwood Formation is a sedimentary sequence of the Bighorn Basin in Big Horn, Hot Springs, Park and Washakie counties, Wyoming, deposited from the late Paleocene into the early Eocene, roughly 56 to 48 million years ago, spanning the Clarkforkian, Wasatchian and Bridgerian North American land-mammal ages; radiometric dating of volcanic tuffs suggests an age range of about 55 to 52 million years, placing the Paleocene-Eocene boundary near its base. It consists of fine-grained mudstone and shale interbedded with medium-grained sandstone and occasional conglomerate, and is exceptionally fossil-rich, preserving trace fossils and remains of mammals, birds, crocodilians, squamates, turtles, amphibians, fish, invertebrates and plants. Notable finds include the early horse Sifrhippus, early primates Cantius and Tetonius, the giant flightless bird Gastornis, and early carnivoran ancestors.