The Cretaceous Dinosaur Nesting Grounds of the Willow Creek Anticline

Exposures of the Upper Cretaceous Two Medicine Formation within the Willow Creek Anticline (WCA) consist of interbedded mudstones and sandstones representing an ancient floodplain. In 1978, Marion Brandvold found baby duck-billed dinosaur bones here. Investigation of the area by paleontologists John R. “Jack” Horner and Robert “Bob” Makela led to the discovery of the first dinosaur nests with associated eggs and babies in the western hemisphere (Horner and Makela, 1979). The close proximity of nests indicated these dinosaurs nested in groups. Examination of the fossils suggested this dinosaur cared for its young after they hatched and inspired its name: Maiasaura (the “good mother reptile”). Thousands of larger bones of Maiasaura were preserved nearby in an extensive bone bed and have been the subject of robust taphonomic and histologic studies. Nearby sites, including “Egg Mountain”, preserve egg clutches of the theropod dinosaur Troodon and the first in ovo dinosaur embryos (Horner and Weishampel, 1988). The WCA preserves not only fossil bones, but also trace fossils from a variety of Cretaceous animals. Nesting structures, footprints, coprolites, gastric pellets, insect pupae, burrows, and more have been discovered here, providing an unparalleled window into this ancient ecosystem (e.g. Varricchio et al., 1997; Chin, 2007).