I gratefully acknowledge D.A. Pearson of the Pioneer Trails Regional Museum (Bowman County, North Dakota), under whose direction the Medicine Pole Hills locality has been excavated, for extensive discussion and for his kind permission to work on these fossils. I am also indebted to J. Oakland, who allowed excavations on his land. D. Pearson and members of the 2001 PTRM field school provided invaluable assistance in the field. L.J. Hickey was helpful in interpreting the local stratigraphy as well as suggesting the name Tuberculacerta; I also thank A. Kihm for helpful discussions about the taphonomy and stratigraphy of the Medicine Pole Hills deposits. L. Gauthier skillfully executed the illustrations shown in Figures 3.1–3, 4–17, 19, and 21; T. Colantonio, Figure 3.6. I am grateful to J. Gauthier, R. Etheridge, K. de Queiroz, and B.-A. Bhullar for discussion. This paper, or parts thereof, also benefited from the comments of J. Botha, K. de Queiroz, J. Downs, W. Joyce, A. Kihm, D. Polly, T. Tsuihiji, M. Wills, and from two thoughtful and anonymous reviewers. Financial support came from a Yale University Fellowship, an NSF Graduate Research Fellowship, a GSA Student Research Grant (#7022-01), and a grant from the ECOSAVE Foundation of the Yale Institute for Biospheric Studies.