We thank A. Aronowsky, J. Hagadorn, and three anonymous reviewers for their comments. P. Jung, R. Panchaud, and A. Heinz (Naturhistorisches Museum Basel) and E. Vokes provided large loans that are greatly appreciated. A number of other individuals and institutions generously lent samples and specimens, and we thank W.D. Allmon, Paleontological Research Institution; A.J. Baldinger, Museum of Comparative Zoology, Harvard University; P. Callomon, Academy of Natural Sciences of Philadelphia; J. Pickering, The Natural History Museum, London; R. Portell, Florida Museum of Natural History; and K. Westphal and E. Leslie, University of Wisconsin Madison Geology Museum. Thanks to L. Collins, A. Budd, J. Jackson, and A. Coates of the Panama Paleontology Project for use of material and data, and exchange of ideas. M. Jones, B. Fitzgerald, and J. Balhoff assisted greatly in data collection. Research was supported in part by the National Science Foundation (EAR-9316363).