We thank Michael Yudell, coeditor of the series in which this book appears, for the invitation to participate in this important initiative. At Columbia University Press we would never have been able to proceed without the enthusiastic support and encouragement of our editor Patrick Fitzgerald and his associates Ryan Groendyk and Brian Smith, active participants in this project. We are also deeply indebted to Kathryn Jorge at CUP, and to Ben Kolstad at Cenveo, for their meticulous and expert attention during the production process. At the American Museum of Natural History, we express our appreciation to Apurva Narechania, Martine Zilversmit, Jeff Rosenfeld, and Michael Tessler for their help with phylogenetic analysis and to Vivian Schwartz for her able editorial guidance. As always, we are both far more than grateful to Patricia Wynne for the wonderful illustrations that impart life to our text. Finally, Erin DeSalle and Jeanne Kelly consistently showed imperturbable good humor during the writing phase; our deepest thanks, as ever, to you both.