EDWIN H. COLBERT is Honorary Curator of Vertebrate Paleontology at the Museum of Northern Arizona in Flagstaff. In addition, he is Curator Emeritus for the American Museum of Natural History in New York and Professor Emeritus at Columbia University in New York. MICHAEL MORALES is Curator of Geology at The Museum of Northern Arizona in Flagstaff, Arizona. ELI C. MINKOFF is Professor of Biology and Chair of the Department of Biology at Bates College in Lewiston, Maine.