This is a comprehensive work on the systematics, relationships and occurrences of all mammal taxa, living and extinct, down through the rank of genus. Since George Gaylord Simpson's 1945 classification, the paleontological record has been greatly expanded, the timescale has been recalibrated, and much debate and progress concerning the theoretical underpinnings of systematization has occurred. Malcolm McKenna inherited the project from Simpson and, with Susan Bell, has constructed an updated hierarchical system that reflects the genealogy of Mammalia.