Can fossil embryos demonstrate evolutionary changes in reproductive modes? How have changes in ocean chemistry in the past affected the development of marine organisms? What can the microstructure of fossil bone and teeth reveal about maturation time, longevity and changes in growth phases? This book addresses these and other issues and document with numerous examples and illustrates how fossils provide evidence not only of adult anatomy but also about the life history of individuals at different growth stages. The central topic of Biology today, the transformations occurring during the life of an organism and the mechanisms behind them, is addressed for the first time in an integrative manner for extinct animals.