Three toed sloth (Bradypus tridactylus). Skeleton of a three toed sloth, or 'Ai', copperplate engraving from Cuvier's 'Ossamens Fossiles' 1812. Cuvier saw that the key to understanding fossils was to relate their bones to animals currently known. In this he was the father of comparative anatomy - a field that was to be crucial to the founding of modern biology. Here a modern sloth provides reference for the even larger bones of the extinct ground sloth megatherium that had been discovered in South America and displayed in Spain. Cuvier was among the first to clarify the reality of extinction.