Brongniart and Cuvier showed how fossils were the key to unravelling the order of the strata in the Paris basin. Their fieldwork discoveries and anatomical reconstructions of fossils of large quadrupeds demonstrated the existence of former, pre-human worlds. These pre-human strata corresponded to levels which to that point had widely been regarded as of relatively recent origin. This is a map that changed the human conception of the age of the world. Their success was made possible by the principles of comparative anatomy. Analogies and homologies between animals of different species allowed Cuvier to reconstruct unfamiliar animals in correct proportions and in their entirety from only a few surviving anatomical fragments.