With the acceptance of the methods and research program developed by Georges Cuvier for the study of fossils, Paleontology had its first Kuhnian paradigm installed. Joseph Leidy began his works under this theoretical and methodological advice and practiced, within the scope of Paleontology, what Thomas Kuhn called normal science. However, with the accumulation of data from his taxonomic works, Leidy identified some issues that could not be answered in light of the cuvierian paradigm. Only the new paradigm, the evolutionism, could answer them, and thus Leidy joined the evolutionary theories of Charles Darwin. This transition process of a naturalist trained under one guidance, and that starts working under a new, is analyzed in this paper, taking into account the peculiarities of application of the framework provided by Kuhn, in a discipline such as Paleontology. Unlike the epistemological rupture predicted by Kuhn, in the paradigm shift in Paleontology many paleontologists have continued to work guided by the old paradigm, but producing usable data by evolutionists. Leidy was one of them, but his transition went further. From 1859, gradually, he began to use relationships of ancestry and natural selection as explanations for the morphological similarities between species that have taken place along the history of life.
Paradigm; Fossils; Evolutionism; Ancestry; Natural Selection