In the late nineteenth century, the European Paleolithic was divided into stages, each of which was characterized by a distinct stone tool type known historically as a fossile directeur, or index fossil. Today, these index fossils are no longer explicitly used to date assemblages because they are known to overlap widely, but they continue to be used as key components in the periodization of the Paleolithic. This study addresses two major questions: (1) How have archaeologists justified retaining these index fossils to distinguish the Lower from the Middle Paleolithic? and (2) Does the diachronic patterning of these tool types support this periodization? The results reveal (1) that the overlap of index fossils was a known problem from the beginning and prehistorians repeatedly modified the classification to accommodate data which documented this overlap without rejecting its fundamental index-fossil-based core and (2) that, while temporal trends agreeing with the Lower/Middle Paleolithic periodization can be identified, they are insignificant compared with the chronological variation observed in each of these tool types. The Lower/Middle Paleolithic periodization should therefore be revised on the basis of a comprehensive examination of multiple lines of evidence, not just lithic typology.