Santa Cruz Province, Argentina, has some of the richest fossil mammal localitiesin the world. However, the absolute and relative ages of its fossil localitieshave long been a source of confusion and debate. In particular, there has beenlongstanding disagreement about the relative ages of the fossils from thewestern part of the province in deposits of the Pinturas Formation compared withthose from the numerous localities of the Santa Cruz Formation along theAtlantic coast. Drawing on recent studies of the tuffaceous sediments in manyclassic fossil localities, and studies of fossil representatives of marsupials,rodents, and primates, we provide a synthesis of the temporal relationship amongfossil localities throughout the province. There is broad agreement between theresults of the tephrochronology and mammalian paleontology. Both tephracorrelations and paleontological comparisons indicate that the lower units ofthe Pinturas Formation are older than the sections of the Santa Cruz Formationpreserved at Monte León and Cerro Observatorio, supporting Ameghino'ssuggestion that part of the Pinturas Formation represents a distinct faunalzone. However, the upper unit of the Pinturas Formation seems to correspond inage with the lower part of the sections at Monte León and CerroObservatorio.