Mammal faunas from western North America exhibit no significant change in species richness with latitude during the Torrejonian (ca. 63–60 Ma) and Tiffanian (ca. 60–58 Ma) North American Land Mammal Ages, in contrast to a strong richness gradient in modern mammalian faunas of the same region today. The latitudinal gradient in oxygen isotope composition of mammalian bioapatite from the Paleocene faunas is similar to that of modern meteoric and surface waters, suggesting that the temperature gradient in the Paleocene was similar to the modern one. The flat richness gradient in the middle Paleocene indicates either different responses to climatic gradients of faunas dominated by extinct clades of placental mammals or distinct ecological processes during the Paleocene diversification of mammals following the end-Cretaceous mass extinction.