Faunal stability during the Early Oligocene climatic crash

Traditional Neo-Darwinism views species as highly flexible entities which adapt to climatic change by gradually evolving new morphologies. Of the 177 species of mammals from the upper Eocene-lower Oligocene (37-30 Ma) White River Group in the High Plains, most species are static for 2.4 million years on average, and some persist much longer. Only three examples of gradualism can be documented in the entire fauna, and these are mostly size changes. Contrary to expectations, most mammalian species show no change during the earliest Oligocene climatic crash (33.2 Ma), in spite of the fact that the vegetation changed from dense forests to open forested grassland, mean annual temperatures dropped 13°C, and conditions got much drier and more seasonal. Only a few mammalian lineages speciated, a few more went extinct, and the vast majority (62 out of 70) persisted through this climatic event with no observable response whatsoever. This evidence shows that organisms are much less responsive to the environment than short-term neontological studies suggest.