In the Cretaceous there were eight New World terrestrial ecosystems that were distinct or only shadowy versions of those present today. Now, 65 Ma later, 12 ecosystems constitute the recognizable subdivisions of the Earth's living and physical envelope. Knowledge of the intervening history is useful for conservation in several regards. It reveals how heterogeneous modern systems are in age and origin (e.g., lowland Neotropical rainforest formed between ca. 60 and 58 Ma; tundra and páramo between ca. 5 and 3 Ma), how individualized the response of component species to previous environmental change has been, and how levels of biodiversity have fluctuated through time. History provides approximate analogs for conditions anticipated in the near future and estimates of the biotic response. In addition, history documents that climatic events originally discovered and formerly associated with the high latitudes also affected tropical regions, and that discovery constitutes a valuable context for assessing previously held views about the stability of tropical communities and environments. There has been a rapid increase in the pace of both natural and human-induced environmental change and biotic response approaching modern times. Blurring the lines between neontology and paleontology, namely, conceptualizing ecosystems over Cretaceous and especially Late Cenozoic time, provides a dynamic view of these systems and furthers realistic strategies to conserve them.