Deinonychus lived in North America during the Early Cretaceous in the regions now known as Montana, Wyoming, Utah, and Maryland (Thulborn, 1990). During this time North America was dominated by an Interior Seaway, which prograded and receded many times during the Cretaceous over the West Central regions. A series of rivers drained into the seaway producing a continuous sequence of overlapping deltas. The region looked much like modern Louisiana with stagnant bayous, estuaries and cypress swamps. The Southern parts of this region were well drained floodplains (Bakker, 1993).
During the Early Cretaceous the plant world was experiencing a turn over from nonflowering gymnosperm plants such as conifers, cycads and ferns to flowering angiosperm plants. The reason for this could be because of the dinosaurs turnover from large herbivores like the Sauropods to the smaller herbivores like the Hadrosaurs. These Hadrosaurs, and others, would heavily browse on the plants and the nonflowering ones would not regenerate as quickly as the flowering plants. Thus a turn over would result as the flowering plants would regenerate after heavy browsing more quickly over the nonflowering plants and take over the vacancies that resulted (Bakker, 1993).