The Scenario

As two voracious Deinonychus mount a frontal assault on the heavily armored nodosaur Sauropelta, a third Deinonychus prepares to attack from the stream bank at the rear. In the left foreground of this scene—which takes place 107 million years ago—a smaller theropod relative of Deinonychus, Microvenator, flees the area, as does a herd of the ornithischian Tenontosaurus in the left middle-ground. In the far distance, a trio of titanosaurs keeps a watchful eye on the conflict as they move away from the stream border.

The Environment

The fossil animals from this scene are preserved in the Cloverly Formation, which is exposed in north-central Wyoming and south-central Montana. Like the slightly older Morrison Formation, these sediments were washed out of the ancestral Rocky Mountains that were being uplifted just to the west. They were deposited across a broad flood plain that bordered a shallow continental sea to the east. Sandstones and conglomerates are commonly preserved in the bottom of ancient stream channels, and finer grained mudstones and siltstones represent flood debris deposited away from the stream channels on the adjacent plain. Much of the finer grained mudstone is made up of altered volcanic ash that presumably erupted out of volcanoes to the west in the ancestral Rockies. The forest contained abundant conifers, and the presence of crocodiles suggests that the temperatures rarely, if ever, dropped below freezing.