<P>The Hell Creek Formation is one of the most fascinating dinosaur graveyard sites ever discovered. Barnum Brown, a renowned American paleontologist, found the first Hell Creek site in Montana. We now know that this formation extends through Montana, North Dakota, South Dakota, and Wyoming. The Hell Creek Formation was laid down between 66.8-66 million years ago. This covers a very interesting span of time, including the last of the dinosaurs up to their extinction at the Cretaceous-Tertiary boundary. </P>
<P>In the Cretaceous period, the land of the Hell Creek Formation looked very different from the dry badlands in the area today. Forests thrived in a warm and moist subtropical climate. A seaway cut through the middle of the United States, stretching from the Gulf of Mexico to the Arctic Ocean. Because of the interaction between land and water in this area, both terrestrial (land) and aquatic animal fossils can be found in the Hell Creek Formation. This, combined with plant fossils, gives us a good idea what the environment was like. </P>
<P>The Hell Creek area was very diverse in the Cretaceous Period. On land, dinosaurs ruled. The first fossil of Tyrannosaurus rex was found here. Large herds of Triceratops roamed the land, along with dromeosaurs, hadrosaurs, pachycephaosaurs, ankylosaurs, ornithopods, and other theropods. Small mammals lived in the shadow of dinosaurs, including some of the first placental mammals. In the water there were turtles, crocodile-like champsosaurs, fish, and amphibians. Birds shared the sky with pterosaurs. </P>
<P>Hell Creek plant life was just as interesting and varied. There were many tall trees, but also low and medium-level shrubs. Flowering plants were well established by then, and there were many plants similar to the ones alive today. Gingko trees, conifers, cycads, moss, and ferns all lived in the area. There were no grasses, but there were willows, maples, magnolias, and sycamore trees. </P>
<P>When the asteroid struck off the Gulf of Mexico devastating extinctions followed. The Hell Creek area lost all dinosaurs (except for birds!) and many other species. Animals that lived in the water did better than animals that lived on land and more cold-blooded animals survived than warm-blooded. Large animals were especially vulnerable. Amphibians, champsosaurs, crocodiles, and turtles survived with little change. Primitive groups of mammals were dominant before the extinction, but placental mammals were more common after. There were still many fish in the habitat, though different species than there were before. </P>
<P>Plants were also affected by the extinction. Many species of flowering plants went extinct. After the impact, paleontologists found metasequoia trees, conifers, sycamore trees, katsura trees, and duckweed. Even though the extinction caused great ecological impact, the general range of species before and after the asteroid were very similar. The big difference is that the non-bird dinosaurs went extinct, leaving many ecological niches empty. It would take millions of years to recover from this loss.</P>