Denali National Park and Preserve contains many fossiliferous geologic units. Most of these are from the warm shallow seas present throughout much of the late Paleozoic and Mesozoic eras. Some of the accreted terranes traveled north from lower latitudes where marine life flourished. Fossils include invertebrates (as the dominant type), marine mollusks, insects, plant debris, and abundant trace fossils. Thousands of trace fossils of fish, pterosaurs, theropods, hadrosaurs, birds, and terrestrial and aquatic invertebrates are preserved in the Cantwell Formation, making this one of the best-preserved Late Cretaceous polar continental ecosystems in the global paleontological record (Brease et al. 2009).