The Bridge Creek assemblage documents major changes in Earth’s climate that took place around 33 million years ago. At that time, the regional climate became 3° to 6° C cooler and precipitation became more seasonal. The ultimate sources of these gradual climate changes are likely attributable to several global scale events, including a global drop in CO2, changes in the Earth’s obliquity, and particularly the separation of Antarctica from Tasmania and South America, which set up the Antarctic circumpolar current and led to the formation of persistent Antarctic continental ice sheets.
Eastern Oregon gradually became drier and more seasonal. The area was covered with hardwood forests, lakes, and swamps which resembled the balmy parts of the Southeastern United States or Southeast Asia. Many of the trees in the ancient forest are related to modern alders, elms, maples, and oaks. They lived alongside the “dawn redwood” (Metasequoia) a conifer species that is still living in eastern Asia. In the Bridge Creek Flora, we find the remains of leaves, fish, amphibians, birds, and insects preserved like pressed flowers in a book. Because it was a lakebed environment, few mammals, other than the occasional bat, are preserved.