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.
Dominant Fossil Found in this Assemblage:
The flora fossils dominate this assemblage and portray a temperate deciduous hardwood forest, similar to those found in Southeast Asia. Trees like Ulmus, Juglans, and Cercidphyllum were host to a species of unidentified bird that lived during the time. Leaf fossils from 110 different species have been discovered in this formation, as well as fruit, seed, and cone fossils representing 58 species.
Swamp dwelling trees like Metasequoia and others of the Cupressaceae family created areas between their roots for fish, amphibians, and salamanders to feed on a family of Phryganeidae (caddisflies) and low flying insects. Metasequoia is the most iconic and prevalent flora fossil to come from the Bridge Creek Assemblage. It is also Oregon's state fossil and still lives today in Eastern Asia.
Mesohippus cf. bairdi was a three toed browsing horse that was likely very agile. Mesohippus’ three toed foot allowed it to dodge and weave effectively in heavily forested environments.
The unidentified bat found in the Bridge Creek Flora fed on either fruits or insects. As there is little known material from this bat, more research and new specimens will bring to light its feeding strategy and a more defined paleoecology within the Bridge Creek Flora.
For a complete list of fossils found at the Bridge Creek Assemblage, email us.
Last updated: November 27, 2022