09 December 2015

9. Bridge Creek Flora

The dawn redwood Metsequoia
Photo by staff of John Day Fossil Beds NM
Location: Oregon, USA
Formation: Big Basin Member of the John Day Formation
Age: Oligocene (33 Ma)

The fossil plants of the Big Basin Member in the John Day Basin are only about a million years younger than the fossils of Florissant, but that brief interval of time encompasses one of the most important periods of change in the Cenozoic.  The clock ticked over from the Eocene to the Oligocene 33.7 million years ago, midway between the deposition of the two lagerstätten, and this boundary marks a significant climatological and ecological shift worldwide.  Global temperature had been slowly decreasing through the Late Eocene, but at the boundary it drops precipitously (though exactly how precipitously is still a matter of some debate).  For the first time in the Cenozoic, ice sheets began to grow in Antarctica and ecological turnover occurred across the face of Earth in an event known in Europe as the Grande Coupure.  The plants of Oregon's Bridge Creek Flora very clearly illustrate this new world order.  Though they grew in the same spot where the forests of the Clarno Formation had existed 11 million years earlier, the Bridge Creek plants draw comparisons not to jungles but to temperate deciduous woodlands. The most abundant fossils from these sites are leaves and needles of trees, some - oak, elm, beech, and several others - compare to species from the Southeast US, while others - including ginkgoes and katsuras - have living relatives native to China and Japan.  The most notable of these plants with Asian affinities is the dawn redwood Metasequoia, a deciduous conifer related, as the name suggests, to redwoods and sequoias.  Metasequoia has the distinction of being one of the very few taxa first described as a fossil and to later be discovered alive and well, in this case in the central Chinese province of Hubei.  Its fossils have been found worldwide, but they are especially common in the Bridge Creek Flora and other John Day Basin sites of similar age, leading to its designation as Oregon's State Fossil in 2005.

Visit: Many of the sites from which the Bridge Creek Flora have been uncovered are protected by John Day Fossil Beds National Monument.  One site, behind the football field at Fossil High School, is administered by the Oregon Paleo Lands Institute and allows public collecting for a fee.
Fossils: As with the Clarno Nut Beds, the best place to see Bridge Creek fossils is at the national monument's Condon Fossil Center, which I will reiterate should be near the top of anyone's list of excellent regional fossil museums.
Is there a relevant book full of gorgeous fossil photos that I can gift to a paleontologically-minded friend?: No, but there should be.

This post is part of my 2015 Paleontology Advent Calendar, a series of vignettes on lagerstätten - sites of exceptional fossil preservation - that document changes in climate and environments through the Cenozoic.  You can see the other posts here.

No comments: