Museum Blog

Fossil leaves on the Interstate

Posted 3/4/2011 12:03 AM by Ian Miller | Comments

The American West is the best place on Earth to be a paleontologist. For the better part of the last 500 million years, sand, silt and mud have accumulated on vast stretches of the continent, entombing spectacular evidence of past life as fossils. These sediments form layers, which are the pages of the epic that is the evolution of multicellular life on Earth. Today, these sediments are sometimes found in small basins sandwiched between or flanking major mountain ranges and other times found in enormous pre-Rocky Mountain deposits dissected by modern mountains and basins.


What's so amazing about being a paleontologist in the American West is that more often than not there are fossils RIGHT under your feet! For instance, the rocks beneath Front Range communities from Fort Collins to Pueblo are absolutely packed with fossils. Just about anywhere you dig you can find fossils of leaves, shells, mammals, reptiles and dinosaurs.


Some of the most exquisite fossil sites in Colorado have been found during road and building construction. The Denver Museum of Nature and Science partners with the Colorado Department of Transportation (CDOT) to dig and collect fossil sites found on State road projects. Just this last Tuesday, March 1, CDOT paleontologist Steve Wallace, museum volunteer Steve Wagner, and I explored a new fossil leaf site found at I-25 and Alameda, right in the middle of the City of Denver. Steve had noticed some nice fossil leaves in slabs of rock turned up by an excavator a few days earlier and we returned to make a representative collection for the Museum.


One of the most remarkable things about the rocks in the Denver Basin (the area from Fort Collins to Colorado Springs to Limon) is that they record the Cretaceous-Tertiary (KT) boundary, which marks the end of the dinosaurs. Because the Denver Basin was filling with sand and mud between 68 and 64 million years ago, it's as if the sediments were a tape recorder running when the Cretaceous ended. We use the fossils in these Cretaceous and Tertiary rocks to understand how life and the earth changed across the KT boundary. All told, we have more than 400 fossil leaf sites in the Denver Basin spanning the KT boundary.  


Though we just collected them last Tuesday, we know that the leaves we dug are Cretaceous in age for two reasons. First, we've extensively mapped the Denver Basin, so we know what age rocks are underfoot based on our maps; and second, these species of plants occur only during the Cretaceous. This is really cool because, despite the fact that we have more than 400 fossil leaf sites in the Denver Basin, our sampling is lopsided and many of our sites occur in early Tertiary rocks. As a result, this site will provide significant information about what Colorado forests were like when dinosaurs roamed Denver.


Right now, all the fossils are drying slowly in tightly wrapped in industrial strength toilet paper (our preferred fossil leaf transport medium!). In a few months, we'll unwrap them and our trained team of Museum volunteers will prepare them, then we'll catalog and begin to study them. In the meantime, stay tuned for updates on this new fossil site.  



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