Friday, March 26, 2010

Hey, Look - Fossil Leaves

So this blog has had a steady diet of paleo lately, and, well, I'm okay with that. My initial thought in the formulation of this post was to pick a random image from the Western Regional Field Course this past summer and make a short little post on it, to keep the readers (all seven!) on their toes.

The above image is from Mesa Verde National Park in the southwestern corner of the magnificently geological state of Colorado. In reality, all states are amazing geologically, even Nebraska (you can't see the Ogalalla, but they still have a Fluvial gem in the Platte).

Mesa Verde may be most well known for the excellent preservation of Anasazi cliff dwellings, but the geology of this park is truly amazing. Perhaps Aucoin will get into the geology-archaeology intertwining in a future post here.

Back to the leaf fossils. The fossil leaves fall into the appropriately names Mesa Verde Group, a late Cretaceous, cliff-forming brown and gray layered sandstones. The importance of the leaf fossils were integral in determining the depositional environment of the Mesa Verde Group.

With the fossil leaves of the Mesa Verde Group, paleontologists or fossil seekers may find other wood debris and a huge variety of plant species, but rarely, if ever, marine fossils. The fossils are so well preserved in some places that the cuticles of the leaves can be peeled off of the rock when freshly split (Raynolds, 2007).

As of Raynolds, 2007, over 100+ seperate leaf species have been identified in late Cretaceous rocks in Colorado. Further investigation led to identifying many of these leaves as drip-tip bearing angiosperms. Drip tipped leaves are common on today's Earth in humid, high rainfall tropical rainforests. the drip-tips relieve the plant of leftover runoff moisture, which can lead to mold and disease in plants.

With the presence of these plant fossils, and the lack of marine specimens, coupled with the grain size and distal relationships (source and sink) of the sediments, the Mesa Verde Group has been labeled a lagoonal type deposit.


Raynolds, Robert G.; Johnson, Kirk M.; Ellis, Beth; Dechesne, Marieke; Miller, Ian M. 2007. Earth History Along Colorado's Front Range: Salvaging geologic data in the suburbs and sharing it with citizens. Denver Museum of Nature & Science, 2001 Colorado Boulevard, Denver, Colorado 80205.

Johnson, K.R., and Ellis, B., 2002. A Tropical Rainforest in Colorado 1.4 Millions Years after the Cretaceous-Tertiary Boundary: Science, v. 296, p. 2379-2383.

Note: The above sources do not refer to leaves of the Mesa Verde Group. The units are similar at Mesa Verde to those of the Castle Rock Flora site, and this was the reason I used these papers. That, and, they were the only source I am aware of.

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