Wednesday, September 8, 2010

Fossil of the Week

9/8 – Flowering Plants of the Green River Formation

If you’ve ever heard of the Green River Formation – an Eocene Lagerst├Ątte found in western Colorado, southwestern Wyoming, and eastern Utah – your first thought is probably about a spectacular fossil fish. Although less familiar, Green River fossil plants are equally spectacular. The PRI collections contain a large number of fossil vertebrates, insects, and plants from the Green River.

The Green River Formation* beds represent about six million years of freshwater lake sedimentation during the Eocene Epoch approximately 48 million years ago. Many of the sediments are so fine grained that they have preserved soft parts not usually found in fossils, including the scales of fish, and the delicate leaves and flowers of trees that fell into the lakes. There are even known cases of insects preserved in the act of eating some of the leaves. In addition to angiosperms (flowering plants like those pictured here), the Green River flora includes fossils of freshwater algae, and terrestrial plants such as ferns, conifers, and horsetails.

Although interesting from a taxonomic and evolutionary point of view, the plant fossils found in the Green River Formation are also particularly useful as climate indicators. They show that despite being centered at about the same latitude as they are today, the Eocene Green River plants were living in the moist subtropics.

In this photo are (clockwise from the left) a tropical hardwood stump, Yellow Point Playa, Wyoming (PRI 2007, Acc 1484, K22079); Schinoxylon Pepper Tree log, Blue Forest, Wyoming (PRI 1999, Acc 1104, K16976); Styrax transversa twig, Garfield County, Colorado (PRI 1979, Acc 722, K6312); Celastrus winchesteri, Staff Tree leaf, Garfield County, Colorado (PRI 1979, Acc 722, K6318); and (at center) a flower, Garfield County, Colorado (PRI 1979, Acc 722, K6188).

Of course, the Green River Formation shows just one way in which fossil plants can be preserved and there are many others. In the Museum of the Earth permanent exhibits, you can see plants from other locations and time periods, including some microscopic flowers that have been burnt and preserved as charcoal.

Text by Ursula Smith (reprinted from “Fossil Focus” in American Paleontologist, Fall 2009)

*For more about the Green River Formation, see Fossils of the Week 10/8/09 - Flower and 7/9/09 - Sycamore Leaf.

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