Fossil of the Week

We usually think of fossils as hard objects - bones, shells, teeth - organisms (or parts of organisms) that are hard to begin with and naturally have excellent fossil records. So, when you consider the leaf on a tree - soft, thin, usually fairly flimsy - you don't really expect it to fossilize often or well. However, as the picture here attests, fossil leaves are actually fairly commonplace. These are almost always in the form of impressions. The leaf itself is actually gone, but it has left its impression in the mud (now rock). These are almost always in the form of "part/counterpart", which means that when the rock containing the impression splits along the plane of the leaf, one side of the leaf (the "part") is revealed on one side of the rock and the other side of the leaf (the "counterpart") is on the facing side of the rock. This particular fossil is a sycamore leaf from the Green River Formation in the western U.S. Today's American Sycamore (Platanus occidentalis) is fast-growing and has been extensively planted as a shade tree. The second oldest tree in the city of Buffalo, New York, is a Sycamore dating to the year 1700. The Green River Formation, spread across Colorado, Wyoming, and Utah, dates to the Eocene Epoch (48 million years ago). It is one of the most famous fossil formations in North America, known for exceptional preservation of soft-parts, including fish, crocodiles, insects, leaves, and even feathers. Insects have been found fossilized whole, preserving delicate wing membranes and spider spinnerets.The leaves of palms, ferns, and sycamores are common in Green River, some showing details of insect damage sustained during their growth.

Text by Paula Mikkelsen