Fossils forever

Fossils are...
not old bones -- at least, not always. These ancient traces of life come in a variety of forms. Some are old bones, more or less altered by chemical processes. Others are old turds (coprolites, technically), or "trace fossils" like footprints or impressions of skin, feathers or scales.

It's a dirty job, but somebody's gotta do it

Dino-hunters look for specimens in gullies, cliffs, and other heavily eroding lands. Craig Pfister extracts a Hadrosaur (duckbill) bone in the Hell Creek Formation of Montana. © Jeff Miller/UW-Madison Office of News and Public Affairs.

Fossilization -- the preservation process-- requires that the organism -- or the footprint or impression -- be covered with sediment before it is eaten or washed away. On land, this often happened when the corpse washes into a river and is covered with sediment. Generally, only hard tissue -- bones and teeth -- survives fossilization, but there were rare cases where some fool got lost in the mountains and ended up in a glacier for long-term storage cold ice. (The Why Files described a slew of several natural embalming techniques in a File on amber.)

These are some of the processes that produce fossils:

	Permineralization occurs when water-borne minerals deposit around the bone structure.
	Petrification happens when minerals replace the bone structure itself. This process can replace the organic molecules one for one and thus can show the bone in microscopic detail.
	A natural cast occurs when flowing water removes all the original bone, leaving an impression of its outside.

Since sediment is essential to fossilization, the place to look for fossils is sedimentary rocks, preferably in rocks that we know formed when conditions were hospitable to dinosaurs. These rock formations should also be rapidly eroding (erosion wipes away surface layers of rock to make life easier for dino diggers). Seriously, sediments accumulate rapidly (at least in geologic terms), and after 64+ million years, you've got a tower of rock. Thus dino-hunters have no choice but to look for specimens in gullies, cliffs, and other heavily eroding lands.

One place where there's plenty of erosion is in the Hell Creek Formation in South Dakota. The Why Files asked ace reporter Brian Mattmiller to describe a season under the sun.

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