My wife and I don't do as much fossil collecting as we did in past years. I am a paleontologist--fossils are what I study. Microfossils. Conodonts--very primitive vertebrates. For fun I collect fossils from the Mississippian age strata around Springfield. These are marine limestones that represent the time in Earth history when crinoids and blastoids were at their peak of abundance and diversity. These fossil groups belong to the Phylum Echinodermata, which includes starfish, sand dollars, sea urchins, as well as many extinct groups. Crinoids live in the oceans still, although blastoids became extinct about 250 million years ago, at the end of the Paleozoic Era.
The limestones around Springfield are almost entirely made of the remains of fossils. Crinoids are most abundant group, but most of the rock consists of their tiny fragments. The most common recognizable parts are pieces of the stems. The stem bits look like lifesaver candies--round with a hole in the center. These stacked up together to made a stem that attached this animal to the sea floor. The whole animal is sorta-kinda like a starfish with a big body and many arms, all attached by a stem to the sea floor. When the crinoid died, the muscles and ligaments holding together all of the pieces of the skeleton rotted, so the skeleton fell apart to little pieces that were spread around by waves and currents. This sediment makes up most of the local Burlington Limestone, a rock formation (rock unit) that is named after the town of Burlington, Iowa. The Burlington Limestone is more than 100 feet thick, and it occurs from central Iowa all the way to southwest Missouri. It was estimated that it took the disarticulated remains of about 17,000 crinoid animals to make one cubic meter of this sedimentary deposit. Thus there were lots of these crinoids living and dying in the oceans for a long time in order to form this rock unit.
Crinoids do not always completely fall apart after death. Usually the stems and arms disintegrate, but sometimes the body (or calyx) was covered with sediment before it fell apart. These fossils are in the rock around here, but you can collect specimens only under special circumstances. If you just look at fresh exposures of the rock--where it was blasted along a highway, for example--you will not find much. You must look on weathered surfaces along the same highway where the soil is in contact with the rock. Here the processes of weathering have caused the limestone fragments to loosen and fall apart, and this weathering process takes hundreds of years. Searching over such weathered rock surfaces will sometimes will result in finding a complete or partial crinoid calyx that is weathering out of the rock. Eventually the weathering process will destroy the fossil too, so some but not too much weathering is what is needed. Finding a good specimen is followed by using a cold chisel and a hammer to chop the fossil out of the rock. Sometimes the process of chiseling out the fossil causes it to fall apart or otherwise break. After a good specimen is collected, the remaining bits of weathered rock must be slowly picked away from the fossil with tools such as dentist's picks. The Burlington Limestone contains many dozens of different genera of crinoids and probably a couple of hundred different species. The collection that my wife and I made over the years contains thousands of specimens and dozens of different species.
There are several other common types of fossils in the Burlington Limestone. Blastoids were mentioned. They are somewhat similar to crinoids in having stems and arms, but they are extinct. Since coming to Springfield in 1974, my family has found 5 echinoids (sea urchins). Brachiopods--a type of shelled animal that kinda-sorta looks like a clam but is unrelated--are also very common and can be seen on most highway cuts in the area. There are many different species of them to be found locally. We find corals rather commonly; most are small "horn corals" that are similar in shape but smaller in size than a cow horn. There are snails, but typically they are not well preserved. Other rare fossils include clams, trilobites, and shark teeth.
I also collect fossils during some of my field research and professional field trips to various parts of the world. Some of the 9-day field trips I lead during Spring Vacation (click on link to GLG 360 for details of these trips) also involve collecting various kinds of fossils. Some of the fossils I collect on these trips are used in illustrating fossil groups on laboratory exercises for GLG 415, my Invertebrate Paleontology class. This class is required for students taking a comprehensive major in Geology (click on link to GLG 415 for more information). Over the years, unfortunately, a number of my best fossil specimens have been stolen or broken.