By: Amy Widdifield, Co-op high school student from Eastwood Collegiate Institute, Kitchener.
When most of us think of fossils, we think of dinosaurs. A fossil is the remains, or some other evidence, of a prehistoric plant or animal. Usually for fossilization, hard materials such as bone or shell are needed. After an organism dies, a quick burial in sedimentary material helps prevent decay, allowing the sediment to harden undisturbed over a long period of time. Very few organisms actually end up as fossils as the condtions needed are rarely found in nature.
There are three main kinds of fossil remains, and many different methods of preservation. The first type of fossil is referred to as unaltered remains. Unaltered remains fall into two categories, soft and hard parts and hard parts only.
In very rare cases the whole animal, even those parts which usually succumb to decay, are preserved. These cases involve special circumstances such as extreme heat, cold, or a unique chemical process. Mammoths have been found perfectly preserved in ice. As well, insects have sometimes been found preserved while encased in amber, which is hardened tree resin. However it is much more common for only the harder parts of the organism to become fossilized.
The second type of fossil is called an altered remain. Rock movement can flatten or distort a fossil, such as a brachiopod shell losing its symmetry from pressure within the rock. Also, circulating water dissolves chemicals from shell and bone leaving them light and spongy. By a method of preservation called replacement, as chemicals are removed, they are replaced by minerals, such as calcium carbonate, silica, and pyrite. This replacement can be complete or only partial. Sometimes this replacement is so exact that the original structure of the plant or animal is preserved. However most often replacement is less than prefect and shows only the general form of the organism.
The last type of fossil remains is called indirect evidence. The actual remains of an organism are not found, but rather some evidence that the plant or animal once lived. Indirect evidence includes tracks, trails, burrows and footprints. Also, there are gatroliths and coprolites, stomach stones and fossil excreta, that help to determine an organism's diet. As well, sometimes an organism is covered in sedimentary material which hardens while the organism itself dissolves away, leaving an empty cavity where the organism used to be, called a cast.
Nearly all fossils that are found are contained within sedimentary rocks, although fossils may be found in volcanic and metamorphic rock. For example, distorted trilobites have been found in phyllite, and tree casts have been found in lava flows. Still, sedimentary rocks usually provide the best information. Sedimentary rocks are divided into different layers or strata which help to determine the age of the fossils found within. If the rock section has not been overturned the rocks on the bottom are the oldest while the rocks on the top are the youngest. Also, there are different kinds of sedimentary rock such as limestone, shale, and sandstone which are all formed differently but each preserve fossils well.
Amateur fossil hunters have always played a major role in the discovery of fossils, some of which have been very important. With the aid of fossils we can begin to piece together the story of prehistoric life and past geography and climate.
In general, fresh exposures of rock, whether man- made or natural, are best for fossil collecting. Remember that exposures are often somewhat dangerous, so be sure to take your time and come prepared. Bring along a knapsack for carrying specimens, enough newspaper to wrap specimens separately, and paper for labeling each specimen. Look around the area for rock surfaces where weathering has exposed fossils. Weathered-out specimens are easy to collect and may be in excellent condition. Should you locate vertebrate bones or any fossil you believe rare, leave it intact and get professional help. Preparation and cleaning should be done at home after specimens have been removed.