No exploration of the geologic time scale is complete without thinking about fossils. Fortunately, kids love fossils. However, there are many common misconceptions – that fossils are fairly common, that every species that once lived must have been preserved in some way, that fossilized skeletons are often found intact. What students rarely understand is that fossils are extraordinarily rare. In fact, the vast majority of species that once lived on the planet vanished without a trace. Each and every fossil is precious because there are so few of them and because each fossil can provide so much information about past life on this planet.
Fossils are the preserved remains of former life or traces of them such as molds, casts, and footprints. The main categories of fossils include:
- Body fossils – mineralized or preserved remains of body parts such as teeth, shells, bones or claws. The most common way body fossils form is by an organism being covered in sediment soon after death. The sediment provides a shield from damage by scavengers, weathering, and decay. The amount of time that passes before the organism is covered determines how much is eventually fossilized – if covered quickly by a sandstorm or landslide, an entire skeleton and even scales or hair may remain, whereas if covered slowly, only the most easily preserved hard tissue such as teeth may remain. As the layers of sediment build over thousands of years, the sediment layers become rock through compaction and cementation. At the same time, the remains of the organism decays and is slowly replaced with minerals that seep through the sedimentary rock, such as calcite, silica, and iron. This process by which body parts are replaced with rock is called permineralization. Different tissues are permineralized at different rates so even the internal details of bones and teeth may be preserved. In addition to permineralization, an animal or plant may become preserved in tar (the La Brea Tar Pits), in ice (several wooly mammoths have been discovered encased in Siberian ice), or in amber (insects are commonly preserved in amber).
- Trace fossils – fossilized evidence of the behavior of past life such as footprints, nests, burrows, eggs, and droppings. Trace fossils are extremely precious because they provide information about the lifestyle and behavior of past life. Do they take care of their young? Do they live singly or in groups? How do they walk? Where do they live?
- Mold fossils – a negative imprint of an organism preserved in stone. While the organism itself may decay without being preserved, the imprint remains in the sedimentary layers that eventually become sedimentary rock.
- Cast fossils – a positive imprint of an organism preserved in stone. If a mold fossil is filled in with additional sediment, this second layer becomes a cast fossil.
Due to the rarity of fossils, the buying and selling of fossils has become a highly controversial subject. Paleontologists are concerned that the commercial fossil trade is harmful to science. Scientists fear that if unique fossils with great scientific importance are purchased by private collectors, then these finds may not be available for scientists to study. On the other hand, private collectors insist that most fossils are being made available to researchers and many are ultimately donated to museums. For a great video discussing the subject, see “Curse of T Rex”, a 1997 Nova special about the discovery of an astonishingly complete T rex skeleton (Sue, now on display at the Field Museum (http://www.fieldmuseum.org/SUE/)) and the battle that ensued between scientists, commercial fossil hunters, the US government, and property owners.
A good understanding of how sedimentary rocks form and of basic stratigraphy principles. Knowledge of the geologic time scale is helpful but not required.