Types of Fossils

Body Fossils

Mostly, when we think of fossils we think of body fossils, where a (large or small) portion of the organism's body parts have been replaced by minerals. Rarely, do the chemical processes fossilize soft tissues, but do readily fossilize hard parts such as the shells of Molluscs (ammonite, nautiloids, clams, and oysters) that are already mostly minerals. So too are partially mineralized bones of vertebrates and already calcified teeth relatively more easily fossilized. Exoskeletons of many arthropod invertebrates are composed of chitin, an organic polymer often combined with a long-chain polymesr that, while less readily, fossilize; where this not the case, there would not be so many insect, trilobite, and many other invertebrate fossils.

The process of fossilization normally involves permineralization. This chemical process begins after burial when carcase become filled with mineral rich groundwater. Minerals precipitate from the groundwater, occupying the empty spaces. This process can occur in very small spaces, such as within the cell wall of a plant cell. In essense, the organisms body/body parts are replced by other environmental minerals (mineralization). Small scale permineralization can produce very detailed fossils. Very complex molecular exchange can also occur in a process as well as diagenesis, also a complex physical-chemical process. Rarely, repeat rarely, soft tissue can leave a mineralized trace in exquisite detail (these are so called soft body fossils that often are found at Lagerstatten fossil sites). Also see fossilization.

Cast or Mold Fossils

Sometimes what was once a body fossils gets dissolved, especially if the fresh water or sea water it is exposed to is has high acidity (PH < 7)or high basicity (PH > 7). The hole that remains is an external cast, hence the name cast fossil.

Ichnofossils, Trace Fossils or Trackways

Ichnofossils (or trace or trackway) fossils are rock records of an organisms locomotion, behavior. Another way to define ichnofossils is sedimentary structures or traces caused by biological activity. Some examples are foot print impressions, or impressions made by burrowing, boring, or feeding activities. Some scientists place stromatolites with bacterial biogenic origin as ichnofossils.

Fossil Resin

A number of plant taxa produce resin, apparently an evolutionary adaptation appearing at least by the Triassic, for sealing damage and protection from insects. Resin is not sap, but rather a natural polymer comprising repeating units of organic chemical units (by the way, DNA is a polymer). Plant resin is not demonstrably different than synthetic polymers we call plastic, as humans mimic those exquisite chemists known as plants -- the same goes for rubber and cotton mimics. Fossil resin originating from plants is distributed worldwide, even in the Arctic. Such fossil biopolymers are colloquially called ambers, though fossil resin is a far better terminology. Most deposits are from Cenozoic time, though the oldest known dates to the Triassic. Amber is erroneously called a gem by a huge fossil trade, though in polished form it can outwardly resemble a gem. However, true gems, are crystals whose atoms are tightly bound by covalent bonds. Polymer bonds in natural resins are much weaker, and highly susceptible to environmental degradation over time (just like synthetic plastics); UV electromagnetic radiation breaks polymer bounds. Chemistry aside, the most alluring aspect of fossil resin is, it initially flowed as a sticky fluid, capturing all manner of life forms in a polymer coffin. The captured organisms are usually called inclusions, and commonly include insects and other small arthropods and invertebrates, as well as bacteria, fungi, plants, and ever so rarely an unfortunate vertebrate such as a lizard. Small fragments of DNA have been detected, though with insufficient nucleic acids to reproduce T-Rex. See fossil resin examples.

Molecular Fossils

Two types of molecular fossils need to be recognize, atomic ratios and fossil cells. The former require counts of a atom ratios in stratigraphic formations (tiny chunks of rock usually). Such molecular fossils have been used as markers of the very earliest evidence of life on earth in stomatolite formations dating to Archaean time some 3.5 billion years ago in Australia and Greenland. The later, "putative fossilized cells, have been found in stromatolites, for example, cyanobacterial cells in 1.9 billion year old Gunflint stromatolites (see J.W. Schopf, Nature, Vol 406, March 7, 2002).

Stromatolites including banded iron

Stromatolites

Index fossils

Index fossils are fossils of genera or species of organisms confined to a certain geologic time range. Thus, if you find it, you know immediately how old is the fossils, and how old is the formation or strata from which it came. Also see: index fossils here.