Index fossils are fossils that only occurred during a specific time frame.
Typically commonly occurring fossils that had a widespread geographic distribution such as brachiopods, trilobites, and ammonites make the best index fossils.
fossil (latin for "obtained by digging") is the preserved remains or traces of organisms (plants, animals, etc) that lived in the distant past.
The convention is that a fossil must predate recorded human history.
Fossilization of an organism requires a unique set of circumstances so that it doesn’t just decay without a trace. Carbonization - Carbonization is a process in which the more volatile substances of the organism (hydrogen, oxygen, nitrogen, etc) are removed, leaving behind the carbon.
These fossils typically appear as a thin, dark film on the rock.
Body fossils include the remains of organisms that were once living (bones, shells, teeth, eggs, etc), while trace fossils are the signs that organisms were once present (footprints, tracks, burrow, coprolites). Permineralization or Petrification - After an organism is buried, minerals carried by water such as silica, calcite or pyrite replace the organic material in the fossil.If a layer of rock containing the fossil is higher up in the sequence that another layer, you know that layer much be younger in age.By combining these principals, relative dating techniques can usually be used to determine the approximate age of a fossil.One example of this is ironstone nodules in the Mazon Creek fossil beds that contain amazingly details fossil plants and insects.One common question that comes up is how is the age of a fossil determined?