Fossils are the preserved remains or traces of organisms from a past geologic age, providing the basis for studying life’s deep history. Fossilization is a rare natural process, typically requiring an organism to be rapidly buried in sediment before it decays. This transformation of organic material into rock creates a patchy but rich record of ancient ecosystems. Paleontologists classify these remnants into two major categories: those preserving the physical structure of the organism and those preserving the evidence of its activity.
Body Fossils: Preserved Organisms
Body fossils are the actual preserved physical parts of an ancient organism, such as bones, teeth, shells, and wood. These remains provide direct evidence of an organism’s anatomy, showing scientists what a plant or animal looked like. Since soft tissues decay quickly, body fossils are usually composed of hard, mineralized parts that resist decomposition.
In rare environments, entire organisms can be preserved with minimal alteration. Examples include woolly mammoths frozen in permafrost or insects encased in hardened tree resin known as amber.
Trace Fossils: Evidence of Life
The second category, known as trace fossils or ichnofossils, records the organism’s behavior rather than preserving the organism itself. These geological records of biological activity offer insights into how ancient life moved, fed, and interacted with its environment. Trace fossils are often more valuable for understanding ecology than for studying anatomy.
Common examples include preserved footprints and trackways, which reveal details about an animal’s gait and speed. Other trace fossils include fossilized burrows, borings, and coprolites (fossilized feces), the analysis of which helps paleontologists determine the diet of an extinct animal.
Common Methods of Fossil Preservation
The preservation of both body and trace evidence relies on specific chemical and geological processes that prevent decay.
Permineralization
One widespread method is permineralization, which is common in porous materials like bone and wood. Groundwater carrying dissolved minerals (such as silica or calcite) seeps into the microscopic empty spaces within the buried material. As the water evaporates, these minerals precipitate and crystallize, filling the pores. This process hardens the structure until it becomes stone-like and denser than the original material.
Molds and Casts
Another common method involves the formation of molds and casts, which occurs when the original organic material dissolves entirely. The mold is the empty cavity left behind in the surrounding sediment, retaining the external shape of the organism. If this hollow mold is subsequently filled by new sediment or minerals, it creates a cast. The cast is a three-dimensional replica of the organism’s exterior shape.
Carbonization
For delicate structures like leaves or soft-bodied organisms, preservation often occurs through carbonization. This process begins with rapid burial in fine sediment under low-oxygen conditions, subjecting the organism to increased pressure and heat. The chemical components of the buried material are distilled away. This leaves behind a thin, black film of almost pure carbon. This carbon film preserves the detailed outline of the organism on the rock layer.