Fossil formation 101

Over the years thousands of fossils have been discovered all over the world, helping researchers, historians, palaeontologists, archeologists and other scientists learn more about prehistoric Earth.

Fossils are evidence of ancient life forms or habitats that have been preserved by natural processes.

The process of fossilisation takes thousands to millions of years, and in order for the process to begin the plant or animal needs to be in the right place at the right time … so to speak.

So what happens?

Fossilisation is a rare occurrence and can only take place if an animal dies in the right circumstances. A woolly mammoth, for example, would have had to die in a location where its body was protected from scavengers and environmental elements. This involves the body being buried in ice, sand or mud, ideally at the bottom of a seabed, riverbed or swamp.

Over time more sediments cover the remains, burying them deeper and deeper. Chemicals and minerals filter into the sediment and permeate the bone (or shell or wood), changing its original mineral composition and thus enabling it to survive for as long as the rock surrounding it does, sometimes even longer.

What palaeontologists dig up is rarely ever the original skeleton of the animal that died millions of years ago.

The end result of fossilisation is a heavy, rock-like copy of the original skeleton. A fossil has the same shape of the original skeleton, but is chemically more like a rock.

Types of fossilisation

There are a variety of ways in which dead animals or plants become fossils or leave behind evidence of their existence.

Permineralisation happens when rock-like minerals seep into the carcass of a dead and buried organism and replace the original organic tissue with silica, calcite or pyrite. This forms a rock-like fossil.

Unaltered preservation is what happens to small insects or plants that are trapped in tree sap, which hardens over time and becomes amber.

Replacement or recrystalisation is when shell, bone or other tissue is replaced with another mineral.

Casts or moulds occur when the original organism dies or is destroyed and the empty space, external cast, that is left behind is filled with other minerals and becomes a cast. Internal moulds, or endocasts, happen when sediments fill the internal cavity of the organism.

Fossils have helped palaeontologists get a glimpse into what life on Earth was like before we came into existence. As discoveries are made, they broaden and refine our knowledge of prehistoric plants and animals – such as, for example, whether or not certain dinosaurs were feathered, what colours they were, how they moved and what they ate.

South Africa’s fossil record dates back some 300-million years.

Click here to watch a video of Professor Bruce Rubidge, of the University of the Witwatersrand, explaining the rich sedimentary record of the Karoo, which allows scientists to observe developments in the evolution of mammal-like reptiles.