Dinosaurs: A Time Team Special
30 December 2001

How fossils form

When a living organism dies it is usually decomposed completely by bacterial action and other processes. Only under certain circumstances will any traces of an organism be preserved or fossilised. In order for this fossilisation to occur it is essential that the organism is buried rapidly, protecting it from the bacteria and oxygen that would otherwise destroy it. Except in extremely rare conditions, soft parts are never fossilised. It is more common to find fossilised hard skeletal remains or trace fossils.

There are several different fossilisation processes. Fossils that have been buried for very long periods of time may undergo two or more processes. Pseudomorphosis is a process in which the original material fills with minerals, producing a cast of the organism with no internal structure or organic material preserved. Distillation preserves organisms as carbon films after certain chemicals dissipate and are then represented as residual organic material. Dissolution processes occur within the rock and leave a mould of the original organism, or a trace fossil. Permineralisation is a process in which wood and bones are preserved in their original state but the cellular structure is filled with minerals. Replacement often occurs after the cell spaces have been filled through permineralisation. Replacement causes the original cell walls to dissolve, leaving visible structure but frequently destroying all organic material.

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