Fossilisation—a visual guide
Fossils are the remains of past life, preserved in rock. Whether animal, bacteria, fungus, plant or protoctid, life forms will leave their trace. A fossil may be a burrow or a track, the mineralised remains of a shell or skeleton, an impression of a soft-bodied organism left in mud or even the remains of the original soft parts of the animal preserved in anaerobic muds or in amber. Of the millions of organisms that die each year, very few will turn into fossils, and those that live in the sea stand a better chance than those that live on the land. Erosion, the effect of bacteria and the action of scavengers quickly destroy potential fossils. There are two processes that are required for fossilisation: 1. the organism has to be quickly entombed into the sediment and 2. compression and percolating minerals are normally required to turn the sediment to rock and the organism into a fossil.
A jumble of damaged crinoid stems (P549495)
After death, the organism falls to the floor of the sea or a lake and is covered by sediment. If this does not happen quickly, they may decay or be broken up. The compression that changes a sediment into a rock may also damage or distort the entombed organism. Land animals and plants are usually destroyed unless their remains are protected in some way, for example, by being covered by wind-blown sand or by falling into a cave where they cannot be scavenged.
The Jurassic scleractinian coral Pseudodiplocaenia oblonga was originally aragonite, which has changed to calcite (P549519)
After burial, chemical changes take place and the original shell material may be replaced by another mineral such as pyrite, calcite or quartz. For example, the shells of some ammonites, gastropods, bivalves and scleractinian corals are made out of the more unstable form of calcium carbonate, called aragonite. However, on burial, aragonite is changed to the more stable form of calcite.
The external mould of a piece of Stigmaria preserving the external features of the organism (P549461)
Often the original organism decays entirely leaving a hollow which acts like a ‘jelly mould’. The hollow is later filled by a mineral or more sediment. This may create an external mould, in which the fossil retains the features of the exterior of the original organism, such as Stigmaria, the root of the giant clubmoss, Lepidodendron.
The internal mould of a goniatite (P549551)
However if the sediment or mineral fills a shell prior to the dissolution of the calcite, then the interior of the shell is fossilised. This leaves a ‘steinkern’ or internal mould such as the illustrated fossil goniatite. The fossil displays the sutures separating the chambers that form the interior of the shell, but not the exterior of the shell.
Charnia, a soft-bodied organism that has left an impression in fine volcanic ash (P550159)
Soft-bodied animals are fossilised extremely rarely. This is because they lack a shell or skeleton, which is the part that normally survives long enough to be fossilised. Biomineralisation first began only 545 million years ago so that older soft-bodied Precambrian fossils are very rare. Charnia lived in the shallow marine waters that covered central England about 600 million years ago. The animal was buried quickly within fine volcanic ash, and compressed so that the entombed Charnia left an impression before rotting away. This impression was preserved when the ash was converted to rock.
A Cretaceous fern preserved in mudstone (P549465)
Plants can also can be fossilised when conditions are right. Leaves might be replaced by clay minerals or carbon. A frond of the early Cretaceous fern, Cladophlebis, fell into a Wealden lake and sank to the bottom, quickly to be covered by mud. In some cases plants such as this would have turn to coal, but here the fern is preserved as a carbonaceous film on a slab of mudstone.
Current activity concentrated graptolites all with the same orientation (P549557)
Just occasionally organisms form exceptionally well-preserved fossils and the deposits are referred to by their German name, Lagerstätten. Very rarely, organisms that fall into sediment with little or no oxygen do not decay by aerobic bacteria and scavengers. In this case the soft parts of the animal may be replaced, cell by cell, perhaps by the mineral apatite, so preserving the organism in great detail. In other cases many animals died at the same time and the action of currents brought them together in vast numbers. In some cases the bedding planes are crowded with fossils such as fish, crinoids, star fish or, in this case, graptolites.
A fly and spider trapped in amber (P549580)
In some cases, insects, arachnids, etc. are found in amber with their bodies, limbs, wings or hairs still perfectly preserved. Cretaceous and Cainozoic ambers are well known for containing small fossils. Sap from trees oozes out of a damaged part of the trunk and drips and flows down the surface of the bark. Any insects unlucky enough to be in its path may be engulfed by the sticky gum, which with time hardens to form amber. In this way the fossils are preserved virtually unchanged since the day the organism died, although the DNA of entombed insects has yet to be found.
<gallery caption="Editing Fossilisation—image gallery"> File:P549495.jpg|A jumble of damaged crinoid stems (P549495) File:P549519.jpg|The Jurassic scleractinian coral Pseudodiplocaenia oblonga was originally aragonite, which has changed to calcite (P549519) File:P549461.jpg|The external mould of a piece of Stigmaria preserving the external features of the organism (549461) File:P549551.jpg|The internal mould of a goniatite (549551) File:P550159.jpg|Charnia, a soft-bodied organism that has left an impression in fine volcanic ash (550159) File:P549465.jpg|A Cretaceous fern preserved in mudstone (549465) File:P549557.jpg|Current activity concentrated graptolites all with the same orientation (549557) File:P549580.jpg|A fly and spider trapped in amber Palaeozoic rocks (549580)