Fossils have been the fascination of humans around the world for thousands of years. Sometimes fossils would be misunderstood for mythical creatures such as dragons or gryphons. Other times, the presence of fossils taught the ancients about geological history. For instance, the Ancient Greek philosopher, Xenophanes understood that fish fossils indicated that areas of Greece were once under water.
Fossils continue to provide hints of the earth’s past and the processes that created life on our planet throughout time. They show us how the continents shifted, how life evolved, the habitats that organisms lived in, how those animals and plants grew and developed over their lifetime, animals’ diets and behaviours, and even what they looked like.
How are fossils actually created?
When most living things die, they go through the natural process of decomposition and don’t leave any trace behind. However, if specific conditions are met, over many thousands or millions of years, a record of their existence can be preserved. Interestingly, fossils can be formed in many different ways, depending on how and where they died, and the environment where their body came to rest.
The basic requirement for fossilization is that remains are buried under sediment, and through a lack of oxygen and protection from the elements, their decomposition and deterioration is slowed. Because of the need for burial, the most likely setting for animals or plants to become fossilized is aquatic environments where the organism can quickly sink into the mud floor or get covered. On land, animals or plants are most likely to get fossilized at the edge of bodies of water where they slide into the water after death or are covered with silt or washed into a water body during a flood. In contrast, fossils are very unlikely to be found in what was previously jungle or mountainous areas.
But, buried by just one layer of sand, mud, silt, volcanic ash, or lava isn’t enough. Fossilization requires many layers of sediment over thousands of years, so that, through the weight of the layers, the lower levels harden into rock.
Permineralisation is the most common way that fossils are created. This is a process that starts with the decomposition of the soft materials – muscle, fat, etc., after predators or scavengers get their fill. As the organism is covered by the immense weight of sediment above it and ground water seeps through the rock, and the bones begin to dissolve. Minerals in the water then replace the original bone and crystalize in the spaces to eventually form rock in the shape of the bones. Over thousands or even millions of years, the minerals are turned into stone.
Examples of fossils created through permineralisation include bones, teeth, shells and wood.
A second process involves impressions and moulds of organisms. This is the way that many marine and fresh water invertebrate animals, such as Ammonites and Trilobites have been fossilized. These types of fossils are created when water dissolves the shell and leaves behind a natural mould in the sediment. If the mould becomes filled with mineral-rich water, it can mineralize and creates a cast of the original bone or shell. Occasionally, the original shell is also preserved; however, this is extremely rare.
Other trace fossils, such as footprints are formed in a similar way. The footprint forms a mould and sediment fills that space to create a cast.
Soft tissues including the intact remains of organisms: their skin, muscle, hair and/or internal organs, can also be preserved in special and uncommon circumstances. This requires rapid burial in material such as ice, volcanic ash, peat bogs or amber which creates a low-oxygen environment and stops the organism from decomposing or being scavenged by other organisms.
The fossilization of amber itself is a process. This begins when trees exude resin from their bark when injured to seal and sterilize the damaged area. If the resin is protected from sun, rain, extreme temperatures, and microorganisms and finds its way into layers of sediment, chemical changes first form copal (young amber) and then heat and pressure drive out terpenes and complete the amber transformation.
Trace fossils can also be formed through any of these processes. These fossils record the activity of an organism and include nests, burrows, footprints and coprolites (better known as fossilized poo!).
It is only when the process of erosion - when the rocks are worn back down and washed away - that these once living organisms are revealed to us from within the stones.
American Geosciences Institute. How do fossils form?
Australian Museum (2021). How do fossils form?
Berkeley University of California. Evolution and Paleontology in the Ancient World https://ucmp.berkeley.edu/history/ancient.html#:~:text=He%20observed%20fossil%20fishes%20and,the%20history%20of%20the%20Earth.
The Natural History Museum, London. How are dinosaur fossils formed?
The Natural History Museum, London. What is Ammonite?
ZME Science (2021). How amber forms -- nature's time capsule