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What is a fossil?
Fossils are physical evidence of prehistoric animals and plants. They tell us about the history of our planet, from climate and evolution to diets and diseases.
There may be more to these prehistoric remains than you ever realised.
When an organism dies and is quickly covered by layers of mud, sand or silt, it has the potential to become a fossil.
Rather than rotting away, some parts of its body are replaced by minerals. In some cases, such as for small soft-bodied animals, this process can take just months, or even days. For larger things, such as big dinosaur bones, fossilisation can take thousands of years.
There’s more than just one type of fossil. Two key types are body fossils and trace fossils.
What is a body fossil?
Fossilised remains of dead plants and animals are known as body fossils. This can include fossil bones, claws, teeth, shells, tree trunks and leaves.
What is a trace fossil?
Other fossilised signs of a plant or animal are called trace fossils. These can include imprints of skin or feathers, eggs and nests, and organic material such as poo, which are called coprolites.
Trace fossils can also be a record of a prehistoric activity, such as footprints and trackways or burrows.
What is a subfossil?
Subfossils are fossils in the making. They’re remains that have begun the process of fossilisation. Minerals have started to replace the biological components.
‘They’re somewhere in the middle of the process,’ explains our dinosaur researcher Professor Paul Barrett. ‘They’re usually quite young fossils, often from Ice Age deposits or younger.’
Both body fossils and trace fossils can be subfossils.
Shark teeth are the most abundant vertebrate fossils. This large tooth belonged to a megalodon, an extinct shark species that grew 15-18m long. It went extinct around 3.6 million years ago.
What is the fossil record?
The fossil record refers to all the fossils that we know from the rocks around us.
‘It goes from when we get the first single-celled organisms, all the way through to the ancestors of humans today and Ice Age mammals,’ explains Paul.
‘The fossil record tells us what plants and animals were around at different times in Earth’s history. Taken together, it’s an encyclopaedia of the history of life.’
Where are fossils found?
You can find fossils anywhere that has rocks of the right age and type. Fossils are usually found in sedimentary rock, which forms when sand, silt, mud or the calcium carbonate shells of sea creatures settle in layers that are then compacted.
‘We have fossils from animals going way back to almost a billion years in time, all the way through to the present,’ says Paul. ‘Everywhere that there are rocks laid down in conditions that allow fossilisation, there's a chance of finding a fossil.’
What kinds of fossils are in an area depends on the location, type and age of the rock. For example, Britain’s White Cliffs of Dover are Late Cretaceous chalk rocks. Chalk is laid down in marine environments. So, in these iconic cliffs we find fossils of ocean-dwelling animals. Ones that lived at the end of the age of the dinosaurs.
To find the right spots for fossil hunting, palaeontologists use geological maps. These show the rock layers in a given area. This can provide clues on whether there are fossils hidden beneath the surface and what kinds of organisms they might be.
The first geological map of Britain was produced by William Smith in 1815. It paved the way for a better understanding of geological time.
What is an index fossil?
Index fossils are used to work out the age of sedimentary rocks. They’re chosen on the basis of being very common fossils that are found only in rocks laid down in a narrow period of time.
When we find an index fossil in rocks in different locations, it tells us that the rocks formed around the same time.
Ammonites make great index fossils due to their rapid evolution and wide distribution. So do trilobites.
Trilobites first appeared around 520 million years ago, and they survived for more than 250 million years. In that time more than 20,000 species evolved.
What can fossils tell us?
People have been finding fossils for a very long time. These objects were often misidentified in the past. For example, ammonite shells were once thought to be coiled snakes turned to stone, and some trilobites were mistaken for butterflies.
Scientists now know what fossils are and can use them to understand the history of life on our planet.
‘Fossils can tell us a lot about not only what an animal looked like but also potentially how it behaved. An animal fossil might include, for example, the preservation of its gut contents, telling you about its diet,’ explains Paul.
Take the spinosaur Baryonyx. The shape of this dinosaur’s skull and teeth suggest the animal evolved to hunt fish. It might have used its enormous claws to hook food out of rivers, similar to how a grizzly bear fishes for salmon. However, the discovery of Iguanodon bones in what’s thought to be fossilised Baryonyx stomach contents, alongside fish scales, suggests this dinosaur’s diet may have been more varied.
We can sometimes see behaviour in fossils too. For example, a fossilised specimen of Citipati osmolskae sitting on a nest of eggs shows us that some dinosaurs were probably protective parents.
Without bones nearby, we can’t be certain what species made these fossilised footprints. But trace fossils such as these can provide behavioural insights. They tell us where an animal was at a particular moment in time and how they were interacting with their environment. © James St John (CC BY 2.0) via Flickr
Fossils may also reveal breakages from fights or other traumas, or even evidence of disease. For example, lesions on sauropod neck bones from Montana have been interpreted as evidence of airsacculitis - an inflammatory disease that still affects birds today.
Many fossils are so small that we need to use microscopes to study them. We use the name microfossil to refer to any that are less than one millimetre across. These can be fossils of whole organisms or fragments of them, such as bits of bone or tiny teeth.
Microfossils may be small, but they can provide us with lots of information about the past. For example, we can learn about past climates from tiny shells.
Dr Lyndsey Fox explains how scientists can learn about the past from ancient microorganisms.
Fossils don’t tell us the whole story, however. Soft tissues and organs don’t usually fossilise. So, while fossils of trilobite exoskeletons and ammonite shells are common, evidence of their soft parts aren’t. Fossils of these parts would allow palaeontologists to gather much more data about the lifestyles and biology of prehistoric animals.
As hard parts, such as bones or shells, are more likely to preserve than soft-bodied organisms, the snapshot of prehistoric life that we get from fossils can be quite biased. It’s also not usually possible to determine an animal’s colour from its fossil, although there are some rare exceptions.
Despite the incomplete nature of the fossil record, fossils provide us with evidence of evolution. We can use them to observe how life on Earth has changed over time. Transitional fossils are particularly useful.
What are transitional fossils?
Transitional fossils are specimens that have traits of their ancestor species and the species that descended from them. They give us clues about how evolution occurred.
The Late Jurassic dinosaur Archaeopteryx is a possible transitional fossil between non-avian dinosaurs and birds, for example. This animal had features in common with modern birds, such as feathered wings and a small body. But it also had dinosaur-like features, such as sharp teeth and a long, bony tail.
This is a cast of the iconic Berlin Archaeopteryx specimen. The London specimen - the first Archaeopteryx skeleton ever recovered - is on display in our Treasures Gallery.
Another example of a transitional fossil is Pakicetus. It’s thought to be one of the first cetaceans - the group of marine mammals that includes dolphins, porpoises and whales. While Pakicetus had four limbs and lived on land like its ancestors, fossils also show that it had evolved ear bones that are unique to whales today. This tells scientists that Pakicetus is very likely part of the evolutionary pathway to cetaceans.
Discover how four-legged, land-dwelling animals evolved into modern-day whales. Find out more about how whales evolved.
Are fossils fragile?
If circumstances are just right and animal or plant remains become fossils, they can survive underground for millions of years. Despite this, fossils can be quite fragile.
‘It varies, depending on the type of rock the fossils are made of and were preserved within,’ explains Paul.
‘Some fossils are really tough, and others are extremely delicate. They can be brittle and snap quite easily, particularly if you remove the rock from around them, leaving just the structures of their shells or skeletons behind.’
Palaeontologists and fossil preparators must be very careful while they’re excavating fossils and preparing them for display or research collections. They use a variety of techniques and tools, including using adhesives, to hold fragile and broken specimens together.
Like animals, plants from our planet’s past sometimes fossilise. You can find these fossil trees in our Hintze Hall. They’re between 25 million and 385 million years old.
Fossils containing certain minerals can be particularly fragile. For example, when crystals of iron pyrite, which is also known as fool’s gold, form in fossils it can have a dramatic effect. Paul describes it as making ‘bones or shells explode in slow motion’.
Pyrite oxidises when it comes in contact with air or moisture, and it can cause a fossil specimen to disintegrate. It’s sometimes known as pyrite decay or pyrite disease.
To protect at-risk fossils, we limit their exposure to air and humidity by storing them in air-tight boxes or bags. Pyrite oxidation can’t be reversed, but the effects can sometimes be neutralised by conservationists.
Is this a fossil?
Have you discovered an object and want to know whether it’s a fossil? Contact our Identification and Advisory Service in the Angela Marmont Centre for UK Nature to learn about your find.
Now you know what they are, learn how to find fossils on the beach.
