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What are fossils

A fossil is any preserved remains, of any once-living thing from a past geological age. Examples include bones, shells and exoskeletons. It can also include impressions or imprints of animals or microscopic oganisms, objects preserved in amber (like in Jurassic Park!), hair, petrified wood, oil, coal, and DNA remnants.

In this activity you will learn about how fossils are made and explore how looking at fossils helps palaeontologists learn about how the animals that are extant (alive today) evolved.

Activity One: How fossils are made

When you imagine a fossil you probably think about bones in rocks, or fossilised shells like the ammonite in the picture above. We are going to start by thinking about how this kind of fossil is made.

Question: Lets begin with how you think fossils are formed. Take a sheet of paper and using words or pictures try to describe how you think fossils are formed, then scroll down to see the answer.

Image of a fossolised mammouth tooth. Brown in colour.
Fossil ammonite
Fossilised shell

Lets find out how it works:

1. An animal dies, its body or skeleton settles on the sea floor or the river bed. It is buried by sediment - sand and mud.

2.  The sediment surrounding the skeleton thickens and begins to turn to stone. The skeleton dissolves leaving a hole in the stone and a mould is formed. Minerals then crystallise inside the mould and a cast is formed.

3.  Over time the fossilised bones will reappear at or rise up to the surface of the earth through natural geological processes.

Activity Two: Why are fossils important?

Palaeontology is the study of the history of life on Earth as based on fossils. Palaeontologists spend a lot of time finding fossils and studying them.


Question: Why do you think that is? What can we learn from fossils? Grab your piece of paper and write down your ideas, then scroll down to find the answer.


Studying fossils helps them learn about when and how different species lived millions of years ago. Sometimes, fossils tell scientists how the Earth has changed. In the early 1800s, Georges Cuvier and William Smith, considered the pioneers of palaeontology, found that rock layers in different areas could be compared and matched by the fossils they contained.

Fossils can also give us evidence of how creatures evolved. For example, palaeontologists deduced that whales evolved from land-living animals. They found fossils of extinct animals that are closely related to whales, which have front limbs like paddles, but similar to legs. They also had tiny back legs. The fact that the front limbs of these fossil animals are in some ways similar to legs, in other ways show strong similarities to the flippers of modern whales, helps scientists piece togther how whales evolved.

Activity Three: Looking at Fossils

You are now going to be a palaeontologist by studying images of fossils very closely and trying to deduce what kind of animal you think they are from, using the clues you can see in the fossils.

Below are two sets of images. The first set are images of fossils from our collections, the second are images of extant creatures - creatures that are alive today.


Look at the images of the extant creatures, remember that fossils are the extinct ancestors or relatives of these plants and animals.

Try to match the fossil with the image of the living creature which is most similar. Then try to imagine what the fossil creature would have looked like when it was alive.

Draw your ideas. Remember it could be a creature that lived in a shell, a plant, a creature from the sea etc – not just a dinosaur!

fossil pair 2.jpg

Pair One

The fossil is of an extinct ammonite, Mantelliceras, that lived in the Chalk Sea. This deep, warm sea covered most of Britain during the Cretaceous period, about 85 million years ago. Most ammonites swam, the shells sinking to the sea bed after death. Some 'uncoiled' ammonites lived on the seabed.
Its modern counterpart is the nautilus, a marine mollusc of the cephalopod family Nautilidae. Ammonites are related to other cephalopods—such as squid, octopuses and cuttlefish—and were early relatives of the nautilus. Both the ammonite and nautilus are marine predators that moved through the water by jet-propulsion.

fossil pair 4.jpg

Pair Two

The fossil is the shell of the oyster Ostrea edulis, a bivalve mollusc from the cool Red Crag sea of the Pleistocene period that covered most of Essex about 2 million years ago.

Next to it is a picture of a modern oyster of the same species.

The fossil was collected in the 1800s at Walton-on-the-Naze, Essex, by the Saffron Walden banker, naturalist and charitable donor, Geroge Stacey Gibson.

fossil pair 3.jpg

Pair Three

The fossil is of an extinct belemnite, Actinocamax, that lived in the Chalk sea during the Cretaceous period, about 85 million years ago. Fossils of this belemnite were found at Cherry Hinton and Harston in Cambridgeshire.
Its modern day counterpart is a squid, another cephalopod. Belemnites were squid-like animals with ten arms, and a simple internal skeleton which we see as the fossil. Unlike modern squids, belemnites didn’t have an extra pair of feeding tentacles. Like other cephalopods such as octopus, squid and cuttlefish, belemnites had beaks, ink sacs and a tail fin.

fossil pair 1.jpg

Pair Four

The fossil is of a sea urchin, Conulus subrotundus, that lived in the Chalk sea. The Chalk sea was deep and warm and covered most of Britain during the Cretaceous period, about 85 million years ago. This fossil was found near Hildersham in the middle chalk rocks of Cambridgeshire and Essex.

Its modern counterpart is also a sea urchin, such as tha the common sea urchin Echinus esculentus.

What have we learned?

  1. Fossils are the remains or traces of plants and animals that lived long ago.

  2. Fossils are formed when an organism's remains dissolve in the Earth. This leaves a hollow mould behind. The mould is then filled in by minerals, leaving a mineral cast of the organism behind.

  3. A palaeontologist is a scientist who studies the fossilised remains of all kinds of organisms.

  4. Ammonites are a group of extinct marine molluscs. They are related to other cephalopods such as squid, octopuses and cuttlefish, and they were early relatives of the modern nautilus.

  5. Belemnites are an extinct group of marine cephalopod, similar to the modern squid.

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