Imagine, you are wandering along a beach. You are looking at the shells and the rocks that have been washed up, and playing in the water. You see a crab washed up on the beach, but you don't think much about it, after all, you've seen many crabs before! This one is a bit bashed up, but you have a closer look anyway. You expect it to be pretty delicate, but when you look closely you discover that it is actually as hard as a rock.
Often, fossilised sea creatures are quite similar to modern creatures and people may overlook them for this reason. This was one of the aspects of my fossil crab which most fascinated the children I showed it to. When they first looked at it they recognised it almost immediately as a crab, albeit a crab which 'had a hard life'. They thought it would be like the crabs they have found on the beach, dry and easily broken, and some were worried that if they held it they might break it. They were fascinated to see that actually, this crab was very hard. Although it could easily have its legs broken off (and many have been lost already), it was not the delicate thing they expected.
Although it isn't a find which is made every day, many people have found fossils of crabs and other sea creatures simply by wandering along a beach! In fact, another, important crab fossil was found in exactly this way and if you would like to learn more about this discovery, click here. Fossilised sea creatures, including shells, are actually the most common fossils in the fossil record, and for a very good reason. Most fossils are formed when a plant or animal dies in a muddy, wet environment. In these places, the animal or plant can be quickly buried in silt and mud, important to the fossilisation process. So sea creatures are in a perfect environment! The faster they are buried the more complete the fossil is likely to be. When things die, particularly animals, it is the natural process for scavengers, or other animals, to feed on them. You've probably seen this yourself, where a dead fish is being eaten by a seagull or a lizard or insect is being eaten by ants. So when silt or mud quickly buries a plant or animal, it protects it. The soft bits of tissue will usually quickly decay, leaving behind the bones and shells. The sediments which build up over the top of the plant or animal harden into rock, and encases the 'potential fossil'. Then, over time the hard parts, like bone and shell begin to decay, and minerals replace the organic parts, little by little in a process called 'petrification'. This is probably what happened with my crab.
Sometimes the bones or shell decay completely, leaving behind a cast of the animal. These casts can be used to make replicas of whatever was fossilised. Usually these replicas are made from plaster, though a mold is (usually) made of the fossil and the plaster replica would be cast from the mold, not the original! Sometimes nature makes the replica without any help from people! The void left behind by the animal or plant might fill up with minerals leaving a rock replica of whatever started the whole process.
Although animals and plants are the most common fossils, all sorts of things can be fossilised - even dinosaur tracks are sometimes found! Similarly, although the fossilisation process above is the most common, there are other way which fossils can be formed. Sometimes an insect or piece of a plant is encased in plant sap, and this hardens over a long period to become what we call amber. Not every piece of amber will have something encased inside, but many do!If you would like to learn more about how fossils are created, click here, here or here.