Trilobite biscuits

There is another reason why one type of animal that lived at this time is often found fossilised. And that's because of how it grew. These animals were trilobites. They were once very common, but they all died out (became extinct) about 250 million years ago. (This is about where your fingers join on to your hand, just before the first dinosaurs appeared on Earth.) Trilobites belong in the same group as insects, spiders, crabs and prawns - the arthropods. All these types of animals have 'segmented' bodies.

So, what is a segmented body? Well, for a trilobite just imagine a packet of biscuits. If you take all the biscuits out of their packet, but leave them resting in their plastic tray, then each biscuit is a bit like a segment of a trilobite. Joined to each trilobite segment was a pair of legs. So a trilobite with ten body segments had ten pairs of legs.

legs. So a trilobite with ten body segments had ten pairs of legs.

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Fortunately, biscuits rarely have pairs of legs, so they remain in their tray. Trilobites, though, probably scuttled over the sand on lots of little legs. Now imagine that your biscuits are chocolate-coated. Six at one end got a bit warm and melted a bit. As they cooled, they stuck together. The rest of the biscuits are loose. Trilobites were like that (although not, so far as we know, chocolate-coated). They have a head formed from six segments joined together as a head shield.

Like all living arthropods, trilobites peeled off their old shells every so often as they grew. A new, slightly larger shell had been growing underneath. Imagine chucking off all your clothes and finding another, new set. That' s what it was like for the trilobite. This means that if a single trilobite shed its skin, say, 20 times during its life, it could leave 20 fossilised heads, hundreds of body segments and 20 fossilised tails - all left in the mud for a palaeontologist to find hundreds of millions of

years later. It's a bit like throwing your clothes on your bedroom floor and leaving them for your mum to pick up - but she never does, and they stay there for ever . . .

If you have to hunt for your food, or you want to avoid being eaten, it's useful to be able to see. Trilobites had the best eyes. Their two eyes were made of hundreds of tiny lenses, each made from a single clear crystal. One American palaeontologist found out that he could attach one of these little crystalline lenses to a special camera and take a photo through it. So, 350 million years after a trilobite was using it to look for its last meal, the palaeontologist was using it to take a photo out of the window of his laboratory. It came out really well, showing that trilobites had great vision. Best of all, I think, is that the photo was of the FBI building across the road. The home of the USA's special police force was being spied on by a trilobite!

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