The best examples of fossil fish anywhere in the world are in Australia, in the Kimberley region of Western Australia. These are called the Gogo fish. You probably imagine that collecting fossils is all about hitting a rock as hard as you can with a hammer and hoping that it breaks in half to reveal a fossil. Well, that's how fossils are usually found. But it's a lot harder to get the best out of the Gogo fish. The fossilised bone is locked inside a hard lump of limestone rock. If you hammer hard enough to break open the limestone, you might break the fossil.
There's another way of getting to the fish fossils. The bone, amazingly, hasn't changed in over 370 million years and is still original bone, so you can put weak acid on it and it won't dissolve. The limestone rock, though, will slowly dissolve away in acid. It's a bit like putting an ice cube in warm water. Over a few minutes it would melt. You can do this for yourself. Put a bit of broken chicken bone in an ice-cube tray - freeze it, then drop it in warm water.
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After the ice has melted you'll be left with just the bone. It's the same with the Gogo fish, only we use weak acid - a bit like strong vinegar (see, fish covered by vinegar again). After two to three months, the limestone has dissolved away and we're left with the perfectly preserved bones.
Although some of the Gogo fish had evolved real bony backbones, many others were still slightly strange. These ones didn't wear their teeth on the outside, they wore something else - their bone! (Like Superman, wearing his underpants on the outside.) They're known as placoderms, meaning 'plated skin'. They have a bony armour covering their head, and sometimes the front part of their body. They often remind me of an overcooked meat pie - fish pie, I suppose that should be.
At this time (near your thumb, remember), sharks were just tiny, not much bigger than snapper. The placoderms ruled the seas. Some grew as long as a stretch limo. We know that these fish ate other fish because of a fossil discovery. After dissolving one of these limestone rocks a palaeontologist found a placoderm head, but it wasn't alone. Stuck in its mouth was the head of a smaller fish. Almost certainly the larger fish's eyes were too large for its belly and it choked to death on the smaller one.
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