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Fish
Sharks and rays are some of the oldest fish: they are groups that have shown remarkably little evolutionary change. Sharks of the Upper Devonian age are hard to distinguish from modern sharks.
Normally, only isolated teeth are found. Because they have evolved so little, they are not used to date rocks.
Bony fish have a long history. Many of them had thick, heavy scales, looking like a kind of armour. They are first found in the Devonian Period, and, although modern
fish are less armoured, they survive to the present
day in examples like the sturgeon.
(Below) Bony fish can leave complete skeletons. Dunkleosteus was a predatory bony fish that could grow to 10 metres. It lived during the Devonian Period.
(Above) Tertiary fish preserved in siltstone.
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