About 550 million years ago, a lot of changes took place on the Earth. At that time, probably the whole of the Earth warmed up. As a result, a lot of ice melted and the sea level rose. It is assumed that the composition of the sea water became calciferous, turning mollusks that lived earlier in the sea into animals with hard protective calcium shells or calcium skeletons. From that time onwards many animals started turning into fossils after death. The first such animals were the trilobites, and these were later followed by the mussels and ammonites, which are related to the octopus.
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