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Cambrian

The Cambrian 545 – 510 million years ago

Following the upheaval and change of Precambrian times the Cambrian marks a transition to quieter, more stable conditions. 545 million years ago the global ice age had come to an end. Shropshire was flooded by a shallow sea, which washed against a rocky coastline. Thick layers of beach pebbles and white sand were built up against the sea cliffs that were once molten volcanic lava.

At about this time, one of the most important events in the history of the earth was taking place. Life had up until now been a relatively simple affair, worms, jellyfish, algae and bacteria, that sort of thing. But all of a sudden in the Cambrian there was a massive explosion in the variety of life that swam in the seas. Creatures with hard outer shells are found as fossils in the rocks of this age. Trilobites began to dominate the seas, as they would for the next 200 million years.

Why did life suddenly radiate across the planet in so many new and different forms? Perhaps it was because during the early Cambrian the huge continent that dominated the Precambrian had broken up into smaller landmasses. This would have created more shallow water environments that could be home to a greater number of animals. Also the severe global ice age had just ended. This would have raised sea levels dramatically, again making more places for creatures to live.

Another effect of the end of the ice age was the influx of cold water. This can cause a feature called upwelling in the oceans, where deep cold water rises to the surface bringing lots of nutrients with it. Simple plankton and algae need these nutrients to survive and a massive upwelling could cause a boom in the number of these tiny plants and animals, which in turn make food for larger animals.

Global temperatures were suddenly higher too, after the ice age; in fact the earth was warmer in Cambrian times than it is today. Maybe this helped fuel the explosion after thousands and thousands of years of ice. Or maybe these creatures had been evolving slowly, before the Cambrian time but only with soft bodies, which don’t make good fossils. So, no fossils, no evidence, and then suddenly these creatures find out how to use minerals and salts from sea water to make themselves hard shells and outer skeletons and these make really great fossils.

The jury is still out on the reasons behind the Cambrian Explosion. If you want to know more about this event check out some of these links:

http://palaeo.gly.bris.ac.uk/Palaeofiles/Cambrian/index.html#interest (an excellent but technical site)

Shropshire has some of the most historically important evidence in the explosion of life and in the naming and dividing of the Cambrian period. Trilobites that were found in this county are internationally important for deciding how the Cambrian is divided into smaller segments of time.

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