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soft bodied, so that their remains are very rarely preserved. The first evidence of more developed life (including sponges, jellyfish, and insects) comes from Australian rocks dated at about 680 million years ago, and by then there are at least four separate major groups of life, or phyla.
The Palaeozoic Era The Cambrian Period
(Below) Cambrian trilobite: Paradoxides.
500 million years ago
570 million years ago
Mass extinction: trilobites greatly reduced
Stromatolites abundant
First gastropods, molluscs, crinoids, trilobites, brachiopods, graptolites
The Cambrian Period marks the sudden change, some time after 570 million years ago, to evolution that included hard shells and other forms of skeleton. Of this period, the most important guide fossils are the trilobites. They occurred all over the earth and evolved quite rapidly, so that their evolution can be used to trace the divisions of Lower, Middle and Upper Cambrian Epochs.
Although trilobites made up, perhaps, three- quarters of all known species, there were many other living things at this time. Among the most common remains are brachiopods and tiny fragments of sponges, known as spicules. Less common, but nonetheless present, are the molluscs and gastropods, as well as small cephalopods. Echinoderms had evolved, but were rare. Corals
do not occur in the Cambrian Period. There was no life on land.
At the end of the Cambrian Period an unknown event happened that caused a large number of families to die out. It is called a mass extinction.
It made nearly half of all trilobite families perish.
(Below) Cambrian trilobite: Lingula.
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(Below) Cambrian trilobites: Elrathria (larger specimen), Ptychagnostus (smaller specimen).