Cambrian: 542–488 million years ago

Lifeless land, teeming ocean

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Illustration of Burgess Shale scene.
Image: John Sibbick
Source: John Sibbick

540 million years ago.
Image: Ron Blakey. Altered by Cally Bennet and Fons VandenBerg
Source: Colorado Plateau Geosystems

Cambrian

542 million years ago the fossil record shows a dramatic expansion of animal life in the sea, known as the ‘Cambrian explosion’. The body plans of some animals that evolved at this time show an ancestry to modern animals. There were also some weird and wonderful creatures unlike any alive today. The ocean teemed with worms, jellyfish, trilobites and brachiopods.

The ‘explosion’ appears in part because some animals had evolved tough body parts, such as shells or exoskeletons, which are more readily preserved as fossils. This reflects a change in the ecology as predators evolved, fuelling the need for defensive features such as spines and hard shells; the predator/prey arms race had begun. One of the most successful hard-bodied groups – the trilobites – first appeared in the Cambrian.

At this time, what we now call Victoria did not exist as land. The edge of the continent was in the west (in present-day South Australia). Offshore volcanoes erupted lava onto the ocean floor and built up chains of volcanic islands. Sand and mud washed off the barren dry land and accumulated in the ocean between the continent and the volcanoes, forming sedimentary rocks. Over millions of years these rocks were compressed, heated and thrust upwards along giant faults. Today they form the bedrock of western Victoria and the remains of these ancient volcanic rocks are called ‘greenstones’.