Explosion of Life
Cambrian and Ordovician periods 440-544 million years ago
The Cambrian Period of geological time was first named by Adam Sedgwick in 1835 and is now known to record a remarkable explosion in the evolution of marine life. Although the origin of life stretches far back into the depths of Precambrian time, the emergence of numerous groups of marine organisms with hardparts is preserved in Cambrian strata from around the world.
Famous amongst such fossils of the Cambrian explosion are the extinct trilobites and one of the largest known specimens can be seen in the Sedgwick Museum display.
However, a much more detailed view of Cambrian seabed life was revealed by the discovery of the Burgess Shale in British Columbia, Canada. Investigation of these rocks by successive generations of Cambridge palaeontologists has revealed a wealth of information about soft bodied organisms of Cambrian age, which are not normally preserved in the rock record.
From Cambrian through Ordovician times the diversity of marine life continued to expand with the appearance of most of the major groups of animals from sponges to vertebrates. The development of widespread shallow seas allowed the evolution of the first modern-looking reefs with corals. However, in detail these ancient reefs are quite different from those of today, being full of unfamiliar organisms such as brachiopods, trilobites and sea-lilies and even their corals are not directly related to modern ones.
Surprising though it may seem, some Ordovician reef limestones now comprise the highest rocks in the world and a small sample is on display in the Museum. It was collected from just below the summit of Mt Everest by Edmund Hillary when he and Tenzing Norgay first conquered the summit on May 29th, 1953. The Earth’s tectonic forces have lifted these rocks some 9 km from below sealevel during the formation of the Himalayan Mountains.
Trilobites such as Paradoxides, were common animals that crawled around on the seabed in Palaeozoic times. They are just one of many groups of marine arthropods (animals with pairs of jointed appendages) that appeared within Early Cambrian times and were part of the 'explosion of life', which produced an abundance of fossil remains within a few million years. Trilobites died out in the end Permian extinction 252 million years ago.
Graptolites were common marine creatures in Early Palaeozoic times, which formed colonies in tiny zooids within a tubular organic skeleton. Although some lived attached to the seabed, most of them drifted around with the current and fed by filtering microscopic food particles from the surrounding seawater. Graptolites first evolved in Cambrian times and died out in Carboniferous times, although a closely related group, the pterobranchs, survive. The name graptolite is derived from 'graptolithus', the Greek 'graptos' meaning 'written' and 'lithos' meaning 'rock'. This name was coined in 1735 by the Swedish naturalist Carl Linnaeus (1707 - 1778).
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