Palaeozoic Era

Palaeozoic Era, first major division of the Phanerozoic Eon of the geological timescale, lasting from about 545 until 248 million years ago. It was preceded by the Proterozoic Eon, and succeeded by the Mesozoic Era. The Palaeozoic Era is divided into six geological periods which, from oldest to youngest, are the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. It began with the most spectacular origin of many different forms of life and ended with the most severe extinction of marine life witnessed by the planet. During the Palaeozoic many forms of unfamiliar animals evolved and became extinct, hence the name Palaeozoic (Greek for “ancient life”). The era opened with a world unrecognizable and topsy-turvy compared with modern geography. Nearly all the land was located in the southern hemisphere, counterbalanced by a huge ocean in the north. Present-day southern continents were welded into one supercontinent called Gondwana positioned such that Antarctica of today straddled the equator and what is now north Africa lay near the south pole. Areas now represented by North America, Europe and the Baltic states, Siberia, and parts of present-day China were all independent blocks of land located close to the equator. By the end of the Palaeozoic all land masses had coalesced to form one supercontinent called Pangaea.

The Cambrian world was warmer and more equable than that of today but the land was barren and life confined to the seas. Within the first 15 million years of the Cambrian, several major invertebrate groups had appeared. Among these were molluscs, brachiopods (see Lampshell), echinoderms and arthropods, including trilobites. Some extraordinary fossil deposits, such as the Burgess Shale of western Canada and the Qiongzhusi Formation of southern China, show that soft-bodied animals like annelids and priapulids lived contemporaneously with the robustly skeletonized animals. Plant life mostly comprised single-celled marine algae.

During the succeeding Ordovician, brachiopods and nautiloid molluscs were very common in coastal seas, which also witnessed the appearance of diverse corals, bryozoans (see Moss Animals) and fishes, while graptolites floated in open oceanic waters. The end of the Ordovician was marked by prolonged glaciation that covered north and central Africa and this led to a major extinction of many species.

The Silurian was marked by a rise in sea level and the establishment of coral reefs, while small land plants fringed the continents and produced early soils to open up niches for primitive insects and spiders.

Inland lakes and rivers of Devonian times became colonized by a great variety of fish-like vertebrates and by the end of the period the first tetrapods with fingers and toes had appeared. Terrestrial plant life flourished with lush vegetation.

In the succeeding Carboniferous, vast swamp forests of towering lycopods and seed ferns decayed to yield most of the world's coal deposits. These Carboniferous forests were home to giant dragonflies, cockroaches, and millipedes as well as early amphibians.

During the Permian the climate gradually became more hostile. Collision between formerly separate continental plates to form Pangaea resulted in mountainous terranes such as the Ural Mountains, deserts in low latitudes and huge ice caps in high latitudes. Extensive glaciation caused a significant lowering of sea levels which, according to some theories, may have been the cause of the huge extinction of marine animal species. Trilobites became totally extinct while bryozoans, corals, molluscs, and brachiopods suffered serious losses, leaving much ecological space for new life in the succeeding Mesozoic Era.