Geology / Background

Life had greatly diversified at the beginning of the Cambrian all over the world, development consolidated during the Ordovician before reaching a major crisis that marks the start of the Silurian. A climatic flip to ice age conditions led (directly or indirectly) to the loss of many early forms of life, wiping out just as many species as the famous Cretaceous Tertiary (K-T) extinction event that finished off the dinosaurs. Some recent theories suggest that life proliferated in the Cambrian following a spate of ice ages in the Pre-Cambrian that covered the whole planet (the Snowball Hypothesis). High stress conditions in isolated communities of life lead to rapid evolution of new species.

From the coldness of the Ordovician Ice Age, the Silurian saw life quickly re-establish itself throughout the warm marine world. As much of the ice caps melted a high sea water level led to the inundation of many low lying areas. Shallow shelf seas gave ideal conditions for marine life. This environment also led to the widespread deposition of limestone throughout the World and this is one of the distinctive features of the Silurian.

It was a significant period for life on Earth as at long last, life came out of the waters and colonized dry land. Previously life was confined to the sea and other areas of permanent freshwater (inland lakes and rivers). Oxygen, a life-giving gas to mammals is a highly toxic chemical to sensitive marine life. To survive exposure to air, plants, followed closely by animals feeding on them, had to solve the step of developing protection from this poisonous gas. Once oxygen could be kept out of the cells, the next step to survive on dry land was the retention of water. They also had to survive under atmospheric pressure, ultra-violet light as well as coping with wide daily temperature fluctuations. It is likely that life moved out slowly from permanently wet areas.

Fish the forebears of amphibians, reptiles, birds and mammals saw a wide expansion and diversification during the SIlurian, including their incursion into freshwater rivers and lakes.

The Silurian period was first mapped out by the pioneering geologist Sir Roderick Impey Murchison in 1835-1839. From extensive examination of outcrops of rocks in English-Welsh borderlands a consistent sequence of rocks based on the occurrence of fossils was traced over large areas. The Silurian system is named after the ancient British Silures tribe who inhabited this area in Roman times. The two other systems that make up the Lower Palaeozoic era (the Cambrian and Ordovician systems) are also both named after ancient British tribes in neighbouring areas. It was in Wales and its borderland that first revealed the progress of early life, seen through the fossil record. A controversy between the two pioneers Sedgwick and Murchison over the designation of 'disputed' rocks in north Wales (belonging either to Sedgwick's Cambrian or Murchison's Silurian) led to Lapworth's invention of the Ordovician System to cover the disputed overlapping rock sequences.

The detailed study of the Silurian in the UK has made a significant contribution to the understanding of the geological processes that occur when two plates collide. In England the sea to the north-west was closing and the associated volcanism, metamorphism and faulting were first worked out, particularly in Shropshire.

One area where life flourished and where Silurian fossils are most easily seen is within fossilised coral reefs. Recognisable ancient ancestors of modern day reef dwelling animals can be seen. The most famous Silurian site in the world is Wenlock Edge in Shropshire, England. The small historic town of Much Wenlock stands at the northern tip of Wenlock Edge in Shropshire. In this area the Silurian saw a general, gradual change from deep water through shallow water into sub-aerial deposition conditions as the Iapetus Ocean closed.

Igneous activity was somewhat less than in previous times although Central Europe saw some activity. In broad terms the Silurian was a period of peaceful onward evolution of life on Earth after the rigours of the Ordovician ice age.

The Silurian System is split into the following series Llandovery, Wenlock, Ludlow and Pridoli. These are fully described on the Silurian System page.