Silurian Period

Silurian Period, third oldest division of the Palaeozoic Era of the geological timescale, spanning an interval from about 448 to 422 million years ago. It is preceded by the Ordovician Period and succeeded by the Devonian Period. The international stratotype (see Stratigraphy) section for the boundary between the underlying Ordovician System is defined in the Midland Valley of Scotland. The boundary between the overlying Devonian System was the first to be agreed internationally and is defined in the Barrandian Basin near Prague in the Czech Republic.

The Silurian was named in 1835 by the English geologist Roderick Impey Murchison after an early Britannic tribe, the Silures, who inhabited western England and Wales, where sedimentary rocks of this age were first studied systematically. Three of the four internationally agreed Series of the Silurian are now named after towns within this area (Llandovery, Wenlock, and Ludlow). The fourth and youngest Series of the Silurian is named after the Czech region of Prídolí.

More than half the world’s land mass during the Silurian was represented by the Gondwanan crustal plate which migrated across the south pole at this time. To the north, a number of other continental plates were present; these now form parts of present-day North America, Europe, and Asia. Laurentia, which today constitutes North America, collided with two smaller palaeocontinents, Avalonia and Baltica, causing closure of the Iapetus Ocean (sometimes referred to as the proto Atlantic Ocean). The exact time that these three continents collided and the ocean finally closed has been debated but full closure is likely to have occurred towards the end of the Ordovician or in the earliest Silurian. There were no major mountain-building episodes (orogenies) during the Silurian. However, the Taconian orogeny of the preceding Ordovician Period (505 million years ago) was probably related to the closure of the Iapetus Ocean. The next major orogeny, the Acadian orogeny, took place during the Devonian Period (408 million years ago).

The Early Silurian is characterized by a return to non-glacial conditions following the glaciation at the end of the Ordovician (see Ice Ages). Warm water masses expanded at the expense of sub-polar masses in two major ocean basins, the Pacific-Arctic region and a Proto-Tethyan basin that covered the area that now represents central Europe. At the same time, some deposits such as limestones were forming in shallow shelf seas at more equatorial locations. Greenland and parts of North America, now within the Arctic Circle, were only about 15° from the equator in Silurian times. Isotopic studies on Silurian limestones reveal that the level of oxygen in the atmosphere was 36-65 per cent of that today. In hot, arid lands, sand dunes formed and evaporite minerals, such as salt and gypsum, precipitated from former inland waters. For example, in the Great Lakes region of North America there are Silurian salt deposits up to 500 m (1,650 ft) thick that are extensively mined.

The expansive Silurian ocean basins hosted a wide variety of planktonic life forms of which the graptolites, now extinct, were the most widespread. These colonial hemichordates evolved rapidly, producing a succession of species that are very useful in correlating deep-water sedimentary rocks around the world. Organic planktonic microfossils such as acritarchs, as well as extinct phosphatic tooth-like microfossils called conodonts, have also been used to correlate both shallow and deepwater sediments. Species of corals, brachiopods (see Lampshell), and trilobites dominated Silurian shallow-water shelf seas but have been used less widely for dating sediments because they mainly lived on the bottom of the sea (benthos) and were not widely dispersed. Instead, the distribution of these, often restricted, faunas has provided geologists with information useful in reconstructing the relative positions of major continental plates. Towards the end of the Silurian, fishes, including the first jawed varieties, became much more common inhabitants of the shallow seas. Rocks deposited at the very margins between the sea and land have yielded fossil spores and tiny branched organic remains from the first vascular land plants. Fossils of the earliest land arthropods have also been discovered in similar sediments.