Silurian

The Silurian is a major division of the geologic timescale that extends from the end of the Ordovician period, about 443 million years ago, to the beginning of the Devonian period. As with other geologic periods, the rock beds that define the periods beginning and end are well identified, however, the exact dates are uncertain by 5 - 10 million years. During this period, the Earth entered a long warm greenhouse phase, and warm shallow seas covered much of the equatorial land masses. Early in the Silurian, glaciers retreated back into the South Pole until they almost disappeared in the middle of Silurian. Silurian witnessed a relative stabilization of the Earths general climate, ending the previous pattern of erratic climatic fluctuations. Layers of broken shells (called coquina) provide strong evidence of a climate dominated by violent storms generated then as now by warm sea surfaces.

Later in the Silurian, the climate cooled slightly, however, in the Silurian-Devonian boundary, the climate became warmer. Subdivisions of Silurian:

Llandovery

The Llandovery epoch lasted from 443 million years ago to 428.2 ± 2.3 million years ago, and is subdivided into 3 stages: the Rhuddanian, lasting until 439 million years ago, the Aeronian, lasting to 436 million years ago and the Telychian. The epoch is named for the town of Llandovery in Carmarthenshire, Wales.

Wenlock

The Wenlock, which lasted from 428 million years ago to 422 million years ago, is subdivided into the Sheinwoodian (to 426 million years ago) and Homerian ages. It is named after the Wenlock Edge in Shropshire, England. During the Wenlock, the oldest known tracheophytes of the genus Cooksonia, appear. The complexity of slightly younger Gondwana plants like Baragwanathia indicates either a much longer history for vascular plants, perhaps extending into the early Silurian or even Ordovician.

Ludlow

The Ludlow, lasting from 422 million years ago to 418 million years ago, comprises the Gorstian stage, lasting until 421 million years ago, and the Ludfordian stage. It is named for the town of Ludlow in Shropshire, England.

Pridoli

The Pridoli, lasting from 418 million years ago to 416 million years ago, is the final and shortest epoch of the Silurian. It is named for the town of Pridoli in the Czech Republic.

The base of the Silurian is set at a major extinction event when 60% of marine species were wiped out. Silurian period sees the appearance of the first plants on land. Algae was in abundance in the seas, as well as marine invertebrates. Coral reefs made their first appearance during this time, built by extinct tabulate (an extinct form of coral) and rugose corals (are an extinct order of coral).

The first bony fish, the Osteichthyes (superclass of fish) appeared, represented by the Acanthodians (sometimes called spiny sharks) covered with bony scales. Fishes reached considerable diversity and developed movable jaws, adapted from the supports of the front 2 or 3 gill arches. A diverse fauna of Eurypterus (Sea Scorpions) - some of them several metres in length, prowled the shallow Silurian seas of North America, many of their fossils have been found in New York State. Brachiopods (also known as lamp shells), bryozoa (tiny colonial animals), molluscs (these range from tiny snails, clams, and abalone to squid, cuttlefish and the octopus), and trilobites (Trilobites are extinct arthropods) were abundant and diverse.

Myriapods became the first proper terrestrial animals. The terrestrial ecosystems included the first multicellular terrestrial animals that have been identified, relatives of modern spiders and millipedes whose fossils were discovered in the 1990s. Myriapods - 4 groups of arthropod - the centipedes, millipedes, pauropods and symphylans - share a number of common features such as a similar body plan consisting of a head followed by an elongate trunk with many legs.

During the Silurian, Gondwana continued a slow southward drift to high southern latitudes, but there is evidence that the Silurian icecaps were less extensive than those of the late Ordovician glaciation.The southern continents remained united during this period.The melting of icecaps and glaciers contributed to a rise in sea level, recognizable from the fact that Silurian sediments overlie eroded Ordovician sediments, forming an unconformity. Other cratons and continent fragments drifted together near the equator, starting the formation of a second supercontinent known as Euramerica.

The vast ocean of Panthalassa covered most of the northern hemisphere. Other minor oceans include two phases of the Tethys - the Proto-Tethys and Paleo-Tethys - the Rheic Ocean, a seaway of the Iapetus Ocean (now in between Avalonia and Laurentia), and the newly formed Ural Ocean.

At the end of Silurian, a series of minor extinction events, including the Lau event, occurred. They were probably caused by climate change or impact events.