Silurian

The Silurian is a geological time interval that forms part of the Paleozoic Era. It follows the Ordovician and precedes the Devonian. It is estimated to have begun approximately 443 million years ago and ended approximately 419 million years ago, making it one of the shorter periods of the Paleozoic.

The name derives from an ancient people of Wales called the Silures. In the 19th century, English geologist Roderick Murchison applied the name to rock strata he studied in Wales, and correlative strata with the same designation were subsequently identified elsewhere in the world.

Boundaries and divisions

The beginning of the Silurian is defined by a major extinction event at the close of the Ordovician, associated with glaciation of the supercontinent Gondwana and a fall in sea level. The Silurian is subdivided into four major divisions: Llandovery, Wenlock, Ludlow, and Přídolí. These divisions are distinguished by changes in graptolite species, marine organisms whose fossils are abundant in rocks of this period.

Climate and seas

Overall, the Silurian was a time of warmer climate compared to the end of the Ordovician. Continental ice sheets diminished, and sea levels rose, creating shallow seas that covered the continents. In these shallow waters formed limestone and reef deposits associated with reef-building organisms.

Marine life

Silurian seas were inhabited by brachiopods, trilobites, crinoids, and cephalopods with simple shells. Graptolites were widespread and important for dating rock layers. Fish with jaws (gnathostomes) also became prominent during this time, representing a major stage in the evolution of vertebrates.

Land

The Silurian is considered the period in which plants bearing vascular tissue and roots appear clearly in the fossil record. A notable example is Cooksonia, a small plant with simple branches. Land animals, including early arthropods and arachnids capable of breathing air, are also recorded from this period, indicating that terrestrial ecosystems were beginning to develop.