Silurian Period: 443 – 419 million years ago

earth 430 million years ago

Earth 430 million years ago

The Silurian system was first identified by British geologist Sir Roderick Impey Murchison, who was examining fossil-bearing sedimentary rock strata in south Wales in the early 1830s.

Murchison named the sequences for a Celtic tribe of Wales, the Silures, inspired by his friend Adam Sedgwick, who had named the period of his study the Cambrian, the Latin name for Wales.

Sir Roderick Murchison

Sir Roderick Murchison

The Silurian period enjoyed relatively stable and warm temperatures, in contrast with the extreme glaciations of the Ordovician before it, and the extreme heat of the ensuing Devonian.

Early in the Silurian, glaciers retreated back into the South Pole until they almost disappeared in the middle of the period.

The rebound of life’s diversity with the permanent re-flooding of continental shelves at the onset of the Silurian saw increased biodiversity within the surviving life forms.

During the Silurian Period land plants and large coral reefs built by tiny coral polyps became widespread.

Silurian Marine Life

Silurian Marine Life

Plant ecosystems were colonized by land animals such as millipedes.

It is also at this time that our first good evidence of life on land is preserved, such as relatives of spiders.

Artist's impression of Silurian fish

Artist’s impression of Silurian fish

A significant evolutionary milestone that occurred during the Silurian period was the appearance of jawed and bony fish.

The first bony fish, the Osteichthyes, appeared, represented by the Acanthodians covered with bony scales; fish reached considerable diversity and developed movable jaws, adapted from the supports of the front two or three gill arches.

The Silurian is also the period when the first known freshwater fish first appeared.

A diverse fauna of Eurypterids (sea scorpions)—some of them several meters in length—prowled the shallow Silurian seas of North America; many of their fossils have been found in New York state.

Leeches also made their appearance during the Silurian Period. Brachiopods, bryozoa, molluscs, hederelloids, tentaculitoids, crinoids and trilobites were abundant and diverse.

Earth entered a long warm greenhouse phase, and warm shallow seas covered much of the equatorial land masses.

Life also began to appear on land in the form of small, moss-like, vascular plants which grew beside lakes, streams, and coastlines.

Cooksonia, the earliest vascular plant, middle Silurian

Cooksonia, the earliest vascular plant, middle Silurian

During the Silurian, supercontinent Gondwana covered the equator and much of the southern hemisphere. A large ocean occupied most of the northern half of the globe.

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 while the melting of icecaps and glaciers contributed to a rise in sea level, recognizable from the fact that Silurian sediments overlie eroded Ordovician sediments.

High sea levels and relatively flat land (with few significant mountain belts) resulted in a number of island chains, and thus a rich diversity of environmental settings.

The continents of Avalonia, Baltica, and Laurentia drifted together near the equator, starting the formation of a second supercontinent known as Euramerica.

When proto-Europe collided with North America, the collision folded coastal sediments that had been accumulating since the Cambrian off the east coast of North America and the west coast of Europe.

This event is called the Caledonian orogeny, a spate of mountain building that stretched from New York State through conjoined Europe and Greenland to Norway.

Some evidence suggests the presence of predatory trigonotarbid arachnoids and myriapods in Late Silurian faeces.

Predatory invertebrates would indicate that simple food webs were in place that included non-predatory prey animals.

At the end of the Silurian period, sea levels dropped again, leaving telltale basins of evaporites in a basin extending from Michigan to West Virginia, and the new mountain ranges were rapidly eroded.

The Teays River, flowing into the shallow mid-continental sea, eroded Ordovician strata, leaving traces in the Silurian strata of northern Ohio and Indiana.

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