Silurian

The Silurian is a geologic period that lasted approximately 23.5 million years. It is the third period of the Paleozoic Era and the third of twelve periods in the Phanerozoic Eon. This era began around 443.8 million years ago. It ended when the Devonian Period began. Scientists identify the period by specific rock beds that mark its start and end. However, the exact dates remain uncertain by a few million years. The start of the Silurian is linked to major extinction events. During these events, up to 60% of marine genera were wiped out.

During this time, life underwent a massive transformation known as the Silurian-Devonian Terrestrial Revolution. This process involved the initial establishment of life on land. First, vascular plants emerged from more primitive land plants. Vascular plants are organisms with specialized tissues to transport water and food. One notable example is the genus Cooksonia, which appeared during the Wenlock stage. Alongside these plants, dikaryan fungi and glomeromycotan fungi began to expand and diversify. Arthropods also made a major move to land. Three specific groups, including myriapods, arachnids, and hexapods, became fully terrestrialized.

In the oceans, a different evolutionary milestone occurred. The Silurian saw the rapid diversification of jawed fish. This group included placoderms and acanthodians, which eventually gave rise to cartilaginous fish. Another group, the osteichthyans, consisted of bony fish. These were further divided into lobe-finned and ray-finned fishes. As these jawed fish became more successful, other groups declined. Jawless fish, such as conodonts and ostracoderms, experienced a sharp decline in numbers. This shift changed the balance of life in the marine ecosystems.

Geologists divide the Silurian into several distinct stages. The first is the Llandovery, which began 443.8 million years ago. This was followed by the Wenlock stage. During the Wenlock, the first air-breathing millipedes appeared in Scotland. Next came the Homerian and the Ludlow stages. The Ludlow stage included the Gorstian and Ludfordian stages. The period concluded with the Přídolí stage. This final stage is named after a locality in the Czech Republic. Each stage represents a specific slice of time in the Earth's history.

The study of the Silurian began in the early 1830s. Scottish geologist Roderick Murchison first identified the system. He was examining fossil-bearing sedimentary rock in south Wales. Murchison named the system after the Silures, a Celtic tribe of Wales. He was inspired by his friend, Adam Sedgwick. Sedgwick had named the Cambrian period after another Latin name for Wales. In 1835, the two men presented a joint paper. This work helped form the basis of the modern geological time scale. However, Murchison and Sedgwick later had a falling out over rock sequences.

This disagreement was eventually resolved by the English geologist Charles Lapworth. He defined a new system called the Ordovician to settle the conflict. This helped clarify which rock beds belonged to which period. Another scientist, Joachim Barrande, also studied these rocks in Bohemia. He divided the Silurian rocks there into eight stages. While some of his classifications were later corrected, he made Bohemia a famous site for studying Silurian fossils. Another name once proposed for this system was the Gotlandian. This name came from the strata on the Baltic island of Gotland.

The physical world was also changing rapidly during the Silurian. The supercontinent Gondwana drifted slowly toward the South Pole. Meanwhile, the continents of Avalonia, Baltica, and Laurentia drifted toward the equator. They eventually collided to form a new supercontinent called Euramerica. This collision caused the Caledonian orogeny. This was a massive period of mountain building. These mountains stretched from New York through Europe and Greenland to Norway. The oceans were also shifting. A massive ocean called Panthalassa covered much of the northern hemisphere.

Climate and sea levels fluctuated significantly throughout the period. The Earth entered a warm greenhouse phase with high CO2 levels of 4500 ppm. This warmth supported shallow seas across many equatorial land masses. However, the climate was not perfectly stable. Scientists see evidence of many drastic fluctuations in the geological record. There were several major events, such as the Ireviken, Mulde, and Lau events. These events involved rapid sea-level changes and minor mass extinctions. These shifts impacted free-swimming organisms, corals, and trilobites. The Silurian was a time of intense biological and geological activity.