What Was the Silurian Period?

The Silurian period is the third of six major periods that make up the Paleozoic era, a geologic division of time that extends from about 542 million years ago to 251 million years ago. The Silurian period itself began at the end of the Ordovician period, approximately 444 million years ago, to the beginning of the Devonian period, about 416 million years ago. Therefore, the Silurian period was a period of time that lasted 28 million years.

At the beginning of the Silurian period were the Ordovician-Silurian extinction events, the second-largest set of extinctions in the history of the planet after the larger Permian-Triassic extinction event, which wiped out 60% of all marine genera. The recovery was rapid, especially among invertebrate faunas. The superficially clam-like brachiopods dominated, making up 80% of total species. Most of the trilobites died off in the recent extinction, and were on the decline throughout the Silurian.

The Silurian period and the Devonian period that came after it were an important time in the evolution of fishes. Fishes dominated the sea, and many species had a curious sort of head armor that is not seen in any extant species. Vascular plants, that is, plants capable of carrying nutrients through their tissues, first appeared on land during the Silurian, although most plants were still non-vascular and only a few centimeters tall. The first steps in the greening of the Earth's surface were underway. These plants were not seed-bearing, and probably did not grow very far from water. They reproduced using spores and direct vegetative growth. The interior of continents were vast, dry, and dead. No true forests existed.

Although there had been some tentative forays of animals onto land before, the Silurian period saw the first true land biota walk on land, in the form of Myriapoda, a subphylum that includes millipedes and centipedes. This subphylum first emerged during the Silurian period, and remains one of the oldest as it is still extant today.

Eurypterids, or water scorpions, and jawless fish spread into brackish and fresh water for the first time. Snails and nautiloids were common organisms. Near the end of the Silurian period, there were some minor extinction events, including the Lau event, caused by either impact events or climate change.