Phanerozoic/Silurian period

The Silurian period existed in the Paleozoic era and occurred between Ordovician and  Devonian periods, about 443 to 417 million years ago. The name Silurian was derived from a Celtic from Wales. During the period the seas fluctuated between transgression and regression due to climate changes and continental buildup. Plants and animals were forced to adapt to live on the land or become extinct.

Geography
In the early Silurian period the continents were clustered around the equator forming the super-continent Godwana, continuing a is a southern drift. The northern cratons and continent fragments drifted towards the equator and merging North America and Eurasia to form the super-continent known as Laurussia. By the end of the period, continents continued to converge with each other causing orogenies such as the Taconic Orogeny and forming mountains. These continental movements were the cause of the periods transgressional phases due to the glaciation that occured. At late Silurian was nearing the end of the larger transgression known as Tippecanoe.

Uncomformities found within different paleocontinents, show the effects of mass glaciation events during the late Ordovician and Silurian periods. The water levels dropped to a maximum of 230 feet draining marine habitats. Other fluctuations of the sea level occurred around 100 to around 165 feet. These events forced marine life to adapt to survive to these conditions.

Subdivisions
The Silurian period is divided into four major events that occurred during that time.

Llandoverian
The Llandoverian Epoch lasted between 446 to 428 million years ago.

Three paleocontinents suggests that at least four global highstands occurred during this epoch. The fluctuation between these events averaged between 2.5 million years apart from each other. Between the Llandovery and Wedlock boundries marks the Irreviken event During the event was a change in carbon and oxygen isotope levels that sparked a mass extinction event.

Wenlockian
The Wenlockian Epoch lasted between 428 to 422 million years ago.

One of the biggest sea level rises occured in this epoch. Also retains a secundo-episode (warmer sea waters and CO2 level rise) that caused a mass extinction

Landlovian
The Landlovian Epoch lasted between 422 to 418 million years ago. Known for the Lau event. The Lau Event declares the date for a mass extinction event of conodont fauna due to a low point in sea level

Also known as the Ludlow epoch, known for a lowstand in sea level. This event caused a mass extinction for conodont fauna.

Pridolian
The Pridolian Epoch lasted between 418 to 417 million years ago.

Climate
Before, the earth witnessed a fluctuation of erratic climate around the globe. Stable warmer climates were introduced in the Silurian period that resulted with melting large polar glaciers in the middle of the Silurian. As a result, there was a significant rise in the sea levels consuming about 65% of North America, as we know today, during the Wedlock and Ludlow epoches.

The warmer climates on earth create more survivable habitats for the current life enabling existing creatures and plants to adapt and flourish.

Life
The warm waters provided by the Silurian period produced good habit for the surviving species of the Ordovian extinction. The survivors flourished during these times and branched out to produce new variations. Among these variations were some of the first migrations from sea to land.

Plants
The fluctuation of sea level caused different adaptations with plants. The late Ordoversian period introduced the first plants on land. The adaptation of these plants during the Silurian involved vascular plants, enabling plants to absorb nutrients from the ground. Seeds were also introduced, to transition sea plants to land by encasing water within the seed to prevent dehydration for survival on land.

Some of the first plants on land were Cooksonia, gymnosperms, Agiaopython, and Rythion. These plants developed resistance to gravity by standing upright. The first plants were small, leafless but had even branching.

Coral
The first appearance of coral reef occured in the Silurian period. Tabulate corals were among the first corals, living in colonies among Silurian rocks. The chains of colonies that the Tabulate corals created looked similarly to Organ pipes. Ragose corals, or horn corals because of the bull-shaped horn, appeared during this time as well. These coral reefs provided protection among the other new species introduced in the Silurian period, fish.

Creatures
Some of the existing biota species from the Ordovian period included many shelled creatures. Among of these creatures were Cephalopods and Eurypterids. Because of the mass extinction cephalopods were smaller in the silurian period, and Eurypterids were Arthropods. Arthropods have an exoskeleton with legs attached, better known as sea scorpions. Crabs and insects were arthropods. Arthropods in the Silurian were the first air-breathing arthropods to exist. were the dominating predator during the period.

Fish
Jaw-boned fish were first introduced during the Silurian period. Although many aquatic species existed in before, fish were known for the bone structure that denoted them as fish. The first fish were jaw-less, and the vertebra was soft and composed of cartilage. By the end of the Silurian, fish developed real bones and real jaws.

Among the first known fish to exist were the Guiyu. This fish existed in the Late Silurian (about 419 Million years ago). Thyestes was a fish that existed between the Wedlock and Ludlow Epochs during the Silurian. They had a bony armor that classifies as Osteostraci