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The Silurian Period: (438 million years ago)

 

The Silurian is a geologic period and system that extends from the end of the Ordovician period (505 million years ago) to the beginning of the Devonian period (408 million years ago). As with other geologic periods, the rock beds that define the period's start and end are well identified, but the exact dates are uncertain by several million years. The base of the Silurian Period is set at a major extinction event when 60% of marine species were wiped out.

 

During the Silurian, The Supercontenent, Godwana, continued to drift southwards to the 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.The melting of icecaps and glaciers contributed to a rise in sea level, recognizable from the fact that Silurian sediments overlie eroded Ordovician sediments, forming an unconformity. Other cratons and continent fragments drifted together near the equator, starting the formation of a second supercontinent known as Euramerica.

 

Life

 

Following the Ordovician extinction event there was a rapid recovery of invertebrate faunas during the Silurian. Tropical reefs are common in the shallow seas of this period, formed by tabulate and rugose corals, stromatoporoid organisms, bryozoa and calcareous algae.   Trilobites, cephalopods, gastropods, and echinoderms. The Trilobites, having reached their acme in the Cambrian and Ordovician, are now in decline. The trinucleids and asaphids are absent, whilst encrinites and illaenids do not survive the end of the Silurian. Planktonic graptolites remain common and diverse. Jawless fish invade brackish and fresh water, as do eurypterids, xiphosurids, scorpions, which may have been semi-aquatic.  rhyniophytes, primitive lycophytes, and myriapods became the first proper land organisms.  At the end of the period Jawed fish appeared for the first time, but they remain insignificant.

 

 

Climate

 

During this period, the Earth entered a long warm greenhouse phase, and warm shallow seas covered much of the equatorial land masses. Early in the Silurian, glaciers retreated back into the South Pole until they almost disappeared in the middle of Silurian. The period witnessed a relative stabilization of the Earth's general climate, ending the previous pattern of erratic climatic fluctuations. Layers of broken shells (called coquina) provide strong evidence of a climate dominated by violent storms generated then as now by warm sea surfaces. Later in the Silurian, the climate cooled slightly, but in the Silurian-Devonian boundary, the climate became warmer.

 

 

The End of the Silurian Period

The Silurian drew to a close with a series of extinction events linked to climate change; however, these were relatively minor compared with those of other geologic periods.

 

Map of the Earth during the Silurian Period

 

 

 

 

 

 

Paleozoic Era

Mesozoic Era