Article 22: The Silurian sees the healing of the Earth. The warm, stable climate provides for one of the most significant developments: the arrival of the first plants to colonize the land.
The Silurian Period of the Earth’s history spans 25 million years, from 445 to 420 million years ago.
The Silurian is a time when the Earth undergoes considerable changes that have important repercussions for the environment and life within it. One result of these changes is the melting of large glacial formations. This contributes to a substantial rise in the levels of the major seas. The Silurian is also a remarkable time in the evolution of fishes, the foreshadow of ‘the age of the fish’ in the next period.
Comparison of two sea level reconstructions during the last 500 Ma.
The scale of change during the last glacial/interglacial transition is indicated with a black bar. Note that over most of geologic history, long-term average sea level has been significantly higher than today!
The Silurian sees the healing of the Earth that recovered from the snowball period. During this period, the Earth enters a long, warm, greenhouse phase. Warm shallow seas cover much of the equatorial land masses.
Early in the Silurian, glaciers retreat back into the South Pole until they almost disappear in the middle of Silurian. The period witnesses a relative stabilization of the Earth’s general climate, ending the previous pattern of erratic climatic fluctuations. Later in the Silurian, the climate cools slightly, but in the Silurian-Devonian boundary the climate becomes warmer again. So, during the Silurian Period, the climate is generally warm and stable, in contrast to the glaciers of the late Ordovician and the extreme heat of the next period, the Devonian.
This warm, stable climate provides for one of the most significant developments to take place during the Silurian Period: the arrival of the first plants to colonize the land. Lichens are probably the first photosynthetic organisms to cling to the rocky coasts of the early continents. When organic matter from decaying lichens joins the action of erosion to wear away rock, the first real soil begins to build up in shallow, protected estuaries. Bryophytes such as moss, hornworts and liverworts first appear in the late Ordovician. The first known plant to have an upright stalk and vascular tissue for water transport, is the Cooksonia of the mid- Silurian deltas. This little plant is a few centimeters high with a branched structure with small bulbous tips. It lacks true leaves, suggesting that the stalk develops to disperse spores and is not itself photosynthetic.
The first known air-breathing animals are arthropods. Millipedes, centipedes and the earliest arachnids first appear in the Silurian. Since arachnids are exclusively predatory, this represents the first terrestrial food web.
‘Arachnida’ from Ernst Haeckel’s Kunstformen der Natur (1904)
During this time there are four continents: Gondwana (Africa, South America, Australia, Antarctica, Siberia), Laurentia (North America), Baltica (Northern Europe) and Avalonia (Western Europe). The supercontinent Gondwana has drifted southward and covers most of the southern latitudes. Much of the northern half of the planet is ocean with two smaller continents, Laurentia and Baltica, near the equator. Micro- continent Avalonia cracks from the northern edge of Gondwana and drifts northward.
Laurentia collides with Baltica, closing the northern branch of the Iapetus Ocean and forming the “Old Red Sandstone” continent. Coral reefs expand and land plants begin to colonize the barren continents.
Beginning in the late Ordovician and continuing throughout the Silurian and into the Devonian, these three northern continents collide, forming the new supercontinent, Laurasia or Euramerica. This collision results in a major mountain building event, known as the Caledonian Orogeny. The hills and mountains of Scotland, Ireland, Wales and the northern Appalachians are remnants of this event, as are the mountains of Sweden and Norway.
Thanks to the University of California, Berkeley, Museum of Paleontology, to Christopher R. Scotese, and to LiveScience