438 to 390 MILLION YEARS AGO
Extensive shallow seas existed around the Paleo-Tethys Ocean region. This allowed massive and widespread coral reefs to flourish.
Earth in Middle Silurian nearly 425 million years ago. Image © 1997 Christopher R. Scotese. As of 2014, an updated map can be downloaded from the Colorado Plateau Geosystems, Inc. web site and created by Professor Ronald C. Blakey of Northern Arizona University (NAU).
While oxygen levels were rising above 25 per cent, the actual pressure in the atmosphere had probably risen to about one-tenth of its present-day value, during which time simple plants like Cooksonia started to adapt to the drier environment of land.
The first plants to take hold on land grew initially in marshy land areas with its primitive root system still embedded in the nearby water supply, and then later in the more desert-like expanses inland.
Wingless insects became the first creatures to crawl onto land, following the vegetation ashore, sometime during the end of this period, or the early Devonian Period.
A trilobite of the Silurian Period. Source: Mackness 1987, p.63.
420 MILLION YEARS AGO
Glaciation appeared at this time despite high levels of carbon dioxide in the atmosphere. Scientists are not too sure why. Perhaps volcanic eruptions have sent debris high in the upper atmosphere to reduce sunlight?
Photograph of a complete fossilized armoured fish of length about 5cm called Athenaegis. This fish appeared during the Silurian period. Source: Long 1995, p.49.
400 MILLION YEARS AGO
The region where the capital city of Australia is located Canberra was once at the bottom of a tropical shallow sea containing reefs, sea shells, corals and beaches along great expanses of land containing volcanoes with plants hugging the shores, rivers and lakes for water and nutrients.