Following the devastating mass extinctions at the end of the Ordovician Period, the glaciers covering the ancient land of Gondwana receded, and another period of intense global warming began. Afterwards, the very first animals started to settle the land, forming the earliest terrestrial ecosystems and jumpstarting a new phase in the evolution of our planet.
Although the Ordovician had seen the first primordial mosses colonise coastal areas around the world and the first curious arthropods had started to explore the land, it wasn’t until the middle of the Silurian that the first terrestrial ecosystems became developed enough to function independently of the sea.
The Silurian is the third geological period of the Phanerozoic aeon and the third of the Palaeozoic Era.
Highlights of the Silurian
- Rapid global warming
- Evolution of the first bony fish
- First vascular plants settle the land
- The first sharks
- Giant fungus dominates terrestrial ecosystems
- First creature to take a breath of air
Global Warming Redefines the Path of Evolution
443.8-million years ago, the glaciers of the Late Ordovician ice age started to melt, and the sea level rose rapidly. Once again, Earth went through an unprecedented period of global warming. At this time, by far the largest continent was Gondwana located in the southeast of the map. The two smaller continents shrank with the rising sea levels, gradually shifting further northwest of the map into the vast Panthalassic Ocean.
During the Early Silurian tiny plants formed mossy growths around the shorelines still heavily reliant on the water.
Earth continued to warm throughout the first half of the Silurian period, eventually reaching an average global temperature higher than it is today.
The Land miniature forests and fungus
Cooksonia is a tiny leafless organism that quickly colonised shorelines in many parts of the world during the middle of the Silurian period. They grew in great abundance. Nonetheless, the largest were no longer than a couple of inches (5 cm), forming expanses of miniaturised ‘forests’ in swampy areas. Cooksonia is most notable for being the earliest known vascular plant (tracheophytes), a group of plants that includes trees and all other land plants that have waxy layers to prevent water from escaping – something that’s essential for land-based life.
While tiny plants were crawling out of the shallows, life in the oceans continued to expand and diversify, with coral reefs stretching far and wide and giving rise to ever more sophisticated ecosystems. Silurian sea life included the first bony fish. Most notable, however, were the eurypterid sea scorpions. The ancestors to sharks also appeared during the Silurian. Other already well-established groups, such as nautiluses, marine gastropods, trilobites and brachiopods, also continued to thrive and diversify.
The latter half of the Silurian was moderately warm, although there was still a southern polar icecap. Oxygen levels were continuing to increase due to the spread of early land plants, and high carbon dioxide levels kept the world in a strong greenhouse climate with high sea temperatures.
Joining cooksonia in its conquest of the land was another now long-extinct clubmoss known as baragwanathia, also a type of vascular plant and one that grew over a metre in length. Like the otherwise unrelated cooksonia, it spread its spores in the wind to reproduce, meaning that it was independent of the oceans.
Giant Mushrooms Take Over
Prototaxites, as it was named, grew up to 26 feet (8 metres) in height and had a trunk-like structure up to 3 feet (1 metre) wide.
The Late Silurian landscape was dominated by spire-shaped pillars of life that were actually some of the largest mushrooms that ever existed.
The Silurian ended 419.2-million years ago. Although terrestrial life was still scarce, and had yet to make a significant impact on regions further inland, that was about to change dramatically. Soon, the alien world that was the Silurian Earth would end up being covered by vast swathes of primordial forests.