The Silurian Period

The Silurian period stretches from 443.4 to 419.2 Ma, it is a relatively short period.

Map of the Palaeo continents during the middle Silurian.
Note the continents on the equator are Laurentia,
Baltica and Avalonia colliding during the Caledonian
Orogeny. Image credit: silurian.stratigraphy.org

But despite it's length this was a time of great biological advances.

During this time Gondwana had continued to drift south and covered much of the southern latitudes. The northern hemisphere was mostly ocean with the exception of two continents, Laurentia and Baltica, during the dawn of the Silurian. As the period passed, Gondwana rifted further, the continent of Avalonia separated and drifted North. The middle Silurian is characterised by mountain building, known as the Caledonian Orogeny. This was when Avalonia, Baltica and Laurentia collided near the Equator. This collision formed the Irish and Welsh Mountains as well as the Grampians, Northern Appalachians and the Norweigian and Swedish Mountain ranges.

The land masses were still at a low altitude and the sea levels remained high. Shallow sea continued to provide new habitats for an evolving fauna. Light had the ability to penetrate the water in the shallow sea environment. The evidence provided for this comes from the great amount of coral reef fossils that we find in the Silurian sediments.

In the early Silurian, the jawless fish discussed in the Ordovician post still dominated. However, as the middle of the period arrived so did the jawed fish. Placoderms, named Romundina, had evolved. Placoderms are armoured fish with cartilaginous skeletons. The evolution of a jaw enabled these fish to become refined hunters in the Silurian waters.

However, fish were still in their infancy. The Eurypterids were the top predators during the Silurian. The sea scorpions had become larger and more common, developing a spiked tail which allowed for the injection of venom into prey.

The climate was also beginning to stabilise after the extensive glaciation during the Ordovician. It is believed that Lichens were the first photosynthetic organisms to move onto land. This would be a logical assumption because the first plants to colonise the land would have needed a softer soil to flourish. Decaying Lichens produced acids that broke down rock into soils. This paved the way for the primitive plant Cooksonia to take control of the banks of deltas and rivers. Cooksonia was the first upright plant to evolve. Cooksonia show it had bulbous tips. This suggests it may not have been photosynthetic due to the lack of leaves, but it is theorised that spores could have been ejected from the stalks. Despite the lack of leaves, this pioneering plant was the first to have vascular tissues.

The fossil of Cooksonia sp. alongside an artist's impression
of the plant. Image credit: tes.com

This revolutionised growth in plants, by growing vertically rather than laterally, plants could start competing better for sunlight with each other as well as the mosses and algae. The fossil of Cooksonia show it had bulbous tips. This suggests it may not have been photosynthetic due to the lack of leaves, but it is theorised that spores could have been ejected from the stalks. Despite the lack of leaves, this pioneering plant was the first to have vascular tissues. 

We also see further diversification of brachiopods, molluscs, crinoids and trilobites. Graptolites, an animal that puzzles palaeontologists on what they may have looked like when alive, are one of the more common fossils of the Silurian, for this reason they are used to date the Silurian strata. This is because they change their morphology so frequently, like ammonites during the Mesozoic. 

Graptolite fossils. Image credit: geology.gsapubs.org

Animals also began to take their first steps onto land by the mid Silurian. Pseumodesmus, a genus of ancient millipede was among the first animals to venture onto land. The predatory arachnids and myriapods that occupied the land are the very start of the terrestrial food web, being the top predators. They may have fed on undiscovered detritivores and mircoscopic grazers, this was a hypothesis that was put forward by Andrew Jeram et al in 1990.

To summise, the Silurian saw a terrestrial revolution as plant and animal both made their first tentative steps onto the barren wastelands. While these organisms battled the elements above the waves, in the oceans life continued to thrive because of shallow sea habitats and a stable climate.