Silurian Earth – The First Breath of Air

The Silurian Period

443.8 to 419.2 million years ago

Ryan Somma

Maritime life thrived throughout the Silurian period, with nautiloids (centre photo) being the most successful and largest predators at the time.

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. In the sixth part of my “A Journey through the History of Earth” series, we’ll be exploring the first of these primordial land-based biomes.

Part 1: Hadean Earth – The Violent Creation of Our World

Part 2: Archean Earth – Signs of Life

Part 3: Proterozoic Earth – The First Animals

Part 4: Cambrian Earth – An Explosion of Evolution

Part 5: Ordovician Earth – Colonising a Barren Land

Part 6: Silurian Earth – The First Breath of Air

Part 7: Devonian Earth – The Age of Fishes and Forests

Part 8: Carboniferous Earth – The Age Bugs

Part 9: Permian Earth – The Age of Amphibians

Part 10: Triassic Earth – The Rise of the Dinosaurs

Part 11: Jurassic Earth – The Land of Giants

Part 12: Cretaceous Earth – The Reign of Tyrants

Part 13: Paleogene Earth – The Rise of Mammals

Part 14: Neogene Earth – Human Ancestors

Part 15: Quaternary Earth – The Age of Man

Despite being by far the shortest of the Palaeozoic periods, lasting less than 25-million years, the Silurian saw one of the most important evolutionary events in the history of our world. 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. Like the Ordovician and the Cambrian before it, the name ‘Silurian’ was inspired by the country of Wales where many fossils dating from this time have been identified. The Silurian period was first described and identified in 1835, and it was named after the ancient Celtic Silures tribe, who were contemporaries of the Ordovices some 2,000 years ago.

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, reaching a peak 590 feet (180 m) higher than they are today. Once again, Earth went through an unprecedented period of global warming, lifting the shackles on evolution and allowing early arthropods and brachiopods (worms) to once again continue their exploration of the land. At this time, by far the largest continent was Gondwana, comprising parts of what is now Antarctica and Australia and located in the southeast of the map. The smaller continents of Siberia and Baltica shrank with the rising sea levels, gradually shifting further northwest of the map into the vast Panthalassic Ocean.

During the Early Silurian, the only known multicellular life that had permanently adapted to life on the land were tiny liverwort-type plants forming mossy growths around the shorelines. Nonetheless, the spread of such organisms formed an essential foundation for the first truly land-based ecosystems. Until then, the primitive terrestrial plant life of the Ordovician and Early Silurian was still heavily reliant on the water.

Oxygen levels in the Earth’s atmosphere continued to rise slowly but steadily thanks to the continued spread of photosynthetic organisms. At the same time, early plant life made its journey from the tidal shallows and gradually spread further inland as it became less dependent on the ocean’s waters for sustenance and reproduction. Nonetheless, oxygen still only accounted for 14% of the atmosphere during the Silurian, which is some 30% less than it is today.

Earth continued to warm throughout the first half of the Silurian period, eventually reaching an average global temperature some 3 °C higher than it is today. As the planet recovered from the ice age, life once again started to thrive and evolve, and the dark times of the Late Ordovician extinction event, one of the most severe in Earth’s history, were long behind.

Miniature Forests Crawl across the Land

MUSE Science Museum

Cooksonia is the by far the best known and iconic plant fossil of the Silurian period. In real life, the plant was extremely small.

Cooksonia is perhaps the most iconic of all Silurian fossils. One of the earliest known true plants, this tiny leafless organism quickly colonised shorelines in many parts of the world during the middle of the Silurian period. Several species have been identified, and it’s widely believed that 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.

Arthur Weasley

Guiyu oneiros is one of the earliest bony fish known. Living during the Late Silurian around 419-million years ago, it was also one of the largest fish of its time.

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, the foot-long (30 cm) guiyu oneiros being one of the largest and best known. Most notable, however, were the eurypterid sea scorpions, a highly successful order of marine predators which are distantly related to arachnids. The ancestors to sharks also appeared during the Silurian, although there is evidence that the earliest sharks had their beginnings in the Ordovician. Other already well-established groups, such as nautiluses, marine gastropods, trilobites and brachiopods, also continued to thrive and diversify throughout the Silurian.

The First Ever Breath of Air

Matteo De Stefano/MUSE

Pneumodesmus newmani is the first known animal to have ever lived permanently on the land. The tiny creature was no larger the a woodlouse, and probably fed on mosses.

In 2004, palaeontologists in Scotland found the definitive evidence of the earliest animal to live on dry land. The fossil was 428-million years ago, and it belonged to a millipede one centimetre long. Even more remarkably, this discovery put back the date of the first terrestrial animal by some 20-million years. Named pneumodesmus newmani after its amateur palaeontologist discoverer Martin Newman, this animal was one of the earliest to breath the air, representing a profound step forward in the evolution of life. Indeed, it might have just been a tiny millipede, but it’s incredible to think that, if it hadn’t been for this enterprising little character, evolution may have taken a very different course.

The latter half of the Silurian was moderately warm, although there was probably still a southern polar icecap covering a part of what is now Africa. 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. These factors combined, along with the essential role played by the lunar tides, to encourage the evolution of larger animals that would eventually migrate out of the tidal shallows and colonise the land to such an extent that they would transform it beyond recognition in the following Devonian period.

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

Mary Parrish

Prototaxites was long assumed to be a primitive plant until it was eventually determined to be a tree-sized fungus.

In the mid-nineteenth century, a bizarre discovery was made of what looked like an extremely ancient fossilized trunk of a conifer dating from the Late Silurian. For almost 150 years, it was assumed to be a very early tree, but the fact that it was much, much bigger than any other terrestrial organism of the time kept everyone baffled. 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.

A century and a half after the its discovery, prototaxites was eventually determined to be a fungus, probably belonging to the nematophyta phylum which included land-based algae from as early as the Cambrian period. A lot of unanswered questions remain surrounding this incredibly bizarre lifeform, but one thing seems certain: the Late Silurian landscape was dominated by spire-shaped pillars of life that were actually some of the largest mushrooms that ever existed.

Conclusion

The Silurian ended 419.2-million years ago with the end of the Přídolí Epoch, so named after a region near the Czech capital Prague where extensive fossils of cephalopods, bivalves and trilobites were found. 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, characterising the beautifully colourful Devonian period that we’ll be exploring in the next episod