Carboniferous Earth – The Age of Bugs

Carboniferous Earth

358.9 to 298.9 Million Years Ago

This picture illustrates some of the most famous inhabitants of the Carboniferous swamp forests, such as the meganeura and various early amphibians and other tetrapods.

After hundreds of millions of years being largely restricted to the oceans, terrestrial ecosystems were well-established by the beginning of the Carboniferous Period. Primeval forests were rapidly crawling further inland, pumping oxygen into the air through photosynthesis. This change to the Earth’s atmosphere was about to profoundly change the course of evolution, as we’ll explore in the eighth part of my “A Journey through the History of Earth” series.

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 the mass extinction of marine life towards the end of the Devonian, the period that preceded the Carboniferous, terrestrial ecosystems were booming. The first amphibians evolved from fish, insects formed from arthropods, and the first primitive trees formed the basis of land-based biomes. Slowly but surely, our world was starting to resemble something with which we would be familiar today. However, as the Carboniferous came along, evolution was about to spiral out of control.

The Carboniferous is a period defined by its extremes. It saw the highest oxygen levels ever known, the largest insects and arachnids and climate change on truly unprecedented levels. It’s a time known for its incredibly volatile nature, where pretty much everything that lived was vying for dominance in a lush rainforest world. Owing to the enormous biomass, much of the coal we rely on today formed during this period. In fact, the name itself means ‘coal-bearing’.

Highlights of the Carboniferous

  • Diversification of insects and arachnids
  • Highest ever oxygen levels
  • Extensive formation of coal beds
  • Decline of trilobites
  • Evolution of the first reptiles
  • Climate change leads to rainforest collapse

Vast Swamp Forests Span a Tropical World

Carboniferous swampMary Evens Picture Library/Alamy

A nineteenth-century painting depicting the characteristic swamps of the early Carboniferous period.

The Carboniferous Period inherited the ancient terrestrial ecosystems responsible for the greening of the land during the late Devonian. Like the period that preceded it, the Lower Carboniferous world was much warmer than it is today, and oxygen levels were rising rapidly as early vascular plants crept across the land. The poles were likely free or almost free of permanent ice. Sea levels also remained high, but the oceans were a very different place after catastrophic marine extinctions destroyed most of the world’s coral reefs during the Devonian.

Despite the devastating effects of the Late Devonian Extinction, marine life quickly recovered as arthropods, molluscs, nautiloids and sharks radiated throughout the warm seas. Where huge armoured monster like dunkleosteus had once ruled, sharks were now the apex predators of the vast Panthalassic Ocean. Among these were the stethacanthids, characterised by their bizarre anvil-shaped crests in place of dorsal fins. Among its contemporaries were the falcatids, males of which had dorsal fins like bayonets, and helicoprionids, sharks with dental arrangements like circular saws.

As life recovered in the Early Carboniferous oceans, vast swathes of swamp forest colonised freshwater regions. Ferns, lycopods, horse tails, club mosses and cycads were thriving, creating lush rainforests across equatorial regions and far beyond. The terrestrial biomass (the mass of organic matter) was spiralling out of control as swampy rainforests covered millions of square miles of land. Their rapid and unhindered evolution continued to pump vast amounts of oxygen into the air, thus changing the course of evolution.

Amphibians Join Terrestrial Ecosystems

archaeothyrisArthur Weasley

The archaeothyris was one of the first synapsids, and a distant ancestor to all mammals and reptiles.

During the Late Devonian, lobe-finned fish developed legs, adapting to an amphibious lifestyle. However, terrestrial animals had yet to evolve to large sizes, which was partly the reason why plant life managed to evolve unhindered with so little to prey upon it. Nonetheless, the Early Carboniferous saw the continued development of true amphibians and other tetrapods, such as the lizard-like pederpes, which grew up to three feet (one metre) in length.

The Viséan, a geological age that lasted from 346.7 to 330.9 million years ago, saw the development of reptile-like amphibians, the distant ancestors to the mammals, birds and reptiles that would follow many millions of years later. They belonged to a clade known as reptiliomorpha, which underwent rapid evolution throughout the Early Carboniferous as amphibious tetrapods colonised the humid swamp forests.

Tetrapods continued to grow throughout the Carboniferous Period, eventually leading to giants like the eryops megacephalus. This creature, whose name means ‘huge head with a drawn-out face’, grew up to 10 feet (3 metres) long, making it one of the largest terrestrial animals of its time.

Later in the Carboniferous, the very first reptiles evolved from early tetrapods. Among the earliest of them was hylonomus, whose name means ‘forest dweller’. It is the oldest known reptile of all, and it grew up to 8 inches (20 cm) long. In terms of appearance, it was akin to a gecko, albeit much larger. From creatures like the hylonomus came the first diapsids in the Late Carboniferous. Diapsids form an extremely diverse group of animals that includes all lizards, snakes, crocodilians, turtles and even the dinosaurs.

Although mammals were still a long way off from appearing, the Late Carboniferous also saw the appearance of the first synapsids, the clade that includes mammals and their distant relatives. Superficially, the earliest synapsids didn’t look much different from diapsids that lived during the time or modern-day lizards. What sets the two clades apart are their skulls types, an important characteristic when taxonomically defining an animal and its place in the tree of evolution.

Photosynthesis Leads to Supersized Bugs

Measuring some 30 inches (75 cm) from claw to stinger, Pulmonoscopius was the largest arachnid that ever lived.

Not since the Great Oxygenation Event of 2.3 billion years ago did oxygen have such a profound effect on the evolution of life on Earth as it did during the Carboniferous. With the unprecedented greening of the land came rapidly rising oxygen levels, peaking at up to 35% during the period. Humans may have trouble adapting to such an oxygen-rich atmosphere today, since we’re used to breathing an atmosphere consisting of only 21% oxygen. However, it was this record high that allowed arthropods to evolve to monstrous proportions. By contrast, the relatively low oxygen levels that we’re used to today place a lower limit on maximum arthropod body size due to the way their breathing systems work.

First among the terrifying Carboniferous creatures was pulmonoscorpius, a scorpion that grew 28 inches (70 cm) long. This beast would have been among the largest terrestrial predators of the Viséan Age in which it lived, likely preying among upon small tetrapods. Among its contemporaries was the equally freakishly large arthropleura, the largest known terrestrial invertebrate of all time. Where the first land-dwelling animal had been a tiny millipede during the Late Silurian, arthropleura had grown into a 7.5-foot-long (2.3 metre) abomination. Nonetheless, despite popular belief, this monster millipede was neither venomous nor predatory, and was instead content with gorging on the vast amount of greenery that existed throughout the Carboniferous.

Although the first flying insects likely evolved during the Devonian Period, it was not until the Late Carboniferous that they really took off, and they certainly did so in style: Meganeura was a prehistoric dragonfly with a wingspan of 26 inches (65 cm), and one of the largest flying insects that ever lived.

As oxygen levels spiralled out of control, it wasn’t just giant bugs that were the horrors of the time. Forest fires would have also been rife, since such high levels of oxygen would allow even damp biomass to catch alight easily. In other words, everything on Earth was a whole lot more flammable, likely to the extent that the sky would have had a bruised reddish tint to it, owing to the ubiquity of forest fires.

Decaying Forests Form the Energy of the Future

This diagram illustrates how coal forms over millions of years following the collapse of swamp forests.

The Late Carboniferous was a particularly volatile time during which the global fluctuated enormously. Temperatures and sea levels plummeted during the second half of the period and, for the first time in Earth’s history, carbon dioxide levels fell to roughly what they were before the Industrial Revolution. Starting with major glaciations around 320-million years ago, the climate fluctuated between cold and arid and hot and humid, leading to multiple minor extinction events along the way.

By far the most important of these extinction events was the Carboniferous Rainforest Collapse, which occurred about 305 million years ago, nearing the end of the period. The vast swathes of coal forests, which spanned the tropical swamplands of the newly formed supercontinent of Pangea, suffered heavy losses.

Despite this event happening hundreds of millions of years ago, it remains extremely important to humanity to this day. The collapse of this enormous coal forest biomass lead to the formation of a dense layer of peat, which eventually turned into coal over many millions of years. It is a sobering thought that much of the energy we rely on today is down to this extinction event. We are quite literally fuelling our civilisation using the remains of incredibly ancient forests and the countless creatures that lived among them.

Conclusion

The Carboniferous began as a hot and humid world of ever-rising oxygen levels only to end with glaciations transforming the climate and redefining the course of evolution. Although the end of the period was not marked by a major extinction event as many other periods are, a great deal of species disappeared as icy deserts crept up from the South Pole into far higher latitudes. Next week, we’ll be exploring the continuation of this time of extreme climate change during the Permian Period, the last of the Palaeozoic periods and a time that ended with the near complete annhiliation of all life on Earth.

Part 9: Permian Earth – The Age of Amphibians

 

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