Around 300 million years ago, Earth was undergoing profound transformations, marking a period of dramatic change in its geological, atmospheric, and biological systems. This ancient world differed significantly from the planet we inhabit today, characterized by immense forces reshaping continents, a distinct atmospheric composition, and the emergence of diverse life forms.
Shaping a Supercontinent
Around 335 million years ago, Earth’s landmasses converged, forming the supercontinent Pangaea. This block formed from the collision of Gondwana, Euramerica (also known as Laurussia), and Siberia. Tectonic plate movements increased during the Carboniferous, shifting continents across the globe.
These tectonic forces resulted in several mountain-building events, known as orogenies, that reshaped Earth’s surface. One such event was the Alleghanian orogeny, which occurred between 325 and 260 million years ago. This event was driven by the collision of Africa (part of Gondwana) with North America (part of Euramerica), forming the Appalachian Mountains.
The Variscan orogeny (also called the Hercynian orogeny) began in the Middle Devonian and continued into the early Permian, with substantial Carboniferous activity. This orogeny resulted from the collision between Euramerica, Gondwana, and the Armorican terrane assemblage as the ancient Rheic Ocean closed. The Variscan mountain belt stretched over 10,000 kilometers, from the Gulf of Mexico across Europe to modern-day Turkey, encompassing ranges like the Massif Central, Pyrenees, and parts of the UK. The mid-Carboniferous Ouachita orogeny was also part of this broader collision, as the South American sector of Gondwana impacted Laurussia’s southern margin. These combined mountain-building events formed the Central Pangean Mountains, which reached heights comparable to the modern Himalayas around 295 million years ago.
A Breath of Ancient Air
Around 300 million years ago, Earth’s atmosphere held a higher concentration of oxygen than today. Oxygen levels peaked at 35%, an increase from 20% at the Carboniferous Period’s start. This oxygen-rich environment allowed some organisms to attain considerable sizes.
The climate was warm and humid, particularly in equatorial regions where much of Pangaea was situated. Evidence from fossil plants, such as absent growth rings, suggests a uniform climate with minimal seasonal variations across these tropical zones. Despite the overall warmth, the late Carboniferous also experienced periods of cooling and glaciation, especially as Gondwana drifted southward towards lower temperature regions.
Abundant plant life, particularly in swamp forests, defined the Carboniferous, giving the period its name. These forests contributed to coal deposit formation. When plants died, their organic matter accumulated in waterlogged, oxygen-poor swamps, preventing decomposition. Over millions of years, this buried plant material transformed into global coal seams under pressure and heat. This carbon sequestration, storing atmospheric carbon dioxide as coal, reduced CO2 levels and contributed to global cooling trends in the later period.
Life’s Flourishing Epoch
Tropical swamps and riparian forests marked the Carboniferous period, dominating the landscape. These ecosystems hosted primitive plant groups thriving in waterlogged conditions. Tree-sized lycophytes like Lepidodendron and Sigillaria grew 30 to 40 meters tall, with scaly-barked trunks. Giant horsetails like Calamites formed dense thickets with hollow, jointed stems over 10 meters tall. Abundant ferns created undergrowth and canopy layers, while seed ferns combined fern-like foliage with seeds. Early gymnosperms, Cordaites (ancestral to modern conifers), also emerged, preferring drier upland environments for seed production.
High oxygen levels allowed Carboniferous insects to attain large sizes. Dragonflies like Meganeura monyi were among the largest flying insects, with wingspans reaching 65 to 75 centimeters. These predatory insects likely fed on smaller insects and amphibians. Millipedes also grew very large; Arthropleura, the largest known land arthropod, could reach 2 to 2.6 meters and weigh up to 50 kilograms. Their wide trackways, over 50 centimeters across, are preserved in the fossil record. This gigantism in insects was facilitated by their passive diffusion respiration system, which was more efficient in an oxygen-rich atmosphere, and by the absence of aerial predators like birds, which had not yet evolved.
The Carboniferous period is sometimes called the “Age of Amphibians” due to their diversification. Early tetrapods, four-limbed vertebrates, became pentadactylous (developed five digits). Labyrinthodonts like the predatory Eryops and Proterogyrinus were common, some reaching almost six meters, and remained closely associated with coal swamps.
An evolutionary milestone was the emergence of reptiles (amniotes) 300 to 320 million years ago. Evolving from amphibian-like ancestors, reptiles developed the amniotic egg, an adaptation allowing them to reproduce on land, independent of aquatic environments. This egg contained a protective shell and internal membranes, providing a self-contained aquatic environment for the developing embryo.
Early reptiles like Hylonomus, a small lizard-like creature, are among the oldest amniotes. Other terrestrial adaptations included thicker, more waterproof skin and more efficient lungs. Amniotes diversified into distinct lineages, including synapsids (leading to mammals) and sauropsids (including modern reptiles and birds). Their ability to thrive away from water allowed them to spread and diversify, especially following environmental shifts at the end of the Carboniferous.