The Carboniferous Period (359 to 299 million years ago) was a time of dramatic biological innovation. Named for the immense carbon-rich coal deposits formed during this time, this era is also known for a remarkable phenomenon: terrestrial arthropods, including insects, millipedes, and spiders, attained gigantic sizes unseen before or since. Understanding how these invertebrates grew so large involves exploring the unique environmental conditions of the Paleozoic world that fostered this evolutionary experiment.
Life in the Coal Swamps
The backdrop for this era of gigantism was a vast, humid, and stable landscape dominated by tropical coal swamps that covered much of the equatorial landmasses. These enormous wetland forests were characterized by dense, lush vegetation thriving in the warm, wet climate. The flora was dominated by towering, scale-barked lycophytes like Lepidodendron and immense horsetail relatives such as Calamites, which could reach heights of 30 meters.
As this massive plant life died, it accumulated in the stagnant, oxygen-poor swamp water, resisting complete decomposition. This process prevented the carbon in the dead plants from returning to the atmosphere, leading to the formation of peat, which would later become the world’s coal reserves. The dense, stable ecosystem of the coal swamps provided abundant shelter and a steady food source for the colossal arthropods that would eventually evolve there.
Portraits of the Largest Species
Among the most impressive inhabitants of these Carboniferous forests were the massive flying insects known as meganeurids, close relatives of modern dragonflies. Meganeura, the largest aerial predator, boasted a wingspan that measured up to 75 centimeters, approximately the size of a seagull. These formidable hunters used their powerful wings and specialized, spiny legs to capture smaller insects and possibly even early amphibians in mid-air.
On the forest floor, the largest terrestrial invertebrate was Arthropleura, a gargantuan relative of modern millipedes. Based on fossilized segments and trackways, this heavily armored arthropod could reach lengths of up to 2.6 meters and weigh around 50 kilograms. Arthropleura is thought to have been a detritivore, likely feeding on the decaying plant matter and spores littering the swamp floor.
Why Oxygen Fueled Massive Size
The primary driver for this gigantism was the unprecedented concentration of oxygen in the atmosphere during the Carboniferous and early Permian periods. Due to the massive burial of plant matter in the coal swamps, atmospheric oxygen levels peaked at around 35%, significantly higher than the 21% present today. This hyperoxic atmosphere released a physiological constraint that limits the maximum size of modern arthropods.
Arthropods do not have lungs or a circulatory system to transport oxygen via blood. Instead, they rely on the tracheal system, a network of narrow tubes that delivers oxygen directly to internal tissues via passive diffusion. As an arthropod’s body size increases, the distance oxygen must travel increases, and the diffusion mechanism becomes highly inefficient.
The elevated oxygen levels effectively increased the partial pressure gradient, supercharging the tracheal system. This allowed oxygen to diffuse much deeper into the tissues of a larger body, compensating for the limitations of a diffusion-based respiratory system. This high oxygen concentration permitted arthropods to bypass the respiratory size limit that restricts their modern descendants, enabling them to evolve into the immense forms observed in the fossil record.
How the Giants Disappeared
The reign of the giant arthropods was tied to the stability of the environmental conditions that created them. The first major factor in their decline was a global cooling and drying trend that began late in the Carboniferous period. This climatic shift led to the fragmentation of the vast coal swamps, reducing the humid habitats and stable plant communities that supported the giants.
The drying climate also contributed to a significant drop in atmospheric oxygen levels later in the Permian period, removing the physiological foundation for gigantism. As oxygen concentrations decreased toward modern levels, the tracheal system could no longer sustain the large body masses of creatures like Meganeura and Arthropleura. Compounding this pressure was the concurrent emergence and diversification of early flying vertebrates, including reptiles and amphibians, which exerted pressure on the large, slow-moving arthropods.