The Carboniferous Period, spanning approximately 359 to 299 million years ago, is named for the vast coal deposits formed globally from immense swamp forests. During this time, the supercontinent Pangea was forming, and the Earth experienced a predominantly warm and humid climate, leading to widespread rainforests and wetlands. These conditions created an environment that fostered the diversification of various life forms, setting the stage for major evolutionary developments.
Giant Terrestrial Invertebrates
The Carboniferous Period is notable for the emergence of exceptionally large terrestrial invertebrates. Many arthropods exhibited gigantism, attributed to higher atmospheric oxygen levels present. Oxygen concentrations during the Carboniferous could reach as high as 35%, significantly higher than today’s 21%. This elevated oxygen facilitated more efficient respiration for insects, allowing them to grow to remarkable sizes.
Among the most iconic examples were the griffinflies, such as Meganeura, which resembled modern dragonflies. Meganeura boasted a wingspan of approximately 65 to 75 centimeters (2.1 to 2.5 feet), making it one of the largest flying insects known. These aerial predators hunted other insects and small amphibians within the lush Carboniferous forests. Another colossal invertebrate was Arthropleura, an ancient relative of millipedes, which could reach lengths of over 1.5 meters (5 feet) and widths of half a meter. Despite their size, Arthropleura were likely detritivores, feeding on decaying plant matter from the abundant swamp vegetation.
Amphibians and Early Reptiles
The Carboniferous Period marked a time for vertebrate evolution on land. Amphibians diversified considerably, becoming the dominant terrestrial vertebrates. These early amphibians, often referred to as labyrinthodonts, retained a strong reliance on water for reproduction, typically needing to lay their eggs in aquatic environments. Examples include temnospondyls, a diverse group of amphibian-like tetrapods that ranged in size and morphology.
Later in the Carboniferous, an evolutionary innovation emerged: the amniotic egg. This shelled egg allowed embryos to develop on land without drying out, freeing vertebrates from their aquatic reproductive ties. The development of the amniotic egg was a step that enabled early reptiles, such as Hylonomus, to become fully terrestrial. These early reptiles, which appeared approximately 320 to 310 million years ago, began to diverge into distinct lineages, setting the foundation for the diverse reptile groups that would dominate later periods.
Diverse Marine Inhabitants
While terrestrial life saw developments, Carboniferous oceans also teemed with diverse inhabitants. Shallow, warm marine waters frequently covered continental areas, providing extensive habitats. Filter feeders such as crinoids, often called “sea lilies,” flourished, forming dense underwater “gardens” and contributing significantly to limestone deposits. Brachiopods, shelled organisms that resemble clams, were also abundant and diverse on the seafloor.
Fish continued to diversify in both marine and freshwater environments. While heavily armored fish from the Devonian Period largely disappeared, modern-looking fish fauna replaced them. Early sharks, such as Cladoselache, were prevalent in marine settings, alongside ray-finned fish. Cephalopods, including nautiloids and the first ammonoids, were also common in the water column.
Environmental Influences on Animal Life
The unique animal life of the Carboniferous Period was profoundly shaped by its environmental conditions. High atmospheric oxygen levels, reaching up to 35%, played a significant role in enabling the gigantism observed in terrestrial arthropods. This oxygen-rich atmosphere supported their respiratory systems. The widespread warm, humid climate facilitated the growth of vast swamp forests across the continents. These dense forests provided abundant habitats and food sources for terrestrial animals. The accumulation of organic matter from these forests led to the formation of extensive coal beds that give the period its name. This unique combination of high oxygen, consistent warmth, and expansive wetlands fostered the substantial diversification and dominance of the period’s characteristic animal groups.