The Carboniferous Period, spanning from approximately 359 to 299 million years ago, marked a significant era in Earth’s history. This geological timeframe was characterized by widespread warm, humid conditions that fostered vast swamp forests across the continents. These dense, lush environments eventually led to the formation of extensive coal deposits, which gave the period its name, meaning “coal-bearing”.
Giants of the Carboniferous Air and Land
The Carboniferous saw the emergence of large terrestrial invertebrates, particularly arthropods, attributed to the high atmospheric oxygen levels of the time. Oxygen concentrations reached up to 35%, higher than today’s 21%. This abundance of oxygen allowed arthropods, which respire through a network of tracheal tubes, to overcome the size limitations imposed by their respiratory system.
Examples included Meganeura, a giant dragonfly-like insect with a wingspan reaching up to 70 centimeters, comparable to the size of a seagull. Another large inhabitant was Arthropleura, a millipede relative that grew to over 2 meters in length and half a meter wide, making it the largest terrestrial arthropod known. These large creatures thrived in the humid forest understories, with Arthropleura likely feeding on decaying plant matter.
The Rise of Land Vertebrates
The Carboniferous Period was an important time for vertebrate evolution, particularly for the colonization of land. Early amphibians diversified considerably and became the dominant terrestrial vertebrates. These early amphibians developed adaptations such as sturdy, weight-bearing limbs with distinct digits and more efficient lungs, allowing them to navigate and breathe in terrestrial environments.
Despite their terrestrial adaptations, many early amphibians still relied on water for reproduction. Some species, like Eryops, grew to substantial sizes, resembling modern crocodiles. The period is referred to as the “Age of Amphibians” due to their ecological prominence.
Toward the late Carboniferous, a key evolutionary innovation emerged: the amniotic egg. This self-contained egg, with its protective shell and internal membranes, allowed vertebrates to reproduce entirely on land, freeing them from aquatic environments. The oldest known undisputed reptile, Hylonomus, appeared around 318 million years ago during this time. This small, lizard-like creature had scales and likely consumed small invertebrates. The development of the amniotic egg was a significant step, paving the way for the later diversification of reptiles, birds, and mammals.
Ancient Aquatic Dwellers
While terrestrial life flourished, the aquatic environments of the Carboniferous Period remained diverse and abundant. Oceans and vast inland seas supported a variety of fish and invertebrate groups. Early sharks became more prevalent, with new groups evolving during this time. An example is Stethacanthus, a shark-like cartilaginous fish known for its unusually shaped dorsal fin, which resembled an anvil or ironing board.
Ray-finned fish also diversified significantly during the Carboniferous. Lobe-finned fish, the lineage from which tetrapods evolved, continued to exist, though their dominance began to wane. Invertebrate marine life was abundant in the warm, shallow seas that frequently covered parts of the continents.
Filter feeders like crinoids, often called “sea lilies,” formed dense “gardens” on the seafloor. Brachiopods, shelled marine creatures that superficially resemble clams, were also common and diverse. Early ammonoids, a group of shelled cephalopods related to modern nautiluses, were present in these ancient waters.