The Carboniferous swamp rainforests of 359-299 million years ago were dominated by giant club mosses (lycopsids such as Sigillaria and Lepidodendron) or scale trees, along with tree ferns, seed ferns (Medullosa), giant horsetails (such as Calamites), and Cordaites (early relatives of conifers) with strap-shaped leathery leaves. These arborescent plants reached close to forty meters high and two meters wide. Just as with the alder tree of today (which creates silt-roots for oxygen intake and whose wood can withstand being under water without rotting), the ancient club moss tree roots were designed to float and were hollow.

The growth of the Carboniferous swamp forests gave off oxygen and removed huge amounts of carbon dioxide from the atmosphere, burying the plants under the swamps and the carbon along with them. Atmospheric carbon dioxide eventually dropped and atmospheric oxygen levels peaked around 35 percent, compared with 21 percent today.

High oxygen levels along with the humid warmth allowed plants and animals to reach sizes not known in today’s atmosphere and to further diversify. Insects, which breathe by the diffusion of air through their exoskeletons, rather than using lungs or gills, exploited the surplus oxygen to grow to immense sizes.

Some scientists suggest that aquatic insect larvae grew large to protect themselves from the overly high oxygen dissolved in the water from the atmosphere. Owlcation tells us that: aquatic larvae of today’s insects have little to no control over how much oxygen they absorb because they absorb it directly through their skin. “Ancient larvae likely functioned the same way as modern larvae and probably were not capable of controlling their oxygen intake well either. These ancient larvae may have grown large to avoid oxygen poisoning, as larger larvae would absorb lower amounts of oxygen relative to their body sizes.”

Gigantic insects thrived in the forest swamps of the Carboniferous. Three-metre long poisonous millipedes (Arthropleura armata) crawled among giant cockroaches and metre-long scorpions (Pulmonoscorpius kirkonensis). Mayflies were abundant and dragonflies (Meganeura monyi) grew to the size of seagulls. Insects were the only animals capable of flight during the Carboniferous Period; birds hadn’t yet evolved and reptile species were still primitive and land-bound. Lack of agile aerial predators allowed flying insects to grow large without being easy targets.

Many amphibians, such as Eryops and Proterogyrinus were predatory species that resembled modern-day crocodiles. Armed with vicious teeth, they reached lengths of almost six metres.

Scientists suggest that these coal forests contributed to their own extinction. Because they were such efficient CO₂ sinks—storing great quantities of atmospheric carbon dioxide in the soil—the Earth eventually cooled. With climate change, the hot, humid environment that supported these giant plants  gave way to the cooler drier environment of the Permian Period, which ushered in the reptiles, and led to what is called Carboniferous rainforest collapse. (The Permian Period also ended an entire age with the greatest mass extinction the Earth has ever experienced).

After millions of years of heat and pressure, the buried remains of giant plants of its ecosystem, were transformed into coal reserves. The anoxic conditions of the swampy environment and the lack of microorganisms to break down the plants (they hadn’t evolved yet!) prevented the plant material from decaying; instead, the dead plant material formed peat (as in our current bogs and fens), which held in the carbon they had taken in. Given the lack of a decay process, no carbon was released and instead this energy was locked in the coal forest for millions of years. Yielding to heat and pressure and sedimentation events from inundating seas, the forest peat biomass transformed to coal and now millions of years later this is being mined and released by humans.  

The industry is pumping those reserves of carbon into the atmosphere as CO₂ and methane. Some suggest that we are returning the Earth to a humid and tropical climate similar to the Carboniferous period.