The Carboniferous Period

The Carboniferous period is divided in two; the Mississippian and Pennsylvanian Carboniferous. Lasting between 358.9 to 298.9 Ma, the Carboniferous is a period of intense coal formation, the name Carboniferous comes from this fact. The Mississippian carboniferous' sedimentology is mostly composed of Carboniferous Limestone, whereas the Pennsylvanian is where we find most of the coal deposits. 

Artist's impression of a Carboniferous landscape. Image credit: Plant Evolution
and Palaeobotany

The sea level here remained low from the Devonian. The levels dropped again in the middle of the period, this caused major marine extinctions, mainly crinoids and ammonoids. Gondwana was still glaciated at this time, but it had little effect on the overall climate, this is evident from the swamplands that were present in the tropics. The formation of Pangaea caused more mountain building in the Appalachians region, this is known as the Alleghenian Orogeny. The Hercynian Orogeny was taking place in Europe at this time. The Eurasian continent came together to push up the Ural Mountains as the supercontinent formed. The Average temperature was twenty degrees Celsius but cooling in the Middle Carboniferous reduced this average to twelve degrees Celsius. The continued glaciations on Gondwana carried on into the Early Permian period. 

The Carboniferous is characterised by a peak in global oxygen content in the atmosphere. This made the atmosphere flammable, forest fires were therefore common. The expanse of forests and abundance of dead plant matter are responsible for the coal deposits. 

Lepidodendron trunk fossils. Image credit: tentree.com

The plant life was similar to that in the late Devonian, including horsetails, mosses and ferns. But there is the emergence of Lepidodendrales, or scale trees that grew to unseen proportions before this time. Cycads and Conifers also evolved in the Pennsylvanian Carboniferous. 

In the oceans there is the appearance of Foraminifers, a group of marine protists. Brachiopods and the Echinodermata had continued success in the oceans. Trilobites are becoming increasingly rare as time progresses. 

Terrestrial animals were evolving rapidly with the new climate. Giant insects are famous from this time period. The 2.6 metre long millipede like Arthropleura foraged on the forest floor whilst the giant dragonfly Meganeura, with it's 75 centimetre wingspan, took to the sky above. Arthropleura is the largest terrestrial invertebrate, and

Artist's impression of Arthropleura and Meganeura. Image
credit: Richard Bizley

Meganeura is the largest flying insect. The size of these animals is due to the moist and oxygen rich atmosphere. 

Amphibians were more diverse in the Carboniferous than they are today, but they could not cope with the change of environment after the Carboniferous Rainforest Collapse. 

Sharks were exploiting the ecological gap left by the extinction of the Placoderms. The radiation of sharks created some weird animals, Stethacanthus had a large flat topped dorsal fin. 

Reptiles make their earliest appearance in the Carboniferous, exploiting the gradual decline of the amphibians. This was alongside the emergence of the synapsids and diapsids. These animals were more advanced through the amniote egg which allowed for further exploitation of the land as these eggs are more protected than the soft amphibian eggs. The earliest reptile to evolve is Hylonomus from the Pennsylvanian.

There are two major events from the Carboniferous. Firstly, there is Romer's Gap. This is a gap in the fossil record from the first 15 million years of the period, it is unknown whether this was a lack of ideal fossilisation environments or an extinction event. Studies by Ward et al. (2006) shows that there was a drop in oxygen levels which indicates an ecological collapse. We do see that the Ichthyostegalian Labyrinthodont amphibians begin to decline early in the Carboniferous and give rise to the reptiliomorph amphibians. The Carboniferous Rainforest Collapse was due to climate change to arid conditions. Reptiles cope better with this environment than the water dependent amphibians as their scales lock in more moisture and the amniote egg does not need to be laid in water to keep moist.