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.

The Devonian Period

An artist's impression of the Devonian landscape. Image credit:
Karen Carr

The Devonian period began 416 Ma and came to a close around 358 Ma. It is commonly referred to as the Age of Fishes, despite plants and insects taking great leaps forward in evolution. 

In the Devonian, Gondwana had begun its drift into lower latitudes, set on a collision course with Euramerica. By the end of the Permian these two continents will collide and form the supercontinent Pangaea. The Caledonian Orogeny was continuing in Euramerica, however, the mountains were being rapidly eroded, this caused vast deposition in shallow ocean basins. The climate was also warming up and so the planet was quite dry. 

Because of the continuation of the formation of shallow sea environments, extensive reef building could be found on the perimeter of the continents, the reefs were continuing to thrive from the Silurian Period. 

The Placoderms that first appeared in the Silurian grew to great lengths, up to 10 metres. This made them the top predators of the oceans. The most famous of which is

Skull of Dunkleosteus. Note the bony extensions at the
front of the mouth, these are not teeth. Image credit:
cmnh.org

Dunkleosteus, this fish did not have crushing teeth instead these were extensions of bone from the armour on the animal's head. The trilobites and brachiopods were joined by the coiled molluscs; these were the first ammonites. By the close of the period we also see the emergence of sharks and rays that diversified from cartilaginous fish. 

Fish also underwent massive evolutionary success. Here ray finned fish (Actinopterygii) and lobe finned fish (Sarcopterygii) had evolved. These fish had evolved true bones, teeth, swim bladders and gills. Actinopterygii have fins supported by thin bones whereas the Sarcopterygii fins were fleshy and had phalanges that were joined to an ulna and a radius on the pectoral fin and a fibula and tibia on the pelvic fins. These two bones where then joined to a humerus on the pectoral fin and a femur on the pelvic fin. You'll notice that these are the same bones that we have in our limbs. This is because Sarcopterygii fish are widely accepted as the common ancestor for all tetrapods. But despite being the more numerous in the Devonian, the Sarcopterygii widely died out, except for the Coelacanth and lungfish that still exist today. 

Plants had taken hold of the land in the Devonian. Ferns, lycophytes and horsetails had evolved from the primitive plants of the Silurian. Plants were evolving incredibly quickly, their size and lifestyle was changing completely. A good example of this is

Artist's impression of Archaeopteris. Note
that the leaves are not true leaves they are
in fact fern-like in appearance. Image credit:
go2add.com

Archaeopteris which grew to a massive 30 metres with a 3 foot diameter. From the fossils of this plant we can see that it shed its fern like branches, a change in lifestyle from the primitive Cooksonia of the Silurian. Archaeopteris was the first deciduous tree. The expansion of greenery across the landscape meant that Carbon dioxide levels fell and Oxygen soared, this was a key characteristic of the following period the Carboniferous. 

The earliest true insect appeared in the Devonian, Rhyniella praecusor was a flightless hexapod that evolved between 412 and 391 Ma. Tetrapods also began to crawl out of the water, the first tetrapods are more closely related to amphibians. Tiktaalik rosae is believed to be the link between the Sarcopterygii and the Tetrapods. This animal was mostly aquatic but had powerful hind limbs that were jointed to a fish-like pelvis. This enabled the animal to propel itself while out of the aquatic environment. It was also able to breathe air through nostrils, an adaptation not previously seen in animals. 

The Devonian period was one of the big five extinctions in geological history (I will cover the big five extinctions in a separate post tomorrow). It is hypothesised that this extinction was two prolonged events rather than a single instantaneous eradication of species. Firstly, the Keilwasser Event which took place in the late middle Devonian. This is where great amounts of corals, the jawless fish went extinct, whilst the number of trilobite species were dramatically reduced. The second event, the Hangeberg Event, took place on the Devonian-Carboniferous boundary. Here the Placoderms and many species of early ammonite were pushed to extinction. Despite 70% of invertebrate life going extinct, vertebrates and plants were relatively untouched by these two events. The extinctions are believed to have been caused by global cooling and the first forest fires caused by Carbon dioxide depletion.