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.