The Devonian Period

Article by: Mia Wroe
Edited by: Lewis Haller, Adam Manning, J. D. Dixon, and Harry T. Jones

The Devonian Period is the fourth period of the Paleozoic Era, spanning from 419.2 million years ago (Ma) until 358.9 Ma. It was named after the county of Devon, in the southwest of England, where the first studies of Devonian rocks took place. The period is characterised by a number of adaptive radiations, both floral and faunal, as organisms colonised the land. 

Sometimes referred to as the ‘Age of the Fishes’, the Devonian was a period of extensive diversification beneath the waves as well as above them. The Early Devonian oceans were dominated by placoderm fish (with bony plates over the front end of their bodies). The Middle Devonian saw the first appearance of cartilaginous fish, the group including sharks and rays, and by the end of the period this group was fairly common. The Devonian also saw the divergence of bony fish into ray-finned fish and lobe-finned fish, distinguished from one another by the structure of their fins. Ray-finned fish have gone on to become the dominant group of fish in the world today, but during the Devonian they were outnumbered by the lobe-finned fish. Today, lobe-finned fish are represented by only two extant groups: lungfish and coelacanths.

Groenlandaspis, an extinct armoured, jawed fish from the Late Devonian. Artwork by Joschua Knüppe.

However, it was from the lobe-finned fish that the first terrestrial tetrapods (vertebrates with limbs rather than paired fins) evolved. The Devonian fossil record has produced a number of species that represent intermediates between the aquatic lobe-finned fish and the first tetrapods/amphibians capable of leaving the water. Perhaps the most famous of these, Tiktaalik roseae represents a species with features of both fish and tetrapods. For example, Tiktaalik had developed wrists that enabled it to hold its appendages in a range of positions; it was capable of doing push-ups. Being able to support its own body with its ‘fins’ on the bottom of shallow, freshwater systems would have been an advantage in navigating the environment in which it lived. The combination of fish-derived characteristics (body scales and lobed fins), coupled with a head, wrist and neck anatomy more comparable to modern tetrapods, made for a very strange animal indeed. 

Although the first vascular plants had colonised the land during the Silurian, it wasn’t until the Devonian that floral terrestrialisation really began to progress. But the plants of the Early Devonian were very different to the plants of today. Little more than a few centimetres in height, early terrestrial plants did not produce flowers, but rather mostly reproduced using spores. As more plants evolved methods of tolerating desiccation, they became less dependent on living by bodies of water and they spread inland. As plants colonised more land, they also provided the decaying organic matter needed to produce the first recognisable soils in a process known as pedogenesis.

Tiktaalik wandering across the land. Artwork by Rebecca Dart. Available at: https://twitter.com/r_dart/status/1017773801013063680.

As the Devonian progressed, so too did the complexity of the plant world. By the Late Devonian, whole forests of plants had evolved, populated with lycopsids, ferns, and progymnosperms. The increase in plant biomass had important impacts on atmospheric composition. The massive increase in photosynthesis levels that came with their expansion increased the concentration of oxygen, which had a vital impact in aiding the terrestrialisation of the tetrapods. However, the resulting reduced carbon dioxide levels may have led to global cooling, which is considered to have contributed to the Late Devonian Extinction – one of Earth’s five big mass extinctions.

The end of the Devonian is marked by a series of extinction events which began with the Late Devonian Extinction (or Kellwasser Event) and ended with the Hangenberg Event. The events occurred over a relatively long time period when compared with the other four biggest mass extinctions through time, so while it is believed that they are both related to climate change, the cause of this change is uncertain. Theories suggesting an asteroid impact of a smaller scale than that of the K-Pg boundary; a huge outpouring of magma in Russia; or the impact of plant terrestrialisation are all potential contributors. The extinctions at the end of the Devonian caused a loss of 50% diversity in vertebrate groups alone, forcing ecosystem restructuring into the Carboniferous. 

Image References
[1] Groenlandaspis, an extinct armoured, jawed fish from the Late Devonian. Artwork by Joschua Knüppe.
[2] Tiktaalik wandering across the land. Artwork by Rebecca Dart. Available at: https://twitter.com/r_dart/status/1017773801013063680.

Information References and Further Sources
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