Climate change as a selection pressure: Late Devonian extinction
Devonian life and climate pre-extinction -
The Devonian was a geological period that occurred during the Paleozoic era from around 359 to 416 million years ago. Primitive life in this period was in the process of colonising the land and before this time there was no organic accumulation in the soils, which were reddish in colour, as the landscape at this time was underdeveloped, probably only colonised by bacterial and algal mats.
By the start of the Devonian, however, early terrestrial vegetation had begun to spread. Evolutionary advances during this period included fish developing rudimentary “legs” for the first time, enabling them to move onto land as tetrapods creating the first amphibians. In addition to this, a wide variety of terrestrial arthropods established themselves on the Earth. The Devonian period also saw plants with seeds reproducing and forming large forests on land for the first time. Off land, primitive sharks increased in number and the first ray finned and lobe finned fish began to evolve. Ammonite molluscs were another species that appeared in this epoch, joining pre existing trilobites and coral reefs. Due to the prominent diversification of fish during this period it is sometimes called “Age of Fishes”, and life in the Devonian seas was dominated by Brachiopods. [1]
The most ferocious fish to arrive in this time were the Placoderms which had powerful jaws with blade-like plates that acted as teeth. The early Placoderms ate molluscs and other invertebrates, gradually becoming large vicious monsters that could slice easily through prey. Other types of bone-plated fish also began to evolve to have different forms of jaws within this time.
During the Devonian period the climate was relatively warm and dry with the two super-continents, Gondwana and Euramerica, located relatively close to each other in the Southern Hemisphere. These warm temperatures on Earth provided the ideal conditions for plants to thrive, allowing them to develop vascular tissues in order to transport water and food through roots and leaves more efficiently, meaning they could grow larger. The plants also developed the ability to produce seeds, meaning they had a new way to reproduce without water and so were therefore able to move inland. This then lead to the spreading of ferns and the first trees, which covered the land providing habitats and food for new and developing creatures. A vast ocean covered the rest of the globe, and the land was comparatively arid, and there was less of an equator-to-pole temperature gradient than today. There were no glaciers until the Late Devonian, when ice began to cover parts of the South Polar region.
Causes of extinction -
The causes of extinction are uncertain, but most theories point to a drop in carbon dioxide levels following the colonisation of plant life on the land. Carbon dioxide levels dropped rapidly throughout the Devonian Period due to chemical weathering, accelerated by the new plant life, which used up carbon dioxide in the atmosphere. And also through carbon storage, where the plants took in carbon dioxide from the atmosphere and converted it into organic carbon storing it in its stems, roots and leaves. As numbers of plants growing on the land rose, the carbon dioxide intake from plants increased resulting in global cooling over time and causing the formation of ice in the polar regions. [2]
Life and climate post extinction -
The new life on land escaped the worst effects of the mass extinction, which was the final chapter of the Devonian Period. The main victims were those living in the sea. Up to 70% of the marine life became extinct, with much of the reef building areas nearly disappearing completely. The major reef-builders: the stromatoporoids, the rugose and the tabulate corals survived the mass extinction, but remained as uncommon species until they evolved into the modern corals in the mesozoic period after around 150 million years. This interesting extinction of the coral and sea floor habitats is related to the widespread anoxia (lack of oxygen) present at this time within the oceans. This halted the development of these types of species. Whilst the sea life diminished, plant species that were flourishing on land after this time of extinction are thought to mirror those found in tropical climates today. [3]
The Devonian was a geological period that occurred during the Paleozoic era from around 359 to 416 million years ago. Primitive life in this period was in the process of colonising the land and before this time there was no organic accumulation in the soils, which were reddish in colour, as the landscape at this time was underdeveloped, probably only colonised by bacterial and algal mats.
By the start of the Devonian, however, early terrestrial vegetation had begun to spread. Evolutionary advances during this period included fish developing rudimentary “legs” for the first time, enabling them to move onto land as tetrapods creating the first amphibians. In addition to this, a wide variety of terrestrial arthropods established themselves on the Earth. The Devonian period also saw plants with seeds reproducing and forming large forests on land for the first time. Off land, primitive sharks increased in number and the first ray finned and lobe finned fish began to evolve. Ammonite molluscs were another species that appeared in this epoch, joining pre existing trilobites and coral reefs. Due to the prominent diversification of fish during this period it is sometimes called “Age of Fishes”, and life in the Devonian seas was dominated by Brachiopods. [1]
The most ferocious fish to arrive in this time were the Placoderms which had powerful jaws with blade-like plates that acted as teeth. The early Placoderms ate molluscs and other invertebrates, gradually becoming large vicious monsters that could slice easily through prey. Other types of bone-plated fish also began to evolve to have different forms of jaws within this time.
During the Devonian period the climate was relatively warm and dry with the two super-continents, Gondwana and Euramerica, located relatively close to each other in the Southern Hemisphere. These warm temperatures on Earth provided the ideal conditions for plants to thrive, allowing them to develop vascular tissues in order to transport water and food through roots and leaves more efficiently, meaning they could grow larger. The plants also developed the ability to produce seeds, meaning they had a new way to reproduce without water and so were therefore able to move inland. This then lead to the spreading of ferns and the first trees, which covered the land providing habitats and food for new and developing creatures. A vast ocean covered the rest of the globe, and the land was comparatively arid, and there was less of an equator-to-pole temperature gradient than today. There were no glaciers until the Late Devonian, when ice began to cover parts of the South Polar region.
Causes of extinction -
The causes of extinction are uncertain, but most theories point to a drop in carbon dioxide levels following the colonisation of plant life on the land. Carbon dioxide levels dropped rapidly throughout the Devonian Period due to chemical weathering, accelerated by the new plant life, which used up carbon dioxide in the atmosphere. And also through carbon storage, where the plants took in carbon dioxide from the atmosphere and converted it into organic carbon storing it in its stems, roots and leaves. As numbers of plants growing on the land rose, the carbon dioxide intake from plants increased resulting in global cooling over time and causing the formation of ice in the polar regions. [2]
Life and climate post extinction -
The new life on land escaped the worst effects of the mass extinction, which was the final chapter of the Devonian Period. The main victims were those living in the sea. Up to 70% of the marine life became extinct, with much of the reef building areas nearly disappearing completely. The major reef-builders: the stromatoporoids, the rugose and the tabulate corals survived the mass extinction, but remained as uncommon species until they evolved into the modern corals in the mesozoic period after around 150 million years. This interesting extinction of the coral and sea floor habitats is related to the widespread anoxia (lack of oxygen) present at this time within the oceans. This halted the development of these types of species. Whilst the sea life diminished, plant species that were flourishing on land after this time of extinction are thought to mirror those found in tropical climates today. [3]
Refrences:
[1] http://www.ucmp.berkeley.edu/devonian/devlife.html
[2] http://paleobiology.si.edu/geotime/main/htmlversion/devonian5.html
[3] http://www.devonianlife.com/
[1] http://www.ucmp.berkeley.edu/devonian/devlife.html
[2] http://paleobiology.si.edu/geotime/main/htmlversion/devonian5.html
[3] http://www.devonianlife.com/