Climate change as a selection pressure: Ordovician - Silurian extinction
The Ordovician–Silurian extinction event was the second largest of the five major extinctions of marine life, behind the Permian extinction. At the time, all known life was confined to the seas and oceans. More than 60% of marine invertebrates died out including two-thirds of all Brachiopod and Bryozoan families. Brachiopods, Bivalves, Echinoderms, Bryozoans and corals were particularly affected.
Ordovician life and climate pre-extinction -
An important evolutionary change in the Ordovician was the evolution of deep-water faunas. Many forms of life moved out of their early shallow-water environments to colonise deeper water and in doing so created new habitats and niches that supported new groups of animals. One of the most important among these emerging new groups were the Graptolites. Graptolites are hemichordates, relatives of modern acorn worms and Pterobranchs, which are distant relatives of all chordates and vertebrates. Another important biostratigraphic group of this time were the Conodonts, which also first appeared during the Ordovician. Conodonts were an enigma until very recently as most Conodont fossils found consisted only of their jaw, the only hard part in their body. The Ordovician environment was generally made up of shallow, warm epi-continental seas, making it favourable for marine life with sea levels high throughout most of this time. Ultimately the Ordovician ended in a brief (300-500 ky), but severe, ice age. Gondwana, particularly Africa, straddled the South Pole and became extensively glaciated and Metazoans were severely affected. About 60% of animal genera became extinct, making this the second most deadly mass extinction of the Phanerozoic.
Causes of extinction -
The movement of Gondwana into the South Polar Region was one of the main contributing factors which caused the mass extinction of this time. It led to global cooling, glaciation and consequent sea level fall. The falling sea level disrupted or eliminated habitats along the continental shelves. Evidence for the glaciation was found through deposits in the Sahara Desert. On the whole a combination of lowering of sea level and glacially-driven cooling were likely driving agents for the Ordovician mass extinction. Mass extinction on Earth at this time also could have been triggered by a star explosion called a gamma-ray burst. A ten-second burst would have stripped the Earth's atmosphere of half of its ozone almost immediately, exposing surface-dwelling organisms, including those responsible for planetary photosynthesis, to high levels of ultraviolet radiation. Volcanism and weathering were the final possible climatic causes for the extinction. Throughout the Late Ordovician out-gassing from major volcanism was balanced by heavy weathering of the uplifting Appalachian Mountains, which drew in CO2. In the Hirnantian stage the volcanism ceased, and the continued weathering caused a significant and rapid drop in CO2. This coincides with the rapid and short ice age.
Life and climate post extinction (early Silurian) -
During the Silurian, the Earth entered a long warm greenhouse phase. Latitudinal variations in climate were relatively similar to today, with glaciers occurring in the higher latitudes (over 65 degrees). Regions of marked aridity occurred within 40 degrees of the Silurian equator and warm shallow seas covered much of the equatorial land masses.
Following the Ordovician extinction event there was a rapid recovery of invertebrate faunas during the Silurian. The now high sea levels and warm shallow continental seas provided a hospitable environment for marine life of all kinds. The biota and ecological dynamics were essentially similar to that of the Ordovician, but more diverse.
Brachiopods were the most common hard-shelled organisms, making up 80% of the total species. Among these, Pentamerids first appeared and were abundant. Rhynchonellids, and the spire-bearing Athyridids and Atrypidids were also common, as were other groups that continued through from the Ordovician. Tropical reefs became a common feature in the shallow seas of this period, formed by tabulate and rugose corals, stromatoporoid organisms, Bryozoa and calcareous algae. Trilobites, cephalopods, gastropods, and echinoderms could also be found in this time. The Trilobites, having reached their acme in the Cambrian and Ordovician eras, began a steady decline in numbers during the Silurian. The Trinucleids and Asaphids were found to be completely absent during the Silurian, whilst Encrinites and Illaenids, despite surviving the extinction event did not survive the entirety of the Silurian. Planktonic graptolites however remained common and diverse. The single-spined Monograptus was the predominant genus, with its species creating useful zone fossils. Jawless fish began to invade salt and fresh water during this period, as did Eurypterids, Xiphosurids and scorpions, which may have been semi-aquatic. Rhyniophytes, primitive Lycophytes, and Myriapods became the first true land organisms. At the end of the period jawed fish appeared for the first time, but they remain insignificant in numbers throughout the rest of the Silurian.
Ordovician life and climate pre-extinction -
An important evolutionary change in the Ordovician was the evolution of deep-water faunas. Many forms of life moved out of their early shallow-water environments to colonise deeper water and in doing so created new habitats and niches that supported new groups of animals. One of the most important among these emerging new groups were the Graptolites. Graptolites are hemichordates, relatives of modern acorn worms and Pterobranchs, which are distant relatives of all chordates and vertebrates. Another important biostratigraphic group of this time were the Conodonts, which also first appeared during the Ordovician. Conodonts were an enigma until very recently as most Conodont fossils found consisted only of their jaw, the only hard part in their body. The Ordovician environment was generally made up of shallow, warm epi-continental seas, making it favourable for marine life with sea levels high throughout most of this time. Ultimately the Ordovician ended in a brief (300-500 ky), but severe, ice age. Gondwana, particularly Africa, straddled the South Pole and became extensively glaciated and Metazoans were severely affected. About 60% of animal genera became extinct, making this the second most deadly mass extinction of the Phanerozoic.
Causes of extinction -
The movement of Gondwana into the South Polar Region was one of the main contributing factors which caused the mass extinction of this time. It led to global cooling, glaciation and consequent sea level fall. The falling sea level disrupted or eliminated habitats along the continental shelves. Evidence for the glaciation was found through deposits in the Sahara Desert. On the whole a combination of lowering of sea level and glacially-driven cooling were likely driving agents for the Ordovician mass extinction. Mass extinction on Earth at this time also could have been triggered by a star explosion called a gamma-ray burst. A ten-second burst would have stripped the Earth's atmosphere of half of its ozone almost immediately, exposing surface-dwelling organisms, including those responsible for planetary photosynthesis, to high levels of ultraviolet radiation. Volcanism and weathering were the final possible climatic causes for the extinction. Throughout the Late Ordovician out-gassing from major volcanism was balanced by heavy weathering of the uplifting Appalachian Mountains, which drew in CO2. In the Hirnantian stage the volcanism ceased, and the continued weathering caused a significant and rapid drop in CO2. This coincides with the rapid and short ice age.
Life and climate post extinction (early Silurian) -
During the Silurian, the Earth entered a long warm greenhouse phase. Latitudinal variations in climate were relatively similar to today, with glaciers occurring in the higher latitudes (over 65 degrees). Regions of marked aridity occurred within 40 degrees of the Silurian equator and warm shallow seas covered much of the equatorial land masses.
Following the Ordovician extinction event there was a rapid recovery of invertebrate faunas during the Silurian. The now high sea levels and warm shallow continental seas provided a hospitable environment for marine life of all kinds. The biota and ecological dynamics were essentially similar to that of the Ordovician, but more diverse.
Brachiopods were the most common hard-shelled organisms, making up 80% of the total species. Among these, Pentamerids first appeared and were abundant. Rhynchonellids, and the spire-bearing Athyridids and Atrypidids were also common, as were other groups that continued through from the Ordovician. Tropical reefs became a common feature in the shallow seas of this period, formed by tabulate and rugose corals, stromatoporoid organisms, Bryozoa and calcareous algae. Trilobites, cephalopods, gastropods, and echinoderms could also be found in this time. The Trilobites, having reached their acme in the Cambrian and Ordovician eras, began a steady decline in numbers during the Silurian. The Trinucleids and Asaphids were found to be completely absent during the Silurian, whilst Encrinites and Illaenids, despite surviving the extinction event did not survive the entirety of the Silurian. Planktonic graptolites however remained common and diverse. The single-spined Monograptus was the predominant genus, with its species creating useful zone fossils. Jawless fish began to invade salt and fresh water during this period, as did Eurypterids, Xiphosurids and scorpions, which may have been semi-aquatic. Rhyniophytes, primitive Lycophytes, and Myriapods became the first true land organisms. At the end of the period jawed fish appeared for the first time, but they remain insignificant in numbers throughout the rest of the Silurian.
References:
http://www.nasa.gov/vision/universe/starsgalaxies/gammaray_extinction.html
http://archives.datapages.com/data/bulletns/1965-67/data/pg/0051/0010/1950/1979.htm
http://en.wikipedia.org/wiki/Ordovician%E2%80%93Silurian_extinction_event#cite_note-15
http://www.nasa.gov/vision/universe/starsgalaxies/gammaray_extinction.html
http://archives.datapages.com/data/bulletns/1965-67/data/pg/0051/0010/1950/1979.htm
http://en.wikipedia.org/wiki/Ordovician%E2%80%93Silurian_extinction_event#cite_note-15