Ordovician radiation
Encyclopedia
The Great Ordovician Biodiversification Event (GOBE or Ordovician radiation) was a diversification of animal life
throughout the Ordovician
period, just 40 million years after the Cambrian explosion
, whereby the distinctive Cambrian
fauna fizzled out to be replaced with a Palaeozoic fauna rich in suspension feeder
and pelagic animals.
It followed a series of extinction events
at the Cambrian-Ordovician boundary, and the resulting fauna went on to dominate the Palaeozoic relatively unchanged.
Marine diversity increased to levels typical of the Palaeozoic, and morphological disparity was similar to today's.
The diversity increase was neither global nor instantaneous; it happened at different times in different places. Consequently, there is unlikely to be a simple or straightforward explanation for the event; the interplay of many geological and ecological factors likely produced the diversification.
, favouring diversification. In addition, the changing geography led to a more diverse landscape, with more different and isolated environments; this no doubt facilitated the emergence of bioprovinciality, and speciation by isolation of populations. On the other hand, global cooling has also been offered as a cause of the radiation, and another alternative is that the breakup of an asteroid led to the Earth being consistently pummelled by meteorites.
The above triggers would have been amplified by ecological escalation, whereby any new species would co-evolve with others, creating new niches through niche partitioning, tropic layering, or by providing a new habitat. As with the Cambrian Explosion
, it is likely that environmental changes drove the diversification of plankton
, which permitted an increase in diversity and abundance of plankton-feeding lifeforms, including suspension feeders on the sea floor, and nektonic organisms in the water column. After the SPICE event
about 500 million years ago, the extinction in the ocean would have opened up new niches for photosynthetic plankton, who would absorb CO2 from the atmosphere and release large amount of oxygen. More oxygen and a more diversified photosynthetic plankton as the bottom of the food chain, would have affected the diversity of higher marine organisms and their ecosystems.
, the GOBE can be considered as the 'filling out' of these phyla with the modern (and many extinct) classes and lower-level taxa.
Taxonomic diversity increased manifold; the total number of marine orders doubled, and families tripled.
In addition to a diversification, the event also marked an increase in the complexity of both organisms and food webs
. Taxa begun to have localized ranges, with different faunas at different parts of the globe. Communities in reefs and deeper water begun to take on a character of their own, becoming more clearly distinct from other marine ecosystems. And as ecosystems became more diverse, with more species being squeezed in to the food web, a more complex tangle of ecological interactions resulted, promoting strategies such as ecological tiering.
The global fauna that emerged during the GOBE went on to be remarkably stable until the catastrophic end-Permian extinction and the ensuing Mesozoic Marine Revolution
.
The acritarch
record (the majority of acritarchs were probably marine algae) displays the Ordovician radiation beautifully; both diversity and disparity peaked in the middle Ordovician. The warm waters and high sea level (this had been rising steadily since the early Cambrian) permitted large numbers of phytoplankton
to prosper; the accompanying diversification of the phytoplankton may have caused an accompanying radiation of zooplankton
and suspension feeders.
The planktonic realm was invaded as never before, with several invertebrate lineages colonising the open waters and initiating new food chains at the end of the Cambrian into the early Ordovician.
Evolutionary radiation
An evolutionary radiation is an increase in taxonomic diversity or morphological disparity, due to adaptive change or the opening of ecospace. Radiations may affect one clade or many, and be rapid or gradual; where they are rapid, and driven by a single lineage's adaptation to their environment,...
throughout the Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
period, just 40 million years after the Cambrian explosion
Cambrian explosion
The Cambrian explosion or Cambrian radiation was the relatively rapid appearance, around , of most major phyla, as demonstrated in the fossil record, accompanied by major diversification of other organisms, including animals, phytoplankton, and calcimicrobes...
, whereby the distinctive Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
fauna fizzled out to be replaced with a Palaeozoic fauna rich in suspension feeder
Filter feeder
Filter feeders are animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feeding are clams, krill, sponges, baleen whales, and many fish and some sharks. Some birds,...
and pelagic animals.
It followed a series of extinction events
Cambrian-Ordovician extinction events
The Cambrian–Ordovician extinction event occurred approximately 488 million years ago . This early Phanerozoic Eon extinction event eliminated many brachiopods and conodonts, and severely reduced the number of trilobite species....
at the Cambrian-Ordovician boundary, and the resulting fauna went on to dominate the Palaeozoic relatively unchanged.
Marine diversity increased to levels typical of the Palaeozoic, and morphological disparity was similar to today's.
The diversity increase was neither global nor instantaneous; it happened at different times in different places. Consequently, there is unlikely to be a simple or straightforward explanation for the event; the interplay of many geological and ecological factors likely produced the diversification.
Causes
Possible causes include changes in palaeogeography or tectonic activity, as well as a modified nutrient supply. The dispersed positions of the continents, high level of tectonic/volcanic activity, warm climate, and high CO2 levels would have created a large, nutrient-rich ecospaceEcology
Ecology is the scientific study of the relations that living organisms have with respect to each other and their natural environment. Variables of interest to ecologists include the composition, distribution, amount , number, and changing states of organisms within and among ecosystems...
, favouring diversification. In addition, the changing geography led to a more diverse landscape, with more different and isolated environments; this no doubt facilitated the emergence of bioprovinciality, and speciation by isolation of populations. On the other hand, global cooling has also been offered as a cause of the radiation, and another alternative is that the breakup of an asteroid led to the Earth being consistently pummelled by meteorites.
The above triggers would have been amplified by ecological escalation, whereby any new species would co-evolve with others, creating new niches through niche partitioning, tropic layering, or by providing a new habitat. As with the Cambrian Explosion
Cambrian explosion
The Cambrian explosion or Cambrian radiation was the relatively rapid appearance, around , of most major phyla, as demonstrated in the fossil record, accompanied by major diversification of other organisms, including animals, phytoplankton, and calcimicrobes...
, it is likely that environmental changes drove the diversification of plankton
Plankton
Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...
, which permitted an increase in diversity and abundance of plankton-feeding lifeforms, including suspension feeders on the sea floor, and nektonic organisms in the water column. After the SPICE event
Steptoean positive carbon isotope excursion
The Steptoean Positive Carbon Isotope Excursion was a geological event which occurred about 500 million years ago at the end of the Cambrian era. The SPICE event was a sudden reversal of the anoxia that had steadily spread throughout the oceans during the Cambrian which also affected the...
about 500 million years ago, the extinction in the ocean would have opened up new niches for photosynthetic plankton, who would absorb CO2 from the atmosphere and release large amount of oxygen. More oxygen and a more diversified photosynthetic plankton as the bottom of the food chain, would have affected the diversity of higher marine organisms and their ecosystems.
Effects
If the Cambrian Explosion is thought of as producing the modern phylaPhyla
Phyla, the plural of phylum, may refer to:* Phylum, a biological taxon between Kingdom and Class* Phylum , in linguistics, a large division of possibly related languagesPhyla, as a singular, may refer to:...
, the GOBE can be considered as the 'filling out' of these phyla with the modern (and many extinct) classes and lower-level taxa.
Taxonomic diversity increased manifold; the total number of marine orders doubled, and families tripled.
In addition to a diversification, the event also marked an increase in the complexity of both organisms and food webs
Food chain
A food web depicts feeding connections in an ecological community. Ecologists can broadly lump all life forms into one of two categories called trophic levels: 1) the autotrophs, and 2) the heterotrophs...
. Taxa begun to have localized ranges, with different faunas at different parts of the globe. Communities in reefs and deeper water begun to take on a character of their own, becoming more clearly distinct from other marine ecosystems. And as ecosystems became more diverse, with more species being squeezed in to the food web, a more complex tangle of ecological interactions resulted, promoting strategies such as ecological tiering.
The global fauna that emerged during the GOBE went on to be remarkably stable until the catastrophic end-Permian extinction and the ensuing Mesozoic Marine Revolution
Mesozoic Marine Revolution
The Mesozoic marine revolution was a fundamental restructuring of marine ecosystems during the Mesozoic era, particularly in the Jurassic and Cretaceous, caused by increased predation pressure....
.
The acritarch
Acritarch
Acritarchs are small organic fossils, present from approximately to the present. Their diversity reflects major ecological events such as the appearance of predation and the Cambrian explosion.-Definition:In general, any small, non-acid soluble Acritarchs are small organic fossils, present from...
record (the majority of acritarchs were probably marine algae) displays the Ordovician radiation beautifully; both diversity and disparity peaked in the middle Ordovician. The warm waters and high sea level (this had been rising steadily since the early Cambrian) permitted large numbers of phytoplankton
Phytoplankton
Phytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν , meaning "plant", and πλαγκτός , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...
to prosper; the accompanying diversification of the phytoplankton may have caused an accompanying radiation of zooplankton
Zooplankton
Zooplankton are heterotrophic plankton. Plankton are organisms drifting in oceans, seas, and bodies of fresh water. The word "zooplankton" is derived from the Greek zoon , meaning "animal", and , meaning "wanderer" or "drifter"...
and suspension feeders.
The planktonic realm was invaded as never before, with several invertebrate lineages colonising the open waters and initiating new food chains at the end of the Cambrian into the early Ordovician.