The Ordovician radiation, or the great Ordovician biodiversification event (GOBE), was an evolutionary
radiation of animal life throughout the Ordovician period, 40 million years after the Cambrian
explosion, whereby the distinctive Cambrian fauna fizzled out to be replaced with a Paleozoic
fauna rich in suspension feeder and pelagic animals.It followed a series of Cambrian–Ordovician
extinction events, 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 ecospace, favoring
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, although the proposed Ordovician meteor event happened at 467.5±0.28
million years ago.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, trophic 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.
== Effects ==
If the Cambrian Explosion is thought of as producing the modern phyla, the GOBE can be
considered as the "filling out" of these phyla with the modern (and many extinct) classes
and lower-level taxa.
The GOBE is considered to be one of the most potent speciation events of the Phanerozoic
era increasing global diversity severalfold.Notable taxonomic diversity explosions during this
period include that of articulated brachiopods, gastropods and bivalves.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 began to have localized ranges, with different faunas at different parts of the
globe.
Communities in reefs and deeper water began 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 into 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 (which 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.
== See also ==
Cambrian explosion Evolutionary fauna
Mesozoic–Cenozoic Radiation