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Ordovician Period (490-443 mya)
Life responds quickly following the Cambrian extinction. In fact,
species exhibit far greater diversity in the Ordovician than in the "explosive"
preceding period. Among the new animal groups to appear are the reef-building
stromatoporoids, bryozoans, corals, and stalked
crinoids. While few fishes appear in the fossil record from this time,
there is fossil evidence that jawless and heavily armored fishes swim the seas.
Later in the period, the first sharks evolve.
In the plant kingdom, pioneering green algae adapt to open-air
living on land. Their thick cell walls prevent them from drying out, which helps
ensure their survival. Catastrophe marks this period's end. Glaciation occurring
over the giant southern landmass, Gondwana, sends chills through ocean communities,
and at least 70 percent of all marine species die out.
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480 mya: Land plants
Green algae, which probably wash ashore by the tides, are the
first creatures other than bacteria to successfully adapt to life on land. Fossils
of spores, which are used in plant reproduction, suggest this happens at least 480
mya. While plants face many challenges adapting to land, they escape their ocean
predators.
480 mya: Fishes
Fishes, which are simply defined as all vertebrates except
those with legs, are members of the chordate phylum. As such, they display
certain characteristic features: a skeletal rod called a notochord, a dorsal
nerve, gills, and a tail. Agnathans, or jawless fishes, are the earliest fishes
and the first true vertebrates. They are bottom-feeders, covered almost entirely
in armor plates. As jaws evolve in the bony fishes and early sharks, jawless
fishes have trouble competing. Hagfishes and lampreys are the only jawless fishes
alive today.
450 mya: Sharks
Sharks, with their cousins the skates and rays, belong to a
group of fishes whose skeletons are made of cartilage, not bone. While sharks are
not plentiful until the Devonian period and later, fossil scales date the earliest
sharks to the late Ordovician. Sharks today are "living fossils"; they have changed
somewhat in appearance over the years, but their predatory ways have not.
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End Ordovician extinction
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Date:
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443 mya
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Intensity:
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2
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Affected:
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Twenty-five percent of
marine invertebrate families, approximately 57 percent of marine invertebrate
genera go extinct
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Hypotheses:
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Glaciation, oxygen depletion
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Summary:
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Over two pulses spaced a
million years apart, highly diverse marine groups die out. As the great
landmass Gondwana passes over the South Pole and is covered by glaciers,
water temperature cools, sea levels drop, and shallow-water communities
suffer most. Then, as glaciers recede, oxygen levels in marine waters sharply
decline. The killing extends to one-third of all brachiopods
and bryozoans, and to numerous groups of trilobites;
graptolites, which resemble saw blades; and conodont
animals, which were soft-bodied creatures that left small, toothlike fossils.
All told, at least 100 marine invertebrate families perish.
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Read more
Springing forward, falling back (443 mya)
Biodiversity increases over time as new species evolve with
slight modifications to ancestral body plans. As with a sprawling tree or bush,
this growth continues until something comes along to stop it. While extinction
halts certain evolutionary lines in their tracks, it also offers opportunities
for new ones to evolve. Without extinction, we might not have the variety of
organisms we see today -- turtles, birds, worms, spiders.
The late Ordovician extinction is indeed a devastating event
for marine animals. But as with the Cambrian extinction, enough species survive
to repopulate Earth. The recovery period is by no means instantaneous, though.
Recovery from mass extinction events typically occurs over 10 million years or
more. But the tree of life grows again, supporting new life forms as well as
older ones.
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