High-Bandwidth Version
Search Evolution  
Click to return to the Evolution Home Page
darwin change extinction survival sex humans religion
Deep Time

Intro | Precambrian Eon | Paleozoic Era | Mesozoic Era | Cenozoic Era

Paleozoic Era: (543-248 mya)

Cambrian | Ordovician | Silurian | Devonian | Carboniferous | Permian



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.

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.


Read more

From water to land: The green invasion (480 mya)

Genetic mutations enable some green algae to survive life-threatening exposure to drying air and direct sunlight, as well as the dangers of cellular meltdown. Those plants with thicker cell walls, which keep their salty contents from mixing with fresh water, can tap the dissolved carbon dioxide they need for photosynthesis. A waxy outer coating stops wind and UV ray penetration that would normally damage the organism.

Ancestral land plants evolve shallow root systems that soak up available water around them. Male reproductive organs release swimming sperm that unite with egg cells in female reproductive organs. This produces small, unencapsulated spores, which develop in puddles or other moist spots. They do not contain much energy supply for the growth of the young, embryonic plant, however.

Green algae begin to occupy freshwater river, lake, and pond systems. These pioneers pave the way for all life that follows on land. Their successors will produce half of the world's oxygen and provide shelter and sustenance to insects and other animals.


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.


Did you know?

Many human characteristics, including vertebrae, bones, limbs, and jaws, first evolved in fishes.


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.

End Ordovician extinction


443 mya




Twenty-five percent of marine invertebrate families, approximately 57 percent of marine invertebrate genera go extinct


Glaciation, oxygen depletion


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.


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.


-> Go to the Silurian Period

Intro | Precambrian Eon | Paleozoic Era | Mesozoic Era | Cenozoic Era

  related web activities  
Origins of Humankind
See the humanlike species that came before us.
A Modern Mass Extinction?
Are we in the midst of one? And if so, did we trigger it?
Life's Grand Design
Are nature's complex forms evidence of "intelligent design?"
  related topics  
  Deep Time/History of Life  
  Evolution of Diversity  
  Evidence for Evolution  
Videos Web Activities Site Guide About the Project FAQ Glossary Site Map Feedback Help Shop