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19.4: History of Paleozoic Life

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    Okay, so you may not recognize the Precambrian. How about the Paleozoic?

    Probably not. You'd see things like these bizarre animals. The creature on the left is much like a large arthropod. The creature on the right had five eyes, and a long nose like a vacuum cleaner hose. These creatures were found as fossils in the Burgess shale.

    Paleozoic Era

    The first period of the Paleozoic Era was the Cambrian. By the beginning of the Paleozoic, organisms evolved shells. Shells could hold their soft tissues together. Shells could protect them from predators and from drying out. Some organisms evolved external skeletons. These are called exoskeletons. Organisms with hard parts also make good fossils. Fossils from the Cambrian are much more abundant than fossils from the Precambrian.

    The Cambrian Explosion

    The warm, humid climate of the early Cambrian was good for life. There was a tremendous increase in diversity. There was also an amazing increase in the number of living things. For these two reasons, this time period is called the Cambrian Explosion.

    With so much diversity, complex ecosystems could develop. All of this life was in the seas.

    The Paleozoic Continues

    Paleozoic seas were full of worms, snails, clams, trilobites, sponges, and brachiopods. Organisms with shells were common.

    The first fish were simple, armored, jawless fish. Fish have internal skeletons. Some, like sharks, skates, and rays, have skeletons of cartilage. More advanced fish have skeletons of bones. Fish evolved jaws and many other adaptations for ocean life. Pictured below is some of the diversity of Earth's oceans (Figure below).

    Mudskippers are fish that are able to walk short distances

    Mudskippers are fish that are able to walk short distances.

    Moving Onto Land

    An organism that lives in water is supported by the water. It does not need strong support structures. It also does not need to be protected against drying out. This is not true of land. Moving from the seas to land required many adaptations.

    Algae had covered moist land areas for millions of years. By about 450 million years ago, plants began to appear on land. Once there were land plants, animals had a source of food and shelter. To move to land, animals needed strong skeletons. They needed protection from drying out. They needed to be able to breathe air. Eventually they developed skeletons, lungs, and the other the adaptations they needed moved onto the land.

    Scale worms are an example of a fossil life from the Cambrian

    Halkieria, or scale worms, are an example of a fossil life from the Cambrian.

    One group of fish evolved into amphibians. Insects and spiders were already land dwellers by the time amphibians appeared.

    Mass Extinctions

    The eras of the Phanerozoic Eon are separated by mass extinctions. During these events, large numbers of organisms became extinct very rapidly. There have been several extinctions in the Phanerozoic. Two mass extinctions stand out more than the others. One took place at the end of the Paleozoic. The other took place at the end of the Mesozoic.

    Permian Extinction

    Between the Paleozoic Era and the Mesozoic Era was the largest mass extinction known. At the end of the Permian, nearly 95% of all marine species died off. In addition, 70% of land species became extinct. No one knows the cause of this extinction. Some scientists blame an asteroid impact. Other scientists think it was a gigantic volcanic eruption. It is possible that multiple things went wrong for life at the same time. The environment was changing, an asteroid impacted, and a giant volcanic eruption took place.


    • In the Cambrian there was an explosion of life. This explosion was in number and in diversity.
    • At the beginning of the Paleozoic, all life was in the seas. Later in the Paleozoic, life colonized the land.
    • To colonize land, animals need to be able to keep themselves together. They need to keep from drying out. They need to be able to breathe air.
    • At the end of the Permian was the largest known extinction in Earth history.


    1. What evolved during the Cambrian that was important for the evolution of life and what is its purpose?
    2. What does a sea creature need to have if it's going to colonize the land?
    3. What brought about the Permian extinction?

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