evolutionary history of vertebrates

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  • Evolutionary History of Vertebrates

    Vertebrates originated about 525 million years ago during the Cambrian Explosion. The first

    vertebrates were the jawless fish. The jawless fish are thought to have relied on filter feeding to capture

    their food, and most likely would have sucked water and debris from the seafloor into their mouth,

    releasing water and waste out of their gills. The jawless fish that lived during the Ordovician period all

    went extinct by the end of the Devonian period. There are species of jawless fish like lampreys and

    hagfish today, but they are only distant cousins of the first jawless species of fish thought to be the first

    vertebrates. After the jawless fish came the armored fish, which developed paired fins. Next, the

    cartilaginous fish, also known as sharks, skates, and rays, have skeletons made of cartilage rather than

    bone and do not possess lungs. The last of the fish classes, the bony fish, includes many modern fish

    species that we see today.

    Amphibians were the first vertebrates to venture out onto land. Early amphibians retained many

    fish-like characteristics but during the Carboniferous period amphibians diversified. They retained close

    ties to water though, producing eggs that lacked a hard protective coating and requiring moist

    environments to keep their skin damp. Additionally, amphibians underwent larval phases that were

    entirely aquatic and only the adult animals were able to live in land habitats.

    The next class of vertebrates is class reptilia. Reptiles arose during the Carboniferous period

    shortly after amphibians and quickly took over as the dominant vertebrate on land. Reptiles did not

    need any kind aquatic habitat, separating them from the amphibians. Reptiles developed hard-shelled

    eggs that could be laid on dry land. They had dry skin made of scales that served as protection and

    helped retain moisture. Reptiles developed larger and more powerful legs than those of amphibians.

    The placement of the reptilian legs beneath the body enabled them greater mobility. All of these

    characteristics eliminated the need to live close to water like the amphibians.

    After reptiles came the class aves, or birds. Sometime during the early Jurassic, a group of

    reptiles gained the ability to fly and these reptiles gave rise to the birds. Birds developed a range of

    adaptations that enabled flight such as feathers, hollow bones, and warm-bloodedness. The last class of

    vertebrates, Mammalia, also developed from reptiles. Mammals developed a four-chambered heart,

    hair covering, and almost all do not lay eggs and instead give birth to live young. The exceptions that do

    lay eggs are the monotremes, an order of mammals that include the platypus and the spiny anteater. All

    monotremes are indigenous to either Australia or New Guinea.