HISTORY OF VERTEBRATES

I. Subphylum Vertebrata

    A. Characteristics:
        1. All diagnostic chordate features.
        2. Vertebral column
            a. Vertebrae initially rode upon or surrounded persistent notochord.
            b. As role of vertebral column increased, role of notochord declined.
        3. Head (cranium)
            a. Sensory organs of head become more prominent.
            b. Anterior end of neural tube enlarged to form brain.
            c. Cranium = cartilage and/or bone that encases or partially encases brain.

    B. Origin of vertebrates:
        1. Cilia/suspension-feeding protochordate (similar to amphioxus):
            a. More muscular pharynx.
            b. Development of cartilage pharynx support bars.
            c. Better feeding allowed more active life style.
        2. Agnathans:
            a. Deposit/bottom-feeders w/ muscular, pump-like pharynx.
            b. Some may have used roughened structures in mouth opening to scrape food
                off of rocks (eg. possibly ostracoderms)
        3. Gnathostomes:
            a. Transitional species may have been raptorial feeders, plucking individual food
                particles from water or off of surfaces.
            b. Favored expansion of pharyngeal pump and mouth closure to prevent escape
                of captured food.
            c. Jaws powered by muscles vastly increased limits of prey size, allowed active
                predatory life style.

II. Vertebrate Classification

    A. Class Agnatha
        1. Order Myxini = hagfishes (eel-like scavengers)
        2. Order Petromyzontiformes = lampreys
        3. Ostracoderms - consists of several extinct orders.
            a. Bony plates form head shield.
            b. Single nasal opening.

    B. Class Acanthodii
        1. First jawed fishes in fossil record.
        2. Rows of spines along top and sides of body.
        3. Smaller, individual bony plates on head.

    C. Class Placodermi
        1. Prominent bony head shield (similar to ostracoderms).
        2. Jaws.
        3. Paired pectoral and pelvic fins.
        4. Notochord prominent, aided by ossified vertebral elements.

    D. Class Chondrichthyes
        1. Cartilaginous skeletons.
        2. Placoid scales (remnants of bone).
        3. No swim bladder; buoyancy provided by:
            a. large, oily liver.
            b. hydrofoil-like pectoral fins.
            c. heterocercal tail.
        4. Includes sharks, skates, rays.

    E. Class Osteicthyes
        1. At least partially ossified skeleton (may be reversed in some groups).
        2. Swim bladder.
        3. Operculum.
        4. Certain lobe-fined fishes thought to be ancestors of amphibians.

    F. Class Amphibia
        1. First tetrapods.
        2. Paired legs (may have been secondarily lost).
        3. At least partially ossified skeleton.
        4. Scales absent in present forms.
        5. Primitive lungs, but usually aquatic larval stages w/ gills.
        6. Labyrinthodonts = oldest known group, some scales, probable ancestors of stem
            reptiles.

    G. Class Reptilia
        1. Scales, dry skin.
        2. Well-ossified skeleton.
        3. Cleidoic eggs, amnion, internal fertilization.
        4. Extinct forms ancestral to birds and mammals.

H. Class Aves
    1. Feathers (allow flight in most species).
    2. Other modifications for flight:
        a. Hollow, light bones.
        b. Air sacs.
        c. Wings.
    3. Cleidoic eggs, amnion, internal fertilization.
    4. Homoiothermic.

I. Class Mammalia
    1. Mammary glands.
    2. Homoiothermic w/ hair for insulation (may be secondarily lost in some).
    3. Placentation (in most), amnion, internal fertilization.
    4. Heterodonty.
    5. Sweat and sebaceous glands.
    6. Anucleate red blood cells.

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