Not sure?  Look it up! EEB/MCB 182: Spring 2007.
Lectures by W. M. Schaffer.

IX. Vertebrate Origins.

Recommended Reading: Chapter 34.

  1. Click to enlarge.
    Figure 1. Deuterostome phylogeny.
    Deuterostomes: Three Principal Groups.
    1. Echinodermata - pentamerally (5-way) symmetric marine forms including sea lilies (crinoids - mostly extinct), sea urchins, sand dollars, starfish, etc..
    2. Hemichordata - Pterobranch worms & acorn worms - the former traditionally placed in a separate group.
    3. Chordata.
      1. Urochrodates - tunicates, sea squirts.
      2. Cephalochordates - lancelets.
      3. Vertebrates - jawless fish, sharks and rays, bony fish, tetrapods.

  2. Exact relationship of echinoderms to other deuterostomes remains uncertain.
    1. Six-gilled "ancestral deuterostome" Figure 34.1 in your textbook may have been a primitive chordate.
    2. Here are some optional references:
      1. Original article published in Science;
      2. Critical rejoinder;
      3. Response to the response.
        4.

  3. Echinoderms.
    1. Internal skeletons composed of calcareous plates lying just below the skin and superficial musculature, and
    2. Water vascular systems composed of calcified, hydraulic canals leading to extensions called tube feet and which function in
      1. Gas exchange
      2. Locomotion
      3. Feeding
    3. See Chapter 34 (especially Figure 34.3 - required) for additional details.
    4. Figure 1 notwithstanding, pentameral symmetry is a almost certainly a secondary feature as evidenced by the fact that echinoderm larvae are bilaterally symmetric.
    5. Question: What are the consequences of primitive echinoderm bilateral symmetry to Figure 1?
      6.

  4. Hemichordates.
    1. Three-part body plan: protosome, collar (mesosome), trunk (metasome).
    2. Acorn worms.
      1. Protosome is a mucous-covered proboscis a burorrowing organ, that captures and directs food to the mouth, which is situated in the collar.
      2. Trunk contains numerous pharyngeal gill slits that facillitate gas exchange.
    3. Pterobranchs.
      1. Proboscis reduced.
      2. Collar bears tentacles.
      3. Gill slits reduced in number or enturely absent.
    4. Believed to be only distantly related to chordates.

  5. Click to enlarge.
    Click to enlarge.
    Figure 2. Archetypal vertebrate.
    Vertebrates. Archetypal vertebrate (Figure 2) was free-swimming and superficially fish-like.
    1. Motion was accomplished by alternate contraction of segmented, " > "-shaped" trunk muscles (the tip of the " > " points forward).
      1. Arranged in dorsal and ventral bundles called myomeres (myotomes).
      2. One myomere per vertebra and situated between them.
      3. Myomeres attach to sheets of connective tissue, called mycomata (myosepta) anchored to the vertebral column.
      4. Within the myocomata, develop the ribs.
    2. Support for the musculature is provided by the vertebral column (backbone).
      1. Through the vertebral centra runs a stiff rod, called the notochord which is the supporting structure in pre-vertebrate chordates
      2. Dorsal to the centra are the vertebral arches through which passes the spinal cord which is hollow.
      3. At the front of the animal,
        4. the spinal cord enlarges to form a hollow brain which, along with various sense organs, access the environment through openings in the skull.
    3. Anteriorly, the muscle mass is dorsal to and encases the visceral organs.
    4. The post-anal tail is comprised principally of muscles and support tissues.

  6. Most vertebrates have paired appendages (pectoral / pelvic) on either side of the body.
    1. First appear as fins in jawed fish (gnathostomes)
    2. In the line of fish leading to tetrapods, paired fins evolve into limbs.

  7. Primitive vertebrates were filter feeders.
    1. Water entered through the mouth and then passed through the gills where food particles were strained out.
    2. Water exited through gill slits on the animal's side near the head.

  8. The gills also function as respiratory organs.
    1. Deoxygenated blood is pumped anteriorly by the ventrally situated heart and then over the gills through vessels called aortic arches.
    2. The reoxygenated blood then flows posteriorly through the dorsal aorta from which it is distributed to the tissues.
    3. The gills and the blood vessels innervating them are supported by a series of cartilaginous and bony gill arches posterior to the mouth.
    4. Starting with the fish belonging to the class Placodermi that arises in the early Devonian, the first two pairs of gill arches are lost, while the third pair is modified to form jaws.

  9. Many steps in vertebrate evolution reflected by embryology.
    1. Examples:
      1. Incorporation of gill arches into the jaw;
      2. fusion of upper jaw with bones encasing the brain and sense organs to form the skull;
      3. conversion of elements of the reptilian lower jaw to ear ossicles in mammals.
    2. Structures start out as one thing and then "change course," and develop into something else.
      1. Thus, two of the mammlian middle ear bones begin life as pieces of the embryonic skull and jaw and then migrate to new positions
      2. Illustrates the proposition that development recapitulates embryonic states of ancestral forms - modern day version of Haeckel's "Ontogenetic Law."
      3. Results from the fact that mutations are less likely to be lethal (or mal-adaptive) if their action occurs later, rather than earlier, in development.
      4. Recall the "Rube Goldberg" character of evolutionary change.

  10. Click to enlarge.
    Figure 3. Amphioxus, a cephalochordate.
     
    Click to enlarge.
    Figure 4. Tunicate larva and adult.
     
    Click to enlarge.

    Figure 5. "Neotenic" theory of vertebrate origins.

    Vertebrate Origins. The vertebrate body plan is clearly anticipated by that of Cephalochordates of which the lancelet, Amphioxus (Figure 3), is a living example.
    1. Manifest segmented muscles, pharynx, gill slits.
    2. Lack skeleton, paired appendages.
    3. Gills used only for feeding.
    4. Despite fusiform shape, lancelets are sedentary. (For a picture of the animal in its burrow, go here.)
    5. Now known from the early Cambrian; To view a mid-Cambrian cephalochordate from the Burgess shale, click here.
    6. From such humble beginnings evolved
      1. All the fishes of the sea, rivers and lakes.
      2. Terrestrial vertebrates (tetrapods) including, amphibians, reptiles, birds and mammals.

  11. Chordate Origins Whence cometh the vertebrates and their chordate ancestors?
    1. One theory holds that Amphioxus-like chordates arose from the neotenic larvae of sessile urochordates such as living tunicates (Figure 4.)
    2. When the larva settles, the "swimming parts," which include a notochord and a dorsal nerve cord, degenerate and are resorbed.
    3. The neotenic theory of chordate origins imagines that the larvae became sexually competent (neoteny) with the consequent loss of the sessile adult phase.
      1. According to this, an important theme of chordate / early vertebrate evolution was progressive integration of the visceral and somatic parts of the body.
      2. These parts respecively derive from
        1. Gill basket
        2. Larval tail
    4. More ambitious schemes (Figure 5) trace the origin of invertebrate chordates and echinoderms back to a common, pterobranch-like ancestor which the 7th edition of your text restores in the hemichordates.

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