Axial Skeleton

Notochord

  • serves as a skeleton but is not made of bone or cartilage

  • serves as a stiffening/structural feature in chordates

construction

  • cells with large vacuoles, surrounded by a layer of collagenous tissue and a layer of connective tissue

  • the two sheaths are functionally the same

  • acts as a hydrostatic skeleton, medicated by vacuole pressure

  • longitudinally rigid (resists compression) and latitudinally flexible (allows for bending along length)

  • Biomechanics

    • moves via alternation muscle contractions

    • functions similarly to swimming flippers in movement

    • side to side movement allows for forward propulsion

  • along with nerves and muscles, it can create (lateral) undulating power strokes

  • Manifestations

    • Urochrodata

      • lose notochord as adults

      • present in plankton

    • Chephalochordata

      • notochord continues beyond the nerve cord at the front of the body (beyond oral opening)

Craniate (origins)

  • Conodont

    • fossils of small teeth from the Cambrian → found worldwide

      • could be used to map (stratographic fossils)

    • Belonged to organisms with small eyes→ Fossil found in the 1990s

    • maybe/possibly craniates

  • “Agnatha” & “Ostracoderms”

    • earliest unambiguous fossils of Craniata date from the late Cambrian to the early Ordovician

    • Ostracoderms

      • have armored-plated bodies (scale-like) with large head shields

        • often had ornamentation on bony plating

      • Tail-like body

        • had notochord and nerve cord

      • Head-like structure

        • nose→ sensing food

        • eyes→ seeing predators/food

        • ears→ allows for course corrections during movement (functioned as a balancing structure)

        • All of this indicated movement for longer distances in open water columns

      • brain

        • allowed for sensory processing

        • developed via the nerve cord getting larger at one end

      • likely moved to their food

      • mobile, impatient, active

        • tendency to be macro-phagus or exhibit predatory behavior

      • larger than tunicates and cephalochordates (Amphioxus)

      • have an optimal/efficient tempo for movement

    • Cyclostomes→ “cylindrical mouths”

      • remaining agnathans

        • lamprey → parasitic

          • has neural arches

            • blocks of cartilage added on top of notochord

            • provide extra anchors for musculature

            • FIRST LEGITIMATE VERTEBRATES

        • hagfish

      • have a notochord and dorsal nerve cord

      • move via ungulatory motion

  • Notochord → hydrostatic stiffening rod

    • allows bounce back after bending

    • resists compression, permits flexion

    • facilitates aquatic locomotion through side-to-side undulation

    • Urochordates: notochord only in larval (motile) stages, in tail only

    • Cephalochordates: notochord persists throughout life & spans the entire body length

    • Craniates: notochord persists throughout life in primitive forms, but only posterior to the midbrain

      • In more advanced forms, the notochord is progressively reduced and replaced by a vertebral column

    • Agnatha (jawless fishes)

      • Hagfishes (Myxini): notochord persists unrestricted

      • “Ostracoderms” only trace impressions of vertebral arch elements; presumably, notochord persisted, largely unrestricted

      • Lampreys (Petromyzontida): notochord persists unrestricted. Arcualia in first few postcranial segments

    • Placoderms

      • Bony vertebral arch elements: neural & hemal arches. In between, the notochord was presumably unrestricted (cartilaginous central possible but not fossilized)