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Comparative Anatomy. Evolution of the Skeletal System: Post-cranial Skeleton. Functional units of the post-cranial skeleton. Visceal skeleton Vertebral column Ribs Sternum. Girdles Paired appendages Unpaired appendages. Evolution of the Postcranial Skeleton. Postcranial Skeleton.

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comparative anatomy

Comparative Anatomy

Evolution of the Skeletal System:

Post-cranial Skeleton

evolution of the postcranial skeleton
Functional units of the post-cranial skeleton.

Visceal skeleton

Vertebral column




Paired appendages

Unpaired appendages

Evolution of the Postcranial Skeleton.
postcranial skeleton
Postcranial Skeleton
  • We need to know a little more about bone.
  • What sorts of forces operate on bony tissue?
    • Compression
    • Tension
    • Shear
    • Torsion
forces operating on bone
Forces operating on bone
  • Examples
    • Compression……. Graviportal limbs of elephants.
    • Shear……………..Greater trochanter of the femur.
    • Torsion…………... Vertebrae & Femur
    • Tension………….. Sternum
forces operating on bone6
Forces operating on bone
  • Bone is living tissue, and accommodates whatever forces are applied to it.
    • As an example, someone who loses a lot of weight quickly will still possess a robust skeleton designed to carry a lot of weight. However, with time the skeleton will reabsorb a considerable amount of tissue and become more gracile.
bone form and function
Bone Form and Function
  • Adaptation of bone form to function occurs at 2 levels:
    • internal bone morphology
    • external bone morphology
  • Two terms are important
    • Stress is a measure of force per unit area.
    • Strain is a measure of the change in shape of a physical body in reaction to an imposed stress.
stress and strain
Stress and Strain
  • Consider a sphere being compressed
    • the compressive stress results in the deformation by strain into an elipsoid.
    • The planes along which maximal compressive and tensile stress occur cross each other at right angles.
    • These same lines fall along the lines of maximal strain.
stress and strain10
Stress and Strain
  • The axes represent trajectories.
  • The horizontal axis (trajectory) shows maximal resistance to tensile deformation.
  • The vertical axis shows maximal resistance to compressive deformation.
forces acting on bone
Forces acting on bone.
  • We can look at cross-sections of bone and determine exactly what kinds of forces were applied to the bone.
    • Note - a bone is not solid in cross section.
    • “force lines” within the bone become ossified for increased strength. These trabeculae represent the trajectories.
  • Consider the compressive forces on the Femur.
stress and strain on a beam
Stress and Strain on a Beam
  • Imagine a beam projecting horizontally from a fixed position.
    • The top of the beam is subject to tensile stress while the bottom is subject to compressive stress.
    • What sorts of trajectories would resist compressive and tensile strain?
stress and strain16
Stress and Strain
  • Note that the trabeculae tend to cross one another at right angles. This is exactly how an engineer would design the bone.
  • Note also that this design permits a hollow center - ideal for pneumatic or hematopoietic functions.
patterns within the appendicular skeleton
Patterns within the appendicular skeleton.
  • The appendicular skeleton is the main system for propulsion and weight bearing. Even in fish, the appendicular skeleton provides propulsion to some degree.
  • Vertebrate limbs consist of a proximal embedded portion (girdle) and a distal free portion.
patterns within the appendicular skeleton18
Patterns within the appendicular skeleton.
  • Limb design is consistent:
    • a single proximal element: propodium
    • 2 intermediate long bones: epipodia
    • typically, 2 rows of small bones, articulating with a series of osseous rays.
  • Deviations from this pattern are generally considered adaptations.
patterns within the appendicular skeleton19
Patterns within the appendicular skeleton.
  • Fins in fish are superficial rayed skeletal structures, and are extremely variable.
    • Provide both protection and locomotion in a variety of ways.
    • Fin structure is related to locomotion.
    • Fins in addition to pelvics and pectorals are used.
  • Lateral undulation is still used.
  • Recall,bones serve 2 functions
    • protect and support soft tissue
    • provide a leverage system for locomotion.
  • As a leverage system, we can think of many joints in terms of the effort arm and load arm. Recall our consideration of the speed ratio and mechanical advantage.
  • A joint that provides a free range of motion is a diarthrosis. (shoulder etc.)
  • A joint with virtually no motion is a synarthrosis. (sutures in skull)
  • A joint with a limited range of motion is an amphiarthrosis. (pubic symphisis)

In an aquatic environment, the water acts as a skeleton. Terrestrial organism often have their mass arranged over only a few points of support.Compare and contrast the articulations of the 2 joints shown here.

changes resulting from terrestrialization
Changes resulting from terrestrialization.
  • What are some of the problems associated with a terrestrial life style?
    • Support
    • Stability
    • Locomotion
    • Respiration
    • Dessication.
  • Note: some of these same issues are faced by aquatic forms as well.
muscular system
Muscular System
  • Divided into 3 subunits
    • somatic
      • derived from myotome, innervated by somatic sensory and somatic motor nerves (= voluntary contral), striated muscle associated with the trunk and appendages
    • visceral
      • derived from hypomere, innervated by visceral sensory and visceral motor neurons (= involuntary control). May be smooth or striated. Includes muscles of digestive tube & throat)
muscular system26
Muscular System
  • Integumentary
    • smooth and striated muscle, intrinsic and extrinsic. Intrinsic muscles are in the dermis (erector pillae). Extrinsic muscles are between the dermis and trunk muscles. It forms a cutaneous sheet, the panniculus carnosus, and a craniocervical sheed, the platysma. The platysma is modified in mammals to from the facial muscles.
evolution of vertebrate locomotion
Evolution of Vertebrate Locomotion
  • There is a shift from axial locomotion to appendicular locomotion
    • (lateral undulation to limb based locomotion)
  • Even within fishes, there is significant evolution with regard to axial locomotion.
    • Consider the shapes of the myomeres. Can you speculate on why they are shaped as they are?

Note: among aquatic forms there is prob-ably a lot of convergence. Note also that the function of the trunk musculature changes from aquatic forms to terrestrial forms: from locomotion to support.

the axial skeleton
The axial skeleton
  • Compare and contrast the axial skeletons of aquatic and terrestrial vertebrates.
  • What differs in the forces applied to them?
  • How do they differ structurally?
Note the regionalization in the vertebral column of the tetrapod. Note the structure of the fish vertebrae.
axial skeleton
Axial Skeleton
  • Terrestrialization has resulted in
    • regionalization of the vertebral column. This is a consequence of the fact that the animal is now supported at only 2 points rather than at all points.
    • Power for locomotion is provided primarily at 1 point rather than at all points.
    • Animal mass is suspended - compare herps and mammals.
axial skeleton35
Axial Skeleton
  • Consider the evolution of the vertebrae.
  • Terrestrialization requires functional changes.
  • Levels of activity are reflected in the structure of the vertebral column.
Notice the difference between the development of the centrum in modern amphibians and that in more derived tetrapods.
Regardless of the means, the end result is consistent. Note: the anuran skeleton is highly derived (apo-morphic)
  • Trend in pectoral girdle has been one of simplification, and divorce from the cranium.
  • In early vertebrates it was involved in sound conduction.
  • Note the robustness and complexity of the anuran pectoral girdle.
pectoral girdle
Pectoral Girdle
  • Evolution of the pectoral girdle involves considerable modification of form and function.
  • Compare and contrast the form of the girdle in Bufo and Felis.
pelvic girdle
Pelvic Girdle
  • How about the pelvic girdle?
    • What is the primary function of the girdle?
    • How does this function change from frogs and salamanders to mammals?
    • What about the structure of the girdle in elephants and dinosaurs.
limb posture
Limb posture
  • Orientation of limbs has changed from fish to mammals.
    • Anterior limb requires posterior rotation and pronation of epipodial elements.
    • Posterior limb requires only anteior rotation.