Comparative Anatomy

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 • 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 • Examples • Compression……. Graviportal limbs of elephants. • Shear……………..Greater trochanter of the femur. • Torsion…………... Vertebrae & Femur • Tension………….. Sternum

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 • 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 • 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 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. • 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 • 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 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. • 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 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 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.

Articulations • 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.

Articulations • 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. • 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 • 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 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 • 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?

Think about metamerism and overlapping myomeres.

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 • 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 • 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 Skeleton • Consider the evolution of the vertebrae. • Terrestrialization requires functional changes. • Levels of activity are reflected in the structure of the vertebral column.

Note the 2 major lineages of vertebral development.

Notice the difference between the development of the centrum in modern amphibians and that in more derived tetrapods.

There are 2 opposing hypothses for the origin of limbs.

Is the evolution of limbs somehow a combination of fin-fold and spine?

There is also some controversy concerning the development of specific elements.

Regardless of the means, the end result is consistent. Note: the anuran skeleton is highly derived (apo-morphic)

Girdles • 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.

CLA=clavicleCLE=cleithrumCO=coracoidIC=interclaviclePC=postcoracoidPT=posttemporalS=scapulaSC=supracleithrumSS=suprascapulaCLA=clavicleCLE=cleithrumCO=coracoidIC=interclaviclePC=postcoracoidPT=posttemporalS=scapulaSC=supracleithrumSS=suprascapula

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 • 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.

IM=IliumIS=IschiumP=Pubis

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.

Comparative Anatomy