The Appendicular Skeleton

Chapter 9

Introduction

• Transitions from water to land and from land to air have allowed for a great diversity of forms within the appendicular skeleton.

• As with many other designs, form closely follows function.• In aquatic animals fins serve different purposes

– Some maneuver, some provide lift, and some are lost or modified • Not all birds use the air the same way, and some don’t fly at

all.• Tetrapods use land differently

– Some crawl, some run, some dig and some climb.

Basic Components

• The appendicular skeleton is composed of paired fins, or limbs, and their girdles.

• The anterior girdle is the pectoral girdle, or shoulder.

• The posterior girdle is the pelvic girdle or hip.

Fins

• Fins are membranous or webbed processes internally strengthened by radiating, thin fin rays.– They are formed from epidermis, but sink inward to the dermis.– In elasmobranchs rays are slender keratinized rods– Fins rays in bony fishes are usually ossified tiny elements that

strengthen the fin web.• The proximal part of the fin is supported by

pterygiophores of two types:– The enlarged basals, within the proximal fin– The slender radials that extend into the middle portion of the

fin.

• Fins occur singly, except for the paired pectoral and pelvic fins.

• The basal pterygiophores of these projecting fins articulate with and are braced with girdles

Limbs

• Limbs, or chiridium, are muscular appendages with well defined joints bearing digits.

• In tetrapods the fore and hind limbs are built on the same pattern, with three regions.– The distal autopodium consisting of the wrist and ankle

• The manus, hand, and pes, foot, are supported at this joint.

– The middle zeugopodium with two supporting elements• The radius and ulna, or tibia and fibula respectively.

– The proximal stylopodium with a single element• The humerus and femur.

Origin of Paired Fins

• The body of fishes are susceptible to deflections from their line of travel.– They may swing (yaw), rock on the long axis (roll),

or buck forward and back (pitch)• Dorsal and lateral fins control the body by

resisting deflections around the center of mass.• Pectoral fins do not produce significant lift,

instead they are used for maneuvering within tight spaces

• As early fishes became more active they would have experienced instability in motion.– This would favor any bony projection that resisted

pitch, roll, or yaw. And led to the evolution of the paired fins.

– The associated girdles stabilized the fins and provided sites for muscle attachment.

• In gnathostome fishes two type of fin developed from two different arrangements of the axis.– The archipterygial and metapterygial fins

Origin Theories

• Gill Arch theory:– Paired fins arose from gill arches– Specifically the endoskeletal girdle arose from the gill arch and the

archipterygial fin from the gill rays– Does not explain evolution of pelvic fins or girdle.

• Fin-Fold Theory:– Paired fins arose from a paired, continuous set of ventrolateral folds

in the body wall that were stiffened by endoskeletal pterygiophores,– Inward extensions of the basals and their fusion around the midline

produced the supporting girdles.– Dermal bone, from the bony armor, later added strength to these

fins.

Phylogeny

Agnathans

• Two early vertebrates from the Cambrian possessed lateral fin folds but lacked paired fins.

• Paired fins are also absent in modern hagfishes and lamprey.

• Ostracoderms had unpaired medial fins and rudimentary pectoral fins– Like sharks they lack a swim bladder and they would have

been denser that the surrounding water– Pectoral fins or spines, along with a flattened head shield,

would have provided some some lift as they swam.

Placoderms

• Both pectoral and pelvic girdles were present.• The pelvic girdle appears to have a single

endoskeletal element.• The more complex pectoral girdle consisted of

various fused dermal elements and braced the scapolucoracoid.

Chondrichthyans

• Early sharks possessed pectoral and pelvic fins that were primarily stabilizers.– They consisted of basal elements and tightly packed

radials.– The girdle was a single basal element.

• In later sharks the paired basal elements of the pectoral and pelvic girdles became extended across the midline to fuse into a U-shaped scapulacoracoid and puboishiac bar respectively.

Actinopterygians

• The pectoral girdle is partly endodermal, but mostly dermal.

• An air bladder, or lung is common.• Fins function mainly in close maneuvering,

adjustments of body position, or breaking.• The dermal shoulder girdle forms a U-shaped

collar of bone around the posterior gill chamber and braces the small scapulacoracoid.

Sarcopterygians

• Called lobed fin fishes in reference to the muscles and internal supportive elements that project from the body to form a fleshy dermal fin.

• Among living genera fins are considerable reduced.

• Ripidistans possessed pectoral and pelvic appendages that internally possess bones above the wrist/ankle that are homologous to those in modern tetrapods.

Tetrapods

• The first tetrapods retained or quickly changed the appendicular skeleton in correlation with locomotion.

• Appearing in the sarcopterygian fish Tiktaalik, and retained by tetrapods was the loss of the attachment of the pectoral girdle to the skull– A feature that allowed increased cranial movement.

• Girdles and limbs became stronger, more robust, and completely ossified.

Pectoral Girdle

• Tetrapods have a shoulder girdle that is structurally and functionally detached from the skull.

• Leaving a dermal shoulder girdle composed of the remaining ventral elements,– The cleithrum and clavicle, and an unpaired interclavicle

that joins both halves across the midline.• In modern amphibians the dermal bones are lost

(salamanders) or reduced (frogs)– The endoskeletal scapulocoracoid is the prominent

girdle element.

• In primitive amniotes, the clavicle and interclavicle persist, but the cleithrum is absent.

• In birds, the paired clavicle usually fuses with the interclavicle to form the furcula.

• A single, unossified scapulocoracoid is soon replaced by two articulated, but distinct elements.– Giving rise to a scapula (dorsally) and coracoid

(ventrally)

Pelvic Girdle

• From its first appearance the pelvic girdle is exclusively endoskeletal.

• In most fishes and early tetrapods it is formed of a single element.

• In modern tetrapods it has three bones:– The Illium, Ishium, and Pubis.

• The attachment of the illium to the vertebral column establishes, and defines the sacral region.

Manus and Pes

• The autopodium has undergone extensive modifications throughout evolution.– There are several digits, beginning with the

metacarpals, or metatarsals, followed by a chain of phalanges.

– The digits rest on a several bones called carpals or tarsals

• Its unusual to find animals with more than 5 (pentadactylous) digits– In many animals the opposite has occurred

• Although the expected pattern of the manus and pes give a starting point when looking at distal limb anatomy.

• The actual morphology is considerably modified by modified by fusions, elongations, eliminations, and additions of new elements.

• In the hindlimb, lateral digits are lost and medial metatarsals are fused into a composite ankle.

• Within amniotes, three ankle joints exist– Mesotarsal Joint is a simple hinge between, found in birds

and dinosaurs.– Intratarsal Joint, line of flexion passes between the calcaneum

and astralagus, found in crocodilians and thecodonts.– A Crurotarsal Joint forms between the shank and proximal

tarsals, found in mammals.

Onto Land

• The musculature of early rhipidistians was probably too weak to supply propulsion or bear the weight of the organism for long periods of time.– Well developed axial musculature allowed for

lateral undulations that propelled the body around pin-like fins.

• Not until the Permian did well developed, terrestrial tetrapods appear

Why Leave Water?

• Different hypothesis have been put forth:– One states that early rhipidistians crawled from pool to pool as

evaporation forced them from their home.• This assumes that the limb was already strong enough for overland

travel

– Another states that movement onto land was an attempt to avoid predation by young animals.• Movement onto land does not require long journeys, just a few steps,

so limbs not need be overly strong

– No one is sure what selection pressure actually led to colonization of land.• But, the fossil record and variation of modern vertebrates clearly

demonstrate its success.

Form and Function

• Changes in the skeletal system are the results of changing demands placed on the different parts of the body.

• On land the main contributors to locomotion are the limbs, not the tail.– Therefore limbs undergo extensive and significant morphological

change.• In addition, the shoulder and hip establish new associations

with the axial skeleton.– In tetrapods the axial column is slung from the shoulder girdle by

muscles and the hip attaches directly to column.• The shoulder moves on the thorax via these muscles which lessons the

wobbling of the head and neck.• The hip is firmly attached and applies strong propulsive forces to the axial

column.

Swimming• Swimming motion in the aquatic environment

is resisted by drag.• Streamlining prevents laminar flow , reduces

drag, and improves performance.• Lateral undulations that pass along the body

wall move the fish through the aquatic environment

• This same mode of transportation still serves most modern amphibians and reptiles.

Terrestrial Locomotion

• The pattern of foot contact is called a gait.• One cycle is complete after all 4 limbs have been

used.• One basic gait is a diagonal sequence, oppositely

opposed feet strike the ground in unison.– A trot is based on the diagonal sequence– The line between the diagonal points of contact pass

through the center of mass, improving support

• Another basic gait is the lateral sequence gait where feet on the same side of the body strike at the same time.– During cycles of locomotion, the center of mass

remains within the supportive configuration, never at the edge.

• This gait occurs in salamanders and reptilians.

• Unlike the gait produced by the fins of bottom walking fishes, terrestrial gaits include significant longitudinal of the stylopodium,

• This makes the limb more active in locomotion.– Contributing a pulling and pushing force against

the ground.– Rotation of the sylopodium can contribute to

locomotion only after the development of a right-angle elbow.

Limb Placement

• Early tetrapods have limbs placed laterally in a sprawled stance, establishes pivot points.

• Locomotion occurs through lateral undulations around this fixed point.

• In terrestrial birds, dinosaurs, and many mammals the trend has been toward cursorial (running) motion

• From the characteristic sprawled position of early tetrapods, modern tetrapods have limbs drawn under the body– A change in posture that increases efficiency of limb swing– This also restricts limb movement to a single, sagittal, plane.

• Early tetrapods with sprawled postures must use an overarm swing after each propulsive stroke to reestablish contact with the ground.– With legs positioned below the body, limb recovery can

be accomplished efficiently using pendulum motion.• In mammals a major shift in the functional

participation of the vertebral column appears.– This is a change from lateral to ventral flexion.– With limbs under the body, lateral motion contributes

little to locomotion.– Loss of ribs from the posterior trunk increases the

flexibility of this region in conjunction with the flexible vertebral column.

• Generally , as locomotion becomes used for more sustained, efficient, and rapid transport on land, many structures become modified.– The digits move forward and more in line with the

limb– Limbs were moved under the girdles that support

them– Vertical flexion of the vertebral column assists in

limb placement

Cursorial Locomotion

• Along with increasing the efficiency of limb motion, many tetrapods become specialized in rapid terrestrial motion.– Rapid locomotion is present in both predator and

prey• The speed attained by a vertebrate is

produced by its stride length and stride rate

• Stride Length:– Increased by increasing limb length; changes in foot

posture

– Increase the distance through which the limbs move while they are off the ground

• Stride Rate– Velocity of travel also depends on the rate at which the

limbs.• Larger, more efficient muscles increase rate of limb motion• Or, lightening of the fore limb reduces mass and allows the

limb to reposition quicker.

Gait

• The gait an animal selects depends on the rate of travel, obstructions in terrain, maneuverability, and body size.

END