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 rightangle 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