chp. 7 the appendicular skeleton

pectoral girdle

• a skeletal structure that supports the anterior (front) limbs and lies posterior to the head.

• It first developed in Devonian fishes (pre-historic) and has been modified in later vertebrates.

Primitive Pectoral Girdle (Devonian Fishes) elements

Composed of 7 paired bones:

• Endochondral bones (endoskeleton):

  • Suprascapula (dorsal)

  • Scapula (middle)

  • Coracoid (ventral)

• Dermal bones (ancestral dermal armor):

  • Postemporal (dorsal)

  • Supracleithrum

  • Cleithrum

  • Clavicle (ventral)

which were devonian fish endochondral pectoral girdle elements?

• Suprascapula (dorsal)

• Scapula (middle)

• Coracoid (ventral)

which were devonian fish membranous bones pectoral girdle elements? (dermal bones)

• Postemporal (dorsal)

• Supracleithrum

• Cleithrum

• Clavicle (ventral)

shark pectoral girdle

• Cartilaginous fishes → cartilaginous pectoral girdle (no dermal bones)

• Made of: suprascapula, scapula, coracoid.

• Similar to the endochondral part of primitive fishes.

modern bony fish pectoral girdle (teleosts)

• Cleithrum = major bone.

• Clavicle and all other dermal bones are lost.

• Coracoid fused with scapula → forms scapulocoracoid.

early tetrapods pectoral girdle

• Pectoral girdle similar to primitive fishes but with key changes:

  • Lost: postemporal.

  • Gained: interclavicle (a singular midventral bone). MEMBRANE BONE

  • Early amphibians:

    • Dermal bones: Cleithrum, Clavicle, Interclavicle.

    • Endochondral bones: Scapula, Coracoid.

• Later tetrapods that went on land: supracleithrum also lost.

interclavicle of amphibians

• Present in amphibians, reptiles, birds, and monotreme mammals.

• Examples:

  • Crocodilians → unpaired interclavicle with procoracoids.

  • Birds → interclavicle forms part of the furculum (“wishbone”).

  • Reptiles + monotremes → retain interclavicles.

fate of clavicle/coracoid

Clavicle + Coracoid roles are linked (both brace scapula against sternum, but usually one is reduced/lost). VENTRAL

• Birds → strong clavicles (furculum), and retain coracoids.

• Most reptiles → only procoracoids, no clavicles.

Coracoid formation (tetrapods)

• Forms from the cartilaginous coracoid plate in lateral body wall.

• Procoracoid from anterior centers, coracoid from posterior.

• In eutherian mammals → both lost.

• the coracoid bone has become reduced to a projection on the scapula

  located over the Glenoid (the shoulder joint) called the Coracoid Process.

coracoid process

small hook-like projection on the scapula (shoulder blade) that sticks out near the shoulder joint.

• It serves as an attachment point for muscles and ligaments of the arm and chest.

👉 In mammals, it’s all that remains of the original coracoid bone.

glenoid

shoulder joint, connects shoulder to humerus (in scapula)

scapula in tetrapods

• Present in all tetrapods with forelimbs.

• Bears part/all of the glenoid.

• Suprascapula usually fused to scapula (except in salamanders and frogs).

pectoral girdle in mammals

Derived from therapsid reptiles (mammal-like reptiles)
- Monotremes still have the same pectoral girdle as did the therapsids.
- the pectoral girdle has been reduced to a scapula (with an acromion and coracoid process) and (typically) a clavicle.

monotremes

mammals that lay eggs

eutherian mammals

every mammal except marsupials and monotremes. have a placenta for babies

scapula in mammals

• Lies posterolateral to ribs.

• Underside: subscapular fossa (smooth to slide against ribs)

• Dorsal side: spine dividing supraspinous & infraspinous fossae. (serves as origin for some shoulder muscles)

• Projections near glenoid: acromion & coracoid process (for muscle/ligament attachment).

therapsid reptile pectoral girdle

"mammal-like reptiles," that lived during the Paleozoic eras and are significant because they are the direct ancestors of modern mammals.

• Endochondral: scapula (with acromion), coracoid, procoracoid.

• Dermal: clavicle, interclavicle.

Monotremes still have the same pectoral girdle as did the therapsids.

mammalian clavicle

Present/absent based on how a mammal uses its anterior limbs.

• Large clavicles → moles (digging), bats (flying).

• Reduced clavicle → cats (pectoral muscles form a muscular sling instead).

• Lost clavicle → ungulates (horses, cows) & cetaceans (whales, dolphins).

pelvic girdle

supports hind limbs

Pelvic girdle in fish

• is poorly developed.

• It usually consists of a pair of Pelvic Plates (cartilaginous or bony) that meet at the Pelvic Symphysis, forming a base for the pelvic fins.

• In cartilaginous fishes and lungfishes, the two pelvic plates fuse into one plate.

• The position of the pelvic girdle varies among teleosts:

  • Some have it just behind and attached to the pectoral girdle.

  • Others have it near the tail base.

• Fish and tetrapod pelvic girdles lack dermal bone!

pelvic girdle in tetrapod embryos

• Cartilaginous pelvic plates develop.

• Each plate has two ossification centers:

  • One forms the Pubis. (cranial)

  • One forms the Ischium. (caudal)

• Another cartilage mass (blastema) forms dorsally, giving rise to the Ilium.

• The ilium, ischium, and pubis grow together and meet at a joint cavity called the Acetabulum, which holds the head of the femur.

acetabulum

holds head of femur and articulates w pubis

• joint cavity where illium, ischium, and pubis grow together

Ilium–sacrum connection

• The ilium braces against the sacrum.

• The sacrum has stout transverse processes (remember sacral vertrebra is fused):

  • 1 pair in amphibians

  • 2 pairs in reptiles

  • 3+ pairs in birds and mammals

• Some tetrapods also have Sacral Ribs, short ribs fused (ankylosed) to sacral transverse processes.

symphysis (in tetrapods)

created btwn 2 pubis bones or 2 ischium bones

• Birds lack a symphysis (so they can lay eggs)

if the symphysis is between the two pubic bones it is the Pubic Symphysis. If the

symphysis is between the two ischia it is the Ischial Symphysis. (ex: ischipoubic bar in sharks)

force distribution in pelvic girdle

From the acetabulum, forces are directed:

• Dorsally against the sacrum/vertebral column.

• Ventrally against the symphysis.

Pelvis (amniotes)

• The sacrum and pelvic girdle join to form a bony enclosure called the Pelvis.

• Pelvic bones are usually fused, but this depends on locomotion.

  • Humans: rigid sacroiliac joint. (synarthrotic)

  • Frogs (anurans): flexible (diarthritic) sacroiliac joint for hopping.

• The pelvis surrounds the caudal end of the body cavity, forming the Pelvic Cavity, which contains urogenital organs and the end of the digestive tract.

pelvic girdle of urodeles (salamanders)

• Weak pelvic girdles, resembling fish.

• Braced against one sacral vertebra by a weak ilium.

• Terrestrial species develop a slender cartilage extending from the pelvic girdle into abdominal muscles, called Prepubic (Ypsiloid) Cartilage.

pelvic girdle of reptiles

• Some have Epipubic and Hypoischial Bones.

• These also appear in monotremes and marsupials.

  • Marsupials use the Epipubic Bone to support the pouch.

epipubic bones

slender bones that extend cranially from the pubic bones. They are found in monotremes, reptiles, and marsupial mammals.

• Marsupials have an epipubic bone to support the pouch.

pelvic girdle of birds

• Expanded ilium and ischium fuse into the synsacrum.

• Pubic bones (CAUDAL) are reduced to long splinters running parallel to the ischium.

• Birds lack a pubic symphysis, allowing space for laying large eggs.

• Ornithischian dinosaurs independently evolved a similar pelvic structure.

  • However, birds evolved from Saurischian (“lizard hipped”) dinosaurs so this is an example of convergence.

os coxa

Ilium, ischium, and pubis fuse completely into a single bone called the Os Coxa or Innominate.

fins

• Fins help fish:

  • prevent rolling,

  • control swimming angle,

  • steer,

  • brake/slow down,

  • and (in the caudal fin) provide thrust.

• Fins can be paired (pectoral, pelvic) or singular (dorsal, anal, caudal).

• Structure: two layers of skin supported by flexible rays radiating from a skeletal base.

fin rays

• Found in the dermis.

2 types:

• Bony fishes: rays = Lepidotrichia (jointed bony scales in series).

  • Distal end may have extra supports called Actinotrichia.

• Cartilaginous fishes: rays = Ceratotrichia (long, hollow rays, like shark spines).

  • May also contain actinotrichia.

  • In elasmobranchs, scales grow into fins for stiffness.

lepidotrichia

type of fin ray found in bony fishes

-consists of jointed bony scales from end to end

• May have actinotrichia to further reinforce fin

actinotrichia

part of fin rays: can be in lepidotrichia or ceratotrichia fin rays to strengthen fin

• made of actinodins

ceratotrichia

fin rays in cartilagenous fishes (chondricythes)

• They are long hollow rays similar to the dorsal spines found in some shark species

• may contain actinotrichia in distal portion to support fin

• In elasmobranchs, scales grow into fins for stiffness.

skeletal base of fins

• Made of Basalia (cartilaginous/bony) and distal Radialia.

• Together called Pterygiophores.

• In some bony fishes, paired fins lack basalia and have reduced radialia.

• Median and dorsal fin bases attach to the vertebral column.

skeletal base for paired or unpaired fins can be composed of either cartilage or bone

basalia

the pterygiophores that attach the fin to the body

• in skeletal base of fin

pterygiophores

one of the cartilaginous or bony elements (as basalia and radialia) by which rays of the fin of a fish are supported

• in skeletal base of fin

radialia

are distal to the basalia and attach to both the basalia and the fin rays

• in skeletal base of fin

paired fins

• Attachment:

  • Pectoral fins → articulate with the glenoid of the pectoral girdle.

  • Pelvic fins → braced against the pelvic plate.

• Types of paired fins (in living fishes):

  1. Lobed fins (Sarcopterygians):

     • Fleshy base with skeleton + muscles, plus distal rays.

     • Fin rays form a paddle.

  2. Fin folds (Chondrichthyes):

     • Broad base with three basalia: Propterygium, Mesopterygium, Metapterygium.

     • In males, basalia form Claspers for reproduction.

  3. Ray fins (Actinopterygians):

     • Teleosts have flexible ray fins with reduced basal skeleton.

• Extinct (4th) type: Spiny fins (Acanthodians).

lobed fins

• a type of paired fin found in Sarcopterygian (lobe-finned fish)

• consists of a fleshy proximal lobe containing the fin skeleton and associated muscles and a membranous distal portion stiffened by fin rays.

• The fin rays have a narrow base and form a paddle-like shape.

• The lobe fin gave rise to the tetrapod limb.

fin folds

• a type of paired fin found in Chondrichthyes (sharks)

• Broad base

• 3 basalia in pectoral fin: Propterygium, Mesopterygium, Metapterygium.

• 2 basalia in the pelvic fin: propterygium and metapterygium

• In males, basalia form Claspers for reproduction.

ray fins

• a type of paired fin found in actinopterygians (bony fish)

• in some teleosts the ray fins have lost skeletal elements to give more mobility.

  • lost components of basal skeleton and show great flexibility

median fins

• Types: Dorsal fins (1 or more) and Anal fins (if present).

• Structure similar to paired fins: proximal basalia, distal radialia, and rays.

• In some male teleosts, the anal fin is modified into a Gonopodium (intromittent organ, e.g., poecilids).

gonopodium

(from median fins) in male teleosts, the anal fin is modified into this

• anal fin in male live-bearing fish, such as guppies, mollies, and platies, used for internal fertilization by delivering sperm into the female's genital opening

caudal fins

classified based on shape and the relationship between the tail, notochord, and vertebral column.

3 types:
- Heterocercal
- Diphycercal
- Homocercal
- Hypocercal (extinct 4th type)

Heterocercal tail

• type of caudal fin

• Vertebral column/notochord extends into dorsal lobe.

Dorsal lobe is GREATER than ventral lobe (ex: sharks)

Diphycercal tail

• type of caudal fin

• Vertebral column barely enters dorsal lobe.

• Evolved from heterocercal tails

Dorsal and ventral lobes EQUAL in size. (Ex: lungfish, coelacanths)

homocercal tail

• type of caudal fin

• Notochord extends deeply into dorsal lobe, enclosed in caudal vertebrae forming a Urostyle.

• Also evolved from heterocercal tails.

Dorsal and ventral lobes EQUAL in size! (most teleosts, mackerel sharks)

hypocercal tail

• type of caudal fin (EXTINCT) Ex: Ichthyosaurs.

• Vertebral column/notochord extends into ventral lobe.

Dorsal lobe is SMALLER than ventral lobe (reverse of heterocercal tail)

tetrapods limbs

• A tetrapod limb has 3 main parts:

  • Propodium = upper arm (forelimb) or thigh (hindlimb).

  • Epipodium = lower arm (forelimb) or leg (hindlimb).

  • Autopodium = manus (hand) or pes (foot).

• Evolutionary trend: increased mobility through more joints.

• Propodium + epipodium are relatively consistent across tetrapods in skeletal lvl.

propodium

upper arm (forelimb) or thigh (hindlimb).

• humerus or femur

epipodium

lower arm (forelimb) or leg (hindlimb).

• radius/ulna or tibia/fibula

autopodium

manus (hand) or pes (foot).

forelimbs

• Propodium = Humerus (similar across tetrapods and early fishes).

• Epipodium =

  • Radius (preaxial, thumb side MEDIAL)

  • Ulna (postaxial, pinky side LATERAL).

radius and ulna r antiparallel

hindlimbs

• Propodium = Femur.

• Epipodium =

  • Tibia (preaxial, MEDIAL) weight-bearing

  • Fibula (postaxial, LATERAL).

    
    Variations:

    • Tibia + fibula may fuse = Tibiofibula (horses, frogs).

    • Fibula reduced (birds).

    • Fibula lost (deer).

    • Tibia + tarsus fused = Tibiotarsus (birds).

• Additional bone: Patella (sesamoid, found in reptiles and mammals).

distal aspect of limbs are most variable among tetrapods.

tibiofibula

fibula unites partially/completely to tibia (ex: horses, frogs)

tibiotarsus

tibia will fuse to tarses (in birds) why birds can’t drive

• fibula is v reduced in size in birds

manus (hand)

manus = autopodium

• 3 Components: Carpus (wrist), Metacarpus (palm), Phalanges (digits).

• Carpus (wrist):

  • Proximal row: Radiale (articulates with radius), Intermedium, Ulnare (articulates with ulna).

    • Extra bone: Pisiform (sesamoid; articulates with ulnare).

    • Human equivalents: Scaphoid (radiale), Lunate (intermedium), Triquetral (ulnare), Pisiform.

  • Middle row: 0–4 Centralia (early tetrapods had 3–4; reduced in reptiles; fused in humans).

  • Distal row: 5 carpals (digits 1–5); humans have 4: Trapezium, Trapezoid, Capitate, Hamate (4+5 fused).

• Metacarpus: 5 metacarpals in generalized hand.

• Phalanges: 2–3 phalanx bones per digit. Humans = 2-3-3-3-3 (pollex = 2).

pollex

2 phalanx bones (distal + proximal) instead of 3 in phalanges (thumb)

metacarpus

palm

• Metacarpals = long bones between carpus and phalanges.

• General tetrapod: 5 metacarpals, numbered 1–5 (thumb → pinky).

• Function: support digits, provide leverage for grasping or locomotion.

phalanges

• Each digit = a Phalange(fingers), made of smaller bones called Phalanx bones.

• General tetrapod: usually 2–3 phalanges per digit. (pollex has 2)

• Human phalangeal formula: 2-3-3-3-3 (thumb has 2 bones, fingers 2–5 have 3 each).

carpus

carpus = cluster of small bones forming the wrist joint.

In the generalized pentadactyl hand, there are three rows of carpal bones:

Proximal row (articulates with radius & ulna):

• Radiale → bone that articulates with the radius.

• Intermedium → middle proximal bone.

• Ulnare → bone that articulates with the ulna.

• Pisiform → sesamoid bone that articulates with the ulnare.

  Middle row:

• Contains Centralia (0–4 bones).

  • Early tetrapods: 3–4 centralia.

  • Reptiles: reduced to 2.

  • Humans: centralia fuses with the scaphoid during development.

Distal row (articulates with metacarpals):

• Typically 5 carpal bones, one for each digit (numbered 1–5).

• Humans have 4 bones here:

  • Trapezium (digit 1/thumb),

  • Trapezoid (digit 2),

  • Capitate (digit 3),

  • Hamate (digits 4 & 5 fused).

proximal row of carpus

(articulates with radius & ulna):

• Radiale → bone that articulates with the radius.

  • Human equivalent = Scaphoid.

• Intermedium → middle proximal bone.

  • Human equivalent = Lunate.

• Ulnare → bone that articulates with the ulna.

  • Human equivalent = Triquetral.

• Pisiform → sesamoid bone that articulates with the ulnare.

  • Present in many reptiles & mammals.

  • In humans, also called Pisiform.

medial row of carpus

• Contains Centralia (0–4 bones).

  • Early tetrapods: 3–4 centralia.

  • Reptiles: reduced to 2.

  • Humans: centralia fuses with the scaphoid during development.

distal row of carpus

(articulates with metacarpals):

• Typically 5 carpal bones, one for each digit (numbered 1–5).

• Humans have 4 bones here:

  • Trapezium (digit 1) thumb

  • Trapezoid (digit 2) index finger

  • Capitate (digit 3) middle finger

  • Hamate (digits 4 & 5 fused) ring and pinky finger

manus modifications for birds

has aerodynamic effect

• Carpals: 2 proximal + 3 fused distal = Carpometacarpus.

• Digits reduced to 3, most clawless.

• Special digit: Alula: the 1st digit (pollex), mobile and feather-bearing. Functions like a "thumb flap" to improve lift and prevent stalling at low speeds.

• manus plays in a role in braking, hovering, and steering.

carpometacarpus

The 3 distal carpal bones will fuse with the metacarpals to form the Carpometacarpus in birds.

(rigid unit that strengthens the wing).

alula

the first finger of a bird that became elongated, slender, and independently mobile

• allows for more precise flight and aids to prevent stalling in air.

manus modifications for bats (mammals)

• Digits: Retain 5 digits. Digits 2-5 are greatly elongated, forming the skeletal supports of the patagium (wing membrane).

• 1st digit (thumb): Free, clawed, used for climbing and grasping.

• Carpals: Several fused into a compound wrist unit for wing strength

manus main component of wing

patagium

wing membrane that assists an animal in obtaining lift when gliding or flying.

manus modifications for Pterosaurs

• Digits: Only 4 digits and 4 metacarpals in manus

  • Digits 1-3 = small, clawed (used for climbing/feeding).

  • Digit 4 = extremely elongated → supports the main wing membrane (patagium).

• Gave them a unique wing structure, different from birds and bats.

like bats, manus is main component of wing

• It is composed of a very elongated phalange and metacarpal embedded in a patagium

manus modifications for flight in scansoripterygid dinosaurs

• Digits 3–5 elongated, especially digit 5, supporting a membranous wing surface (patagium)

• more of bat-like wing