Appendicular Skeleton

Appendicular Skeleton Consists of

Paired Fins (or limbs) and Girdles

Paired fins

Limbs or fins that are arranged in pairs, providing stability and locomotion in aquatic animals.

Girdles

Structures that support paired fins or limbs, such as the pectoral and pelvic girdles.

Pectoral girdle

The structure that supports the pectoral fin or forelimb

Pelvic girdle

The structure that supports the pelvic fin or hindlimb. Hips

Fins

membranous or webbed processes, internally strengthened by thin, radiating fin rays

Fin rays

Slender, segmented elements that strengthen the fins. They initially form at the interface of dermis and epidermis (like scales) but then sinks into the dermis. Dermal in origin

Ceratotrichia

dermal fin rays of elasmobranchs that are slender and elastic, resembling keratin. Hair-like structures extending down within the fin that gives it flexibility.

Lepidotrichia

series of tiny ossified or chondrified elements in bony fishes that strengthen the web. Individual elements making up the rays, segmented.

Pterygiophores

supports the proximal (attached to the body) part of the fin in both chondrichthyans and bony fishes. Leads into the fin rays

Basals

enlarged, proximal, pterygiophore at the base of the fin

Radials

slender, distal, pterygiophore towards the middle of the fin

Single fin types

Dorsal, caudal, anal

Paired fin types

pectoral and pelvic. phylogenetic precursors for arms and legs

chiridium

limb, muscular appendage that has well-defined joints with digits at the distal end

Limbs of tetrapods: Autopodium

The distal end of a limb, including the wrist/ankle, manus/pes (instead of hand and feet) and digits.

Why manus and pes instead of hands and feet

Hands can only technically be used for primates because indicate grasping. Feet can only technically be used for bipeds because they indicate standing

Limbs of tetrapods: Zeugopodium

The middle region of a limb, consisting of bones like the ulna and radius or tibia and fibula.

Limbs of tetrapods: Stylopodium

The proximal region of a limb, containing the humerus or femur.

Glenoid fossa (cavity)

depression in the pectoral girdle that articulates with the humerus. Head of humerus connects with pectoral girdle (scapula)

Acetabulum

A deep socket in the pelvic girdle that articulates with the femur. Nice deep round structure where the femur head connects

Basic components of manus and pes

the digits begin proximally with the metacarpals/metatarsals and lead into the chain of phalanges. They rest on the carpals/tarsals.

Bones of the manus

• several phalanges (3 or 2 in the thumb)

• 5 metacarpals

• carpals

Carpal bones

radiale (connect with the radius), 3 centrales (in between), and ulnare (connect with the ulna)

Bones of the pes

• cannon bone: formed by a fusion of metatarsal III and IV (in bovids and cervids)

• tarsometatarsus: fusion of lower tarsals and metatarsals (in birds)

• calcaneum: fibulare bone (connects with the fibula)

• astragalus: tibiale bone (often fuses with intermedium)

Phylogenetic precursors

Structures that evolved into modern limbs, such as pectoral and pelvic fins.

origin of paired fins: deviations from the line of travel

may swing side to side (yaw), rock about the long axis (roll), or buck forward or backward/up and down (pitch).

Pelvic fins were for stabilization and pectoral fins are for locomotion

Archipterygial fins

chain of basals (gray) run down the middle of the fin, radials (white) project outward to support the preaxial (anterior) and postaxial (posterior) sides of the evenly. Found in sarcopterygians. Turn into limbs

Metapterygial fin

the basals are located posteriorly and most radials project to the preaxial side of the fin. Likely evolved first because they are common in gnathostomes.

Gill-Arch Theory

A hypothesis suggesting that paired fins and girdles evolved from gill arches. Girdle arose from gill arch, archipterygial fin arose from gill rays. Does not explain pelvic fins and girdle because it doesn’t make sense spatially.

Fin-Fold Theory

A theory proposing that paired fins arose from a paired, continuous ventrolateral fold in the body wall that became stiffened by the endoskeletal pterygiophores. Basals extended inward and fused to form girdles and dermal bone was later added. Supported because ancient lampreys look they have folded fins and because of embryonic studies.

Shift to Tetrapods

Stylopodium appears first (humerus/femur). It branches out to the preaxial (radius/tibia) and postaxial (ulna/fibula) forming the zeugopodium. Postaxial subdivides into the autopodium (manus/pes), preaxial contributes to carpals/tarsals.

Modifications of pectoral and pelvic appendages

claspers: modifications to the pterygiophores of the pelvic fins in males, used in mating in sharks

pelvic girdle moved forward to reside with the pectoral girdle in teleosts

Chondrichthyes phylogeny

• Have 3 enlarged basal pterygiophores in pectoral fins called metapterygium, mesopterygium, and proterygium

• Metapterygial stem of the pelvic girdle consists of postaxial series

• coracoid bar (pectoral girdle) doesn’t articulate with the skull

• ischiopubic bar (pelvic girdle) is a single solid element and doesn’t articulate with the axial skeleton. Free floating, associated with the body wall

• fin rays called ceratotrichia

Actinopterygii phylogeny

• pectoral girdle forms a u-shaped collar of bone just behind the gill chamber and articulates with the skull

• scapulocoracoid is the connection point with the fin, acts as a support

• pelvic girdle has shifted forward under the pectoral fins

• have lepidotrichia fin rays

• have metapterygial fins

Bone types of the pectoral girdle in Actinopterygii (bony fishes)

cleithrum, clavicle, supracleithrum, postcleithrum (not in all species), and scapulocoracoid

cleithrum

largest element of the girdle, articulates with scapulocoracoid

clavicle

meets with the cleithrum and the other clavicle (at the symphysis)

supracleithrum

articulates with the cleithrum and posttemporal (skull) bone. Dorsal

Sarcopterygii phylogeny

• early lobe-finned fishes contain fin elements that are similar in structure to limbs of early tetrapods

• extant coelacanths and lungfishes fins are reduced compared to ancestors

• pectoral girdle contains cleithrum, clavicle, postcleithrum (swapped out supracleithrum), and scapulocoracoid

• pelvic fin articulates with a single girdle bone, assiciated with the body wall, free-floating

• extinct species had interclavicle that was retained in the pectoral girdle of later tetrapods

Tetrapods phylogeny

early tetrapods quickly acquired modifications associated with locomotion on land. Robust bones to withstand gravity

Amphibians pectoral girdle

• now firmly attached to the axial skeleton, provides ventral support. No direct connection to the skull because we now have a neck

• consists of clavicle, scapula, procoracoid (scapulocoracoid split), cleithrum and interclavicle (not all have these)

Amphibian forelimb

• pectoral girdle aticulates with the forelimb at the glenoid cavity

• Bones of the limb are humerus (upper arm), radioulna (fusion of radius and ulna to support weight with jumping), and the manus

Amphibian pelvic girdle

• Salamanders have puboischia (fused pubis and ischium) and ilium

• Anurans have ilium, ischium, and urostyle (instead of pubis)

• joins medially at the symphysis

• articulates with the femur in the hindlimb at the acetabulum

Urostyle

additional articulation between the vertebral column and sacrum, providing support and helping to maintain posture

Amphibian hindlimb

• leg articulates with the pelvic girdle at the acetabulum

• bones include femur, tibiofibula (fusion to provide strength and absorb shock for the big jumping motions), and pes

• no patella

Reptiles + Birds pectoral girdle

generally consists of scapula, procoracoid, coracoid, clavicle, and interclavicle. (no cleithrum in amniotes)

Crocodilian variation of pectoral girdle

lacks clavicle but still has interclavicle (very small and near the sternum)

Snake variation of pectoral girdle

not present

Turtle variation of pectoral girdle

clavicles and interclavicles are incorporated into the plastron (entoplastron)

Bird variation of pectoral girdle

clavicles fuse with the interclavicles to form the wishbone (furcula).

side note: giant sternum is called the keel

Reptile forelimb

consists of humerus radius, ulna, and manus

Bird forelimb

forelimb lengthened and went through a reduction and fusion of digits for flight. Have radius, ulna, radiale, ulnare, fused metacarpals, 3 phalanges

os coxa

having a 3 ilium, ischium, and pubis bones fused together

Reptile + Bird pelvic girdle

• os coxa

• pubis bones join at the pubic symphysis

  • not in birds, they have a nice open passage for eggs to be able to move through the birth canal

Reptile hindlimb

bones include the femur, patella, tibia, fibula, and pes

Bird hindlimb

• tarsals fuse with tibia to create tibiotarsus

• 2nd layer of tarsals fused with the metatarsals to become tarsometatarsus

• Walk on their toes

• still have femur, patella, fibula, and phalanges

Mammals pectoral girdle

• clavicle reduced in size

• scapula becomes dominant

• the coracoid fuses to the scapula to become the coracoid process

• monotremes still have interclavicle

Mammal forelimb bones

humerus, radius, ulna, manus

mammal pelvic girdle

os coxa

mammal hindlimb bones

femur, tibia, fibula, pes, and patella

Manus and pes modifications

• ancestral state is pentadactyly (5 digits)

• marine mammals have polyphalangy (multiple bones per digit (phalanxes) instead of just 3 or 2) to extend the hand and increase surface area to move themselves through the water

• ungulates have fusion of phalanxes that reflects how they move

Swimming form and function

• streamlined form to promote lamellar flow, reduce turbulence and drag

• lateral undulations used to produce a back thrust and propel the animal

Secondarily aquatics

• loss of hindlimbs (whales and dolphins), modified postures in seals and sea lions where they hold the hindlimbs together so that it functions like a single caudal tail when they swim

• Aquatic birds evolved robust forelimb bones to create more surface area and better push themselves through the water

Gait

pattern of contact (footfalls) with substrate (land) during locomotion. 2 types: diagonal or lateral sequences

diagonal sequence

diagonally opposite feet strike the ground in unison. Most common gait in tetrapods. Reptiles and salamanders do this. Provides stability in sprawling posture

lateral sequence

feet of the same side strike the ground in unison. Common in mammals. Erect posture, evolved in running

Cursorial locomotion

Running

Cursorial locomotion speed is determined by

• stride length: influenced by foot posture (plantigrade or digitigrade).

  • digitigrade increases the stride length because it increases limb length

• stride rate: rate at which limbs are moved

  • short limbs increase stride rate but decreases stride length

  • lighter distal portions of legs will increase rate by decreasing mass

Saltatorial locomotion

Hopping

saltatorial locomotion favors

enlarged feet on the propulsive limbs, center of mass towards the rear, large/robust hindlimbs, long tail to act as counterbalance (except bunnies)

Scansorial locomotion

Climbing in trees (type of arboreal)

Scansorial locomotion favors

claws coming off digits, small body size, prehensility, suspensory structures

prehensility

appendage that can wrap around something. ex) porcupine tail or sloth claws

Brachiation locomotion

swinging in trees (type of arboreal)

Brachiation locomotion favors

broad thorax to accommodate lots of muscles for swinging and hanging, forelimbs longer than hindlimbs, mobile shoulder joint (ball and socket joint with robust range of motion), either no tail or prehensile tail (semi brachiators)

Fossorial locomotion

digging or burrowing

Fossorial locomotion favors

large claws or incisor teeth to help them dig through the sediment, short, broad forelimbs (like little shovels/paddles), forelimbs splayed to the side (sprawling posture), vibrissae (whiskers), small eyes, no pinnae (outer ear) so dirt doesn’t get into it

Aerial locomotion

Movement through the air, including gliding/parachuting and flying.

Gliding/Parachuting favors

a patagium (flap of skin)

ex) bats, sugar gliders, flying squirrels, scary flying snake

Flight favors

wings (with or without feathers) from modified forelimb, streamlined body (lamellar flow), large sternum to accommodate flight muscles, decreased mass (help with lift), wing shape reflects type of flight