Appendicular Skeleton

Consists of:

• Paired fins (or limbs)

• Girdles: braces that support the fins/limbs like hips

  • Pectoral girls (shoulder)- supports the pectoral fin or forelimb

  • Pelvic girdle (hip)- supports pelvic fin or hindlimb

Fins:

• Membranous or webbed processes, internally strengthened by thin, radiating fin rays

• Fin rays initially form at interface of dermis and epidermis (like scales) but then sink into the dermis. Dermal in origin

  • Elasmobranchs → dermal fin rays are slender and elastic, resembling keratin (ceratotrichia), hair-like structures extending down within the fin that gives it flexibility.

  • Bony fishes: series of tiny ossified or chondrified elements that strengthen the web (lepidotrichia), individual elements making up the rays, segmented

• The proximal part of the fin is supported by pterygiophores (proximal = attached to the rest of the body). Seen in both chondrichthyans and bony fishes

  • Enlarged basals (proximal), at the base of the fin

  • Slender radials (distal), towards the middle of the fin

  • The it leads into the fin rays

Fin types:

• Single:

  • Dorsal

  • Caudal

  • Anal

• Paired: phylogenetic precursors for arms and legs

  • Pectoral

  • pelvic

Limbs

• A limb (chiridium) is a muscular appendage that has well-defined joints with digits at the distal end

• Limbs of tetrapods share the same pattern recognized regions

  • Autopodium- distal end; ankle-wrist, manus/pes (instead of hand and feet), digits

    • Not hands because “hands” indicate grasping- can only be used in primates

    • Not feet because “feet” indicates standing- only bipedal

  • Zeugopodium: middle region; ulna and radius or tibia and fibula

  • Stylopodium: proximal region; humerus (forelimb) or femur (hindlimb)

• Glenoid fossa (cavity)- depression within the pectoral girdle that articulates with the humerus. Head of humerus connects with pectoral girdle (scapula)

• Acetabulum- deep socket in the pelvis that articulates with the femur. Nice, deep, round structure where the femur head connects

Basic components of the manus and pes (autopodium)

• The digits begin proximally with the metacarpals/tarsals and lead into the chain of phalanges

• Digits (phalanges) rest of the carpals (bones of the wrist) or tarsals (bones of the ankles)

• Bones of the manus include:

  • Several phalanges (3 or 2 in the thumb)

  • Five metacarpals

  • Carpals

    • Radiale (connect with the radius)

    • 3 centrales (in between the ulnare and radiale)

    • Ulnare (connect with the ulna)

• Bones of the pes include:

  • Cannon bone: formed by a fusion of metatarsal III and IV

  • Tarsometatarsus: in birds

  • Calcaneum: fibulare bone

  • Astragalus: tibiale bone (often fuses with intermedium)

Pectoral and Pelvic fins are the phylogenetic precursors to limbs

Origin of paired fins

• The streamlined body of a fish is susceptible to deviations from the line of travel

  • May swing side to side (yaw)

  • Rock about the long axis (roll)

  • Buck forward or backward (up and down) (pitch)

• Experiments demonstrate the role of fins in stabilizing body position

• Pelvic fins are stabilization

• Pectoral fins are locomotion

2 fundamental types of fish fins:

• 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) side of the fin evenly

• Metapterygial fin: the basals are located posteriorly and most radials project to the preaxial side of the fin

Gill-Arch Theory

• Proposed that paired fins and their girdles arose from the gill arches

• Says that girdle arose from the gill arch and the primitive archipterygial fin arose from the gil rays

• While this could explain, the evolution of the pectoral fins/girdle, it does not explain the appearance of the pelvic fins and girdle

• Doesn’t make sense spatially because the pelvic girdle is too far from the pharyngeal slits

Fin-Fold theory

• Proposed that paired fins arose within a paired, but continuous, ventrolateral fold in the body wall that became stiffened by the endoskeletal pterygiophores

• The basals extended inward and eventually fused to form the girdles

• Dermal bone was later added to the pectoral girdle

• This theory has more support because ancient lampreys and hagfish look like they have folded fins and embryonic studies

It is likely that metapterygial fins evolved first as they are common in gnathostomes

Archipterygial are found in sarcopterygian and likely evolved from modified metapterygial fins

• Turns into limbs

Shift into tetrapods

• Stylopodium appears first, leading to the humerus or femur

• The stylopodium branches out to the preaxial (radius or tibia) and postaxial (ulna or fibula), forming the zeugopodium

• The postaxial element subdivides to form the autopodium (manus or pes)

  • Preaxial element only contributes a few carpals/tarsals but none of the digits

Basic configuration is generally the same in the pectoral and pelvic appendages

Some modifications:

• Caspers: modifications to the pterygiophores of the pelvic fins in males; used in mating, (in sharks)

• In many teleosts, the pelvic girdle associated with the pelvic fins has moved forward to reside with the pectoral girdle

Phylogeny

• Chondrichthyes

  • Pectoral & Pelvic girdles

    • Modern sharks have 3 enlarged pterygiophores at the base of the pectoral fins (the basals: the metapterygium, mesopterygium, and propterygium)

    • Metapterygial stem of pelvic girdle consists of a postaxial series

    • Coracoid bar is the pectoral girdle, Does not articulate with the skull

    • Ischiopubic bar is the pelvic girdle, single solid element. Does not articulate with the axial skeleton. Associated with the body wall. Separate, free floating

      • Female shark because it doesn’t have the claspers

  • Fin rays are ceratotrichia

  • Actinopterygii have lepidotrichia rays

• Actinopterygii

  • Pectoral girdle

    • Forms a U-shaped collar of bone just behind the gill chamber.

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

    • Bone types:

      • 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. Different from chondrichthyes. Dorsal

      • Postcleithrum is not in all species

• Pelvic girdle

  • Has shifted forward to insert under pectoral fins

• Bony fish have metapterygial fin.

• Shift to sarcopterygii has archipterygial fin

• Sarcopterygii

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

  • While extant coelacanths and lungfishes, show skeletal specializations, their fins are significantly reduced

  • Pectoral girdle

    • Cleithrum

    • Clavicle

    • Postcleithrum (swapping supracleithrum for this)

    • Scapulocoracoid

  • Eusthenopteron (now extinct) had an additional bone (interclavicle) that was retained in the pectoral girdle of later tetrapods

  • The pelvic fin articulates with a single girdle bone. Associated with the body wall, looks like it's floating

• Tetrapods

  • 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, which provides ventral support. No direct connection to the skull because we have a neck now

    • Consists of clavicle, scapula, procoracoid (scapulocoracoid split into these 2), cleithrum (not all have this, and interclavicle (not all have this)

  • Forelimb

    • Pectoral girdle articulates 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 with frogs), and the manus

  • Pelvic girdle

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

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

      • Urostyle: additional articulation between the vertebral column and sacrum, providing support and helping to maintain posture. 

    • Join medially at the symphysis

    • Articulate with the femur in the hindlimb at the acetabulum

  • Hindlimb

    • Leg articulates with the pelvic girdle at the acetabulum

    • Bones include the femur, tibiofibula (fusion provides strength and is the shock absorber for the big jumping motions), and pes

    • Don’t really see a patella

• Reptiles + Birds

  • Pectoral Girldle

    • Generally consists of the scapula, procoracoid, coracoid, clavicle, and interclavicle. Cleithrum goes away as we shift to amniotes

      • Not present in snakes

    • Variation in the girdle

      • Crocodilians: lack clavicle but still have interclavicle (very small and near the sternum)

      • Turtles: clavicles and interclavicles are incorporated into the plastron (entoplastron)

      • Birds: clavicles fuse with the interclavicles to form the wishbone (furcula)

        • Have giant sternum called the keel

  • Forelimb

    • Consists of humerus, radius, ulna, and manus

    • In birds, the forelimb lengthened and went through a reduction and fusion of digits for flight. Radius, ulna, radiale, ulnare, fused metacarpals, 3 phalanges

  • Pelvic girdle

    • Have ilium, ischium, and pubis bones and they are now called os coxa if all 3 are there and they are all fused together

    • Pubis bones join at the pubic symphysis

      • This is not the case in birds. Nice open passage that allows for eggs to move through the birth canal

  • Hindlimb

    • Bones include the femur, patella, tibia, fibula, and pes.

    • Modified in birds because there is a fusion. 

      • Tarsals fuse with tibia to create tibiotarsus. 2nd layer of tarsals fused with the metatarsals to become the tarsometatarsus. They are walking on their toes

      • Still have femur, patella, fibula, and phalanges

• Mammals

  • Pectoral girdle

    • Clavicle reduced in size

    • Scapula becomes dominant bone

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

        • Monotremes still retained the interclavicle

  • Forelimb

    • Humerus, radius, ulna, manus

  • Pelvic Girdle

    • Os coxa

  • Hindlimb

    • Femur, tibia, fibula, pes, patella

  • Manus and Pes modifications

    • Ancestral state appears to be pentadactyly (5 digits)

    • Marine mammals exhibit polyphalangy (multiple bones per digit (phalanxes) instead of 3 or 2). Extending the hand to increase surface area and move themselves through the water

    • Ungulates have gone through the opposite trend. Fusion of phalanxes that reflects how they move

Form and Function

• Swimming

  • Streamlined form is favored. Promote lamellar flow, reducing turbulence and drag

    • Secondarily aquatics: loss of hindlimbs, modified postures. Seals and sealions

      • 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 

  • Lateral undulations used to produce a backthrust and propel the animal (not in secondarily aquatics)

• Terrestrial locomotion

  • Pattern of contact (footfalls) with substrate during locomotion is referred to as an animal’s gait

  • 2 basic gaits:

    • Diagonal sequence: diagonally opposite feet strike the ground in unison. Most common gait in tetrapods. Reptiles and salamanders. Opposite feet going into footfall. 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. Enhanced by erect posture

    • Speed is determined by:

      • Stride length: influenced by foot posture

        • ex) changing from plantigrade (entire foot on ground) to digitigrade (toes on ground) increases the stride length because it increases limb length.

          • Unguligrade is just tippy toes. Ungulates

      • 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

    • Favors:

      • Enlarged feet on the propulsive limbs

      • Center of mass shifted toward the rear of the body

      • large/robust hindlimbs

      • Long tail to act as a counterbalance (most times, bunnies are weird)

  • Scansorial locomotion: climbing in trees (type of arboreal)

    • Favors:

      • Claws coming off digits

      • Small body size

      • Prehensility: appendage that can wrap around something (porcupine tail or sloth claws)

      • Suspensory structures 

  • Brachiation locomotion: swinging in trees (type of arboreal)

    • 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 a prehensile tail (semi brachiators)

  • Fossorial locomotion: digging or burrowing

    • Favors

      • Large claws or incisor teeth: help them dig through the sediment

      • Short, broad forelimbs: like little shovels (paddles)

      • Forelimbs splayed to the side: sprawling posture again, not erect

      • Vibrissae (whiskers): can’t rely on eyes

      • Small eyes

      • Often lack pinnae (outer ear): so that dirt doesn’t get into it

  • Aerial locomotion

    • Gliding or Parachuting

      • Favors:

        • A patagium (flap of skin): bats, sugar gliders, flying squirrels, creepy flying snake

    • Flight

      • Favors:

        • Wings (with or without feathers) from modified forelimb

        • Streamlined body: enhances lamellar flow, less turbulence

        • Large sternum to accommodate flight muscles

        • Decreased mass: to help with lift and making the body more aerodynamic

        • Wing shape reflects type of flight