Appendicular Skeleton (Midterm 2)

What’s the appendicular Skelton

• Pectoral Gridle

• Pelvic Gridle

• Appendages

Rough Regions of Limbs (Arms/Legs)

Manus/Pes Region

Antebrachium/Crus Region

Brachium/Thigh Region

Bones of the appendages (Arms/Legs)

Phalanges

Metacarpals/Metatarsals

Carpals/Tarsals

Antebrachium/Crus bones

Brachium/Thigh Bones

The Physics of moving muscle against force

There is a:

• Effort (Muscle)

• Fulcrum (Joint)

• Load (the weight/force you are moving against)

To understand fulcrums, we should follow…

We should follow the joints and see where muscles attach

The way that humans’ legs have evolved (stronger anterior thigh muscles and posterior calf muscles) make us the best at what?

It makes humans one of the best runners (NOT the fastest, but we have the best endurance)

Fin Fold Hypothesis

The Fin Fold Hypothesis is an evolutionary theory that explains how the paired fins of early fish (which later evolved into limbs like arms and legs) developed from a continuous fold of skin

• The Jaw is likely gill arch, but Fins are more complicated

• Suggests they are modified from continuous fins

Fin Fold Hypothesis Vs Gill Arch Hypothesis

• The fin-fold theory suggests limbs evolved from longitudinal, segmented flank folds.

• The gill-arch theory proposes pectoral fins and girdles derived from ancestral gill support structures.

Fin Fold has the best support for…

Pelvic structures

Mixed evidence, maybe both (Fin-Fold Hypothesis and Gill-Arch Hypothesis) happened for…

Pectoral structures

Originally cartilaginous, then latter ossifying supporting structures provide…

Provide pulling Support on the smaller fin portions

pterygiophores

The bones or cartilages with which the base of the rays of the median fins articulate; the connecting points for the dorsal and anal fin rays

• Basal and Radial

Basal Pterygiophores

Basal pterygiophores are the proximal, often larger, elements articulating with the girdle

• Basals provide primary attachment

Radial Pterygiophores

radial pterygiophores (radials) are smaller, distal elements extending from the basals to support the fin rays

• radials allow for fin flexibility and movement

Basal and radial pterygiophores

Basal and radial pterygiophores are bones (or once were in sharks)

Lepidotrichia

In Osteichthyes (bony fish), dermal bone forms the fin rays

Ceratotrichia

Ceratotrichia cartilage (in Chondichthyes) form the fin rays

The three basal Pterygiophores are AKA

AKA Tribasal Bones

The three Basal Pterygiophores become…

three bones of the arm and leg

General Pathway for the Evolution of Limbs

• Modification of pterygiophores into wrist/hand bones

• Loss of fin rays

Pectoral Griddles in Mammals Vs Fish

• Fish: Pectoral Griddle is attached to the skull

• Mammals: Pectoral Griddle is free floating

Pelvic Griddles in Mammals Vs Fish

• Fish: Pelvic griddle is free floating

• Mammal: Pelvic griddle is attached to spine

Pectoral Griddle Vs. Pelvic Griddle

• The Pectoral Griddle (shoulder girdle) consists of the scapula and connects the arms to the axial skeleton

  • Includes the Scapula, Ribcage, and muscles

• The Pelvic Gridle connects the lower limbs to the axial skeleton

  • Includes the sacrum and pelvis

Osteichthyes Pectoral Gridle

Sacropterygian Fish Pectoral Gridles

Tetrapod Pectoral Gridles

The humerus becomes more prominent

The clavicle becomes dominant ventral attachment

The scapula/o-coracoid becomes dominant dorsal attachment

why is coracoid a dumb name?

Early Osteichthyes had

• Posttemporal

• Postcleithrum

• Supracleithrum

• Cleithrum

• Clavicle

• Interclavicle

• Scapulocoracoid

Fish Pectoral

• the shoulder is essentially the back of the head

• these are mostly dermal bones covering neurocranium

Amphibian Girdle

• Dermal bones like cleithrum disappear/shrink

• Endochondral bones like scapula, coracoid, humorous, ulna, radius becomes more important (still have procoracoid)

• Disarticulated from skull

• Larger emphasis on cartilage instead of endochondral bone

• Increases flexibility at the loss of strength

Reptilian Girdle

• mostly retain significantly larger robust bones and more ossification than amphibia

  • (Except snakes)

Reptilian Pectoral

• the addition of a dermal bone interclavicle provides more rigidity, not seen in amphibia

Avian Pectoral Girdle

• a long, thin scapula is typical of archosaurs

• this scapula change allowed greater dexterity, speed, and power

• later, exapted for flight stabilization

  • “priming the pump” for flighted bird’s existence

Keel on the Sternum

• They are in Aves

• gives ample surface area for pectoral muscle attachment

Furcula

• They are in Aves

• they are formed from fused clavicles, and elongated scapula, and a attached coracoid bone to enable flight

Why is it important to have a keel and furcula in Aves?

the muscle movements involved are extreme and the skeleton must be able to resist these movements

Avian Pectoral Girdle

• Reduction to 3 digits (“middle finger primacy”)

  • fusion of metacarpals

• a complete lack of dexterity = very stiff wing shape

• the pectoralis muscles serve to adduct the humerus for downward flap

• Weaker supracoracoideus muscles loop through the triosseal canal to abduct the humorous

• The muscles that raise the arms are NOT in the expected arrangement in aves

Flighted Pectoral Girdle

• Birds fly with their arms/wrists

• Bats fly with their fingers/shoulders

Avian Pelvic Girdle

• fused synsacrum keeps things lightweight

• relatively short femur becomes less important

• Tibiotarsus is fusion of tibia and proximal tarsal

• Tarsometatarsus is fusion of distal tarsals (ankle) and metatarsals

the monotremes retain basal traits like:

(Mammal Pectoral Girdle)

• the monotremes retain basal traits like

  • smooth scapula

  • interclavicle

  • coracoid

In therians

(Mammal Pectoral Girdle)

• the endochondral Sternum appears and takes the role the dermal interclavicle played

The loss of a coracoid means…

(Mammal Pectoral Girdle)

The loss of a coracoid means nothing is holding the scapula + arm in place except:

• Clavicle

• Muscle

• Hope

Different Mammal Pectoral Girdles

• Some are well developed

  • Ex) Monkeys

  • Allows for high mobility of forelimbs

• Some are reduced

  • Ex) Cats

  • Smaller clavicle, which results in a greater stride

• Some are completely gone

  • Ex) horses

  • Clavicle is absent, which allows for greater speed, better shock absorption, and smoother movement

most mammals don’t have…

the level of fusion seen in aves, but exceptions occur