SKELETAL SYSTEM: APPENDICULAR SKELETON

I.	BASIC COMPONENTS

	A.	Paired fins:
		1.	Skeletal fin supports: (pterygiophores)
			a.	Basals - within proximal part of fin.
			b.	Radials - extend support from basals into middle of fin.
		2.	Fin rays:  from radials out to margins; keratinized rods, 							cartilages, or bones.

	B.	Limbs - forelimbs & hindlimbs built on same skeletal plan:
		1.	Stylopodium = upper limb, closest to body; single element.
		2.	Zeugopodium = middle region; two supportive elements.
		3.	Autopodium = distal ends of limbs; wrist, ankle, digits.
			a.	Forelimb: manus = wrist, palm, fingers.
			b.	Hindlimb: pes   = ankle, sole, toes

	C.	Associated girdles: pectoral & pelvic.

II.	ORIGIN OF PAIRED FINS:

	A.	Selection probably favored any body projection that would resist pitch, roll, or yaw.
	B.	Two basic fin types: different arrangements of metapterygial stem (axis)
		1.	Archipterygial fin = metapterygial stem down middle of fin; endoskeletal radials 
			project outwards.  eg. lungfishes
		2.	Metapterygial fin = metapterygial stem of basals is posterior; radials project 
			toward preaxial side of fin.  eg. Squalus

	B.	Gill-arch Hypothesis:  
		1.	Endoskeletal girdles arose from a gill arch and fin elements arose from gill rays.

		2.	Problems:
			a.	Does not explain appearance of pelvic girdle at distance from gill arches.
			b.	Presence of dermal bone in pectoral girdle.
			c.	Different embryologies of pectoral girdle and gill arches.

	C.	Fin-fold Hypothesis:
		1.	Paired fins arose within paired, continuous ventrolateral folds in body wall that 
			were stiffened by a transverse series of endoskeletal pterygiophores (basals and 
			radials).  Additional stability came from inward extension of basals and their 
			fusion at midline to form girdles.

		2.	Dermal bone added later to pectoral girdle, from overlying dermal armor.

		3.	Evidence:
			a.	Many fossils of early fishes possess hints or presumed remnants of fin 					folds.
				eg. some primitive ostracoderms; paired row of spines in Acanthodians.
			b.	Paired fins of embryonic sharks develop together from continuos thickening 
				of ectoderm along body wall.
			c.	Dermal fin rays of many living fishes develop from modified rows of scales 
				during embryogenesis.

	D.	Evolution of tetrapod limbs:  (discuss selective pressures)
		1.	Developmental pattern similar to fins:
			a.	Major elements arise on postaxial side of limb; asymmetrical.
			b.	Distal elements arise by subdivision and budding of postaxial primordia to 
				form autopodial elements.
			c.	Preaxial element gives rise to few wrist or ankle elements, no digits.

		2.	Limb assymmetry likely due to retention of postaxial metapterygium of fishes.

III.	TETRAPOD LIMBS

	A.	Significance of girdles:
		1.	Attach limbs to axial skeleton (directly or indirectly).
		2.	Pelvic - transmits force generated by hindlimbs to axial skeleton.
		3.	Pectoral - shock absorber/sling for front of body.

	B.	Pectoral girdles: modifications of basic pattern seen in early bony fishes:
		1.	Endochondral elements:
			a.	Coracoid - ossification centers may split to produce the following:
				1)	Procoracoid - homologous to coracoid of fishes & amphibians.
				2)	Posterior coracoid (may be called coracoid)
			b.	Scapula

		2.	Dermal elements:
			a.	Clavicle.
			b.	Interclavicle.
			c.	Cleithrum - remnant of fish girdle in primitive amphibians.

		3.	Depression (glenoid fossa) articulates with humerus.

		4.	Trends:
			a.	Fishes - reduction in endochondral bones.
			b.	Tetrapods - reduction or loss of dermal elements.

	C.	Pelvic girdles: endochondral bones only; no dermal elements.
		1.	Arose from pterygiophores supporting fins.

		2.	Fishes: pair of ischiopubic plates; not attached to vertebral column.

		3.	Tetrapods - composed of three endochondral bones: ilium, ischium, pubis.
			a.	Ilium: articulates with sacral vertebra(e).
			b.	These elements fuse into single innominate bone.
			c.	Deep socket (acetabulum) articulates with femur.

		4.	Posture and locomotion correlate with shape of pelvic girdle.

	D.	Limbs - generally pendactylous.
		1.	Early tetrapod specimens had >5 digits, therefore, the early vertebrate pattern 
			was polydactylous, but later stabilized at 5 digits.
		2.	May have been two lineages, one leading to amphibians (5 hind, 4 front) and the 
			other leading to amniotes (5 digits per limb).
		3.	Many verts. have <5 digits due to loss or fusion of phalanges.