Skeletal System
Updated 4 November 2004
A. Cartilage - gelatinous matrix (may be
impregnated with calcium salts or various fibers);
1. Relatively soft and deep
2. Primarily embryonic tissue in most vertebrates
3. Fate
a. remain
b. replaced by bone
c. reabsorbed
B. Bone - relatively hard, deep or surface
1. Endochondral (replacement) bone - ossification of a
cartilage precursor
2. Dermal bone - ossifies directly in dermis (no
cartilage precursor); evolutionary trend toward sinking into the body
(exoskeleton®endoskeleton)
a. fish - covers entire body (scales)
b. tetrapods - relatively restricted distribution in most
recent forms (e.g., gastralia, osteoderms,
some axial and appendicular bones); extensive in
armadillos, turtles
Cranial skeleton (3
ontogenetic and phylogenetic components); slide 1
(color-coded in text)
A. Chondrocranium (=neurocranium;
cartilaginous braincase); slide 2, slide 3
1. Important ossifications: sphenoids,
ethmoids (turbinals),
occipitals, sclerotic bones (slide 4)
B. Splanchnocranium (=visceral skeleton” - support gills); slide 5
(epibranchials, ceratobranchials)
1. Derived: jaws (palatoquadrate
& mandibular cartilages), hyomandibular
2. Phylogenetic derivatives; hyomandibular
becomes stapes, larynx, hyoid, mammalian ear bones; hyoid VIDEO
3. Jaw suspensions (slide 6)
C. Dermatocranium (enclose or replace chondrocranium
or visceral skeletal parts); slide 7
1. patterns (slide 8
(color-coded in text); composite skull (slide 9)
1. Chondro- poorly known (ostracoderms)
or open (cyclostomes - trough like); similar to embryonic structure
2. Splanchno- support gills, filter feeding
3. Dermato- heavy, no pattern (ostracoderms);
none (cyclostomes)]
B. Placodermi (slide 10)
1. Chondro- poorly known
2. Splanchno- gills, jaws (palatoquadrate-mandibular
joint)
3. Dermato- large heavy plates, no pattern
C. Chondrichthyes (slide 11)
1. Chondro- complete, solid, calcified (DEMO)
2. Splanchno- jaws (palatoquadrate-mandibular
joint); gills (spiracle); jaw protrusion shark (slide 12);
VIDEO
3. Dermato- none
D. Osteichthyes
1. Chondro - actinop/sarcop:
well ossified, most kinetic (movement between upper jaws and chondrocranium - feeding advantage) (slide 13)
a. crossopterygian cranial kinesis -
characteristic ossification in 2 distinct units (similar to tetrapods)
2. Splanchno- quadrate/articular
jaw joint (original jaws regress, replaced by dermal bone), hyomandibular,
gills
3. Dermato- actinop: numerous,
patterns but homologies uncertain; crossopterygean:
pattern for tetrapods)
a. "new" dermal jaws (slide 14)
4. Teleost jaw protrusion - liberation of bony elements
(slide
15)
E. Tetrapods
1. Chondrocranium well ossified (portions)
2. Jaws firmly attached to braincase (stapes),
hyoid, larynx
3. Gills in amphibian larvae only
4. Pectoral girdle loses attachment with skull
5. Many have some form of cranial kinesis (akinetic in lissamphibians,
turtles, crocodilians, mammals); READ - reptilian cranial kinesis
(diversity)
6. Phylogenetic trends
a. greater ossification (secondarily reduced in lissamphibians)
b. reduction in # of bones (slide 16)
c. chondrocranium & dermatocranium
join as one functional unit (slide 17,
slide
18)
d. development of secondary palate (slide 19)
e. development of dentary/squamosal
jaw joint and additional auditory ossicles in mammals
(slide
20 (Diarthrognathus
- double articulation)
1. Components (slide 21)
a. general: centrum,
arches, processes
b. arches - neural (dorsal), hemal
(ventral)
c. processes - spinous,
transverse, hemal, zygapophyses
(torsion-tetrapods; + additional zygosphene/zygantrum
[snakes])
d. centrum (constitutes the body); high variation in
structure due to extent of ossification and extent of notochord replacement
2. Relative emphasis on the two ossifications of
each centrum (pleurocentrum,
intercentrum); slide 22
a. aspidospondly (separate); emphasize one or the other (or
equal emphasis) in different groups of labyrinthodonts
b. holospondly (fused, mostly derived from pleurocentrum) amniotes
3. Articulatory surfaces (slide 23);
high variation in centrum is phylogenetically
useful only in a very general way (due to adaptation, convergence); little
pattern
4. Regional differentiation (slide 24)
a. fish: trunk, caudal (caudal fin supported by
neural and hemal arches (slide 25)
b. most tetrapods:
trunk (ant-cervical, post-sacral); caudal (fracture planes; autotomy
video)
c. greater regional differentiation in
crocodilians, birds, mammals (slide 26)
d. cervical (specialized: atlas, axis; slide 27);
development of neck®greater head
movement (+lose connection to pectoral girdle)
e. trunk (thoracic, lumbar; fusions: dorsal
bone-birds)
f. sacral (fusions: e.g., synsacrum,
DEMO)
g. caudal (fusions: e.g., urostyle,
pygostyle, coccyx)
B. Ribs and sternum
1. Fish - dorsal and ventral ribs (slide 28);
no sternum; on all vertebrae
2. Tetrapods
a. two-piece dorsal ribs in amniotes; attach to sternum? (slide 29)
b. ossified in birds (uncinate
processes, slide 30)
c. trend toward greater restriction in
distribution (slide 31)
d. ribs external to pectoral girdle in turtles
3. Sternum structure (slide 32);
lost in many limbless forms; fused in some
4. Misc: gastralia,
plastron (DEMO); dermal bone, no relationship to ribs
C. Appendicular Skeleton
1. Paired appendages (slide 33)
a. fins (composed of pterygiophores
[proximal basals, distal radials] + fin rays); 2 types:
1) median fins (caudal, dorsal, anal)
2) paired fins (pectoral, pelvic); internally
supported by skeletal girdles
b. limbs (paired only; composed of hindlimb and forelimb each with 3 regions
2. Fish girdles
a. shark: slide 34
(no dermal bone in either girdle)
b. actinop: slide 35
(dermal contributions to pectoral; skull attachment)
c. sarcop: slide 36;
dermal contributions to pectoral; archipterygial vs. metapterygial fin structure (slide 37)
d. Eusthenopteron - tetrapod pattern;
homologies (slide 38;
slide
39); proposed origin of tetrapod limb
3. Tetrapod girdles
a. primitive (crossopterygean):
slide
40
b. pectoral girdle not attached to skull
c. pelvic girdle attached to vertebral column
d. pectoral: slide 41;
slide
42
e. pelvic: slide 43;
slide
44; slide 45
4. Tetrapod limb structure
a. earliest tetrapods polydactylous (5 digit condition became norm early, slide 46;
basis of comparison)
b. READ: note variation in limb bones (especially digital/phalangeal
reduction & multiplication)
READ: Form and Function