EEB 3273/Schwenk
LAB 2-3: AXIAL AND APPENDICULAR SKELETON
Preparation
H & W, Chap. 5 (small print)
H & W, Chap. 6 (small print; ignore bones of the hands & feet)
Background—Together, the axial (body axis or midline) and appendicular (bilaterally symmetrical
girdles and limbs) skeletons compose the body, or postcranial skeleton (as opposed to the head
skeleton). The postcranial skeleton comprises the vertebral column, ribs, the pectoral and pelvic
girdles, and their associated fins and limbs. It serves as a storehouse for various minerals necessary for
metabolism, protects internal organs, provides rigid structure for the body, and due to its segmented,
hinged nature, combines with associated muscles for movement. In contrast to the rigid exoskeleton of
arthropods, the internal skeleton is stronger and has allowed vertebrates to reach much larger body sizes
while remaining relatively agile. The axial and appendicular skeletons are composed mostly of
endochondral bone and cartilage. A few dermal bones are, however, associated with the pectoral
girdle in some vertebrates.
Today's Lab
This lab is composed of three major parts. First, we examine the modifications seen in the axial skeleton
relating to life in water (fishes) vs. life on land (tetrapods), comparing the latter to secondarily aquatic
tetrapods (e.g., cetaceans). Second, we explore the morphology of the pectoral and pelvic girdles and
appendages in fishes as compared to tetrapods. Third, we consider some of the modifications of the
tetrapod body skeleton associated with four ecomorphological types: cursorial (running), fossorial
(digging), scansorial (climbing), and volant (flying). The bone names for which you are responsible are
listed in this handout.
I. AXIAL SKELETON
A. RELATIONSHIPS OF THE VERTEBRAL COLUMN
1) Fishes (H & W, Fig. 5-2)
The vertebrae of fishes aid in the production of lateral undulations for swimming. The entire vertebral
column is subjected to similar stresses and, therefore, shows little regional differentiation. It primarily
resists compression caused during contraction of the segmented body musculature for swimming. Since
fishes are buoyant in water, their axial skeleton does not need to resist the shear forces created by gravity.
Two basic types of vertebrae can be identified: trunk (body) and caudal (tail).
Material:
Examine the vertebrae of a shark and a tuna (derived bony fish), as well as a mounted bony fish skeleton.
Identify:
trunk vertebrae
caudal vertebrae
centrum
neural arch
neural canal
hemal arch
hemal canal
notochord (where present)
EEB 3273, Lab 2, Page 1
ribs
median fin
tail fin
2) Non-Mammalian Tetrapods (H & W, Fig. 5-4)
As tetrapods moved onto land, the body skeleton became increasingly important for support and limbed
locomotion. In response to gravitational stresses, tetrapods show more ossification of the body skeleton
than most fishes and the development of intervertebral articulations (pzygapophyses + others in some
groups) that help to interlock to stiffen the column and prevent shear. The vertebral column also shows
further regional differentiation/specialization into cervical (neck), trunk, sacral (fused vertebrae where
the pelvic girdle attaches) and caudal vertebrae.
Material:
Consider how the transition to terrestrial life has reshaped the axial skeleton in amphibians (a frog, Rana,
and a salamander, Necturus) and reptiles (lizard, alligator and bird skeletons).
Identify:
cervical vertebrae
trunk vertebrae
sacral vertebrae/sacrum
caudal vertebrae
centrum
neural arch
neural canal
neural spine/process
atlas c.1
axis c.2 (not specialized)
prezygapophyses
postzygapophyses
transverse processes
hemal arch (only in caudal vert. of crocs)
ribs
uncinate process (birds)
sternum
3) Mammals (H & W, Fig. 5-5, 5-6, 5-7)
Mammals show the greatest degree of regionalization in the vertebral column. The vertebral column is
vital not only for locomotion and protection of the spinal cord, but it also serves as an integral part of
diaphragmatic respiration. The vertebral column is differentiated into cervical, thoracic, lumbar, sacral,
and caudal regions (thoracic + lumbar = trunk). Be sure to be able to identify vertebrae from the five
different spinal regions in mammals, as well as the terms listed below.
Material:
Examine an articulated cat skeleton, the isolated vertebral column of a horse (be sure to orient this
correctly), whale vertebrae, and the cervical vertebrae of another small cetacean (dolphin), plus any other
material present.
Identify:
cervical vertebrae
thoracic vertebrae
lumbar vertebrae
sacral vertebrae/sacrum
caudal vertebrae
centrum
neural arch
neural canal
neural spine/process
atlas c.1
axis c.2
dens/odontoid process
prezygapophyses
postzygapophyses
pleurapophyses
transverse processes
intervertebral disc (may not be preserved)
ribs
sternum
Zygopophysis Hint: postzygapophyses feel bad about being stuck in back so they are down-and-out;
prezygapophyses are happy about being in front and are therefore up-and-in (in reference to the direction
that the articular facets—the smooth surfaces where they make contact—face)
EEB 3273, Lab 2, Page 2
B. RIBS AND STERNUM (H & W, Fig. 5-5, 5-6, 5-7, 5-8)
In terrestrial (tetrapod) vertebrates, ribs serve a much more important role in support and locomotion than
in fishes. They protect the viscera and prevent the trunk from compressing when the body is lying on the
ground. Furthermore, in amniotes (reptiles and mammals) the ribs have become vital to the expansion of
the thoracic cavity during breathing. As a result, the ribs of birds and mammals show various
modifications that allow for the insertion of intercostal (between rib) muscles. Some ribs have costal
cartilages that connect them ventrally to the sternum (part of the axial skeleton in the ventral midline).
This connection forms a strong but flexible box which is helpful in absorbing the intense shock produced
in the pectoral girdle by running quadrupeds as the forelimbs strike the ground. In some climbing and
digging mammals, some ribs become flattened blades that nearly overlap, or even fuse together, creating a
more rigid thorax that resists dorso-ventral bending (e.g., anteaters). Birds have uncinate processes on
their ribs that extend posteriorly, overlapping rib behind. This also helps to stiffen the rib cage to resist
the stresses of flight.
In most vertebrates, ribs are associated with nearly all vertebrae, including the cervicals (with the
exception of c.1 and c.2, the atlas and axis used for head movement). However, a diagnostic feature of
mammals is that there are no ribs on all 7 cervical vertebrae (monotremes have fused ribs on c. 3-7).
This is the principal way to distinguish a cervical from a thoracic vertebra in a mammal—there are rib
facets on thoracic vertebrae and no rib facets on cervical vertebrae.
Nearly all mammals have the same number of cervical vertebrae (7). This includes giraffes with very
long necks and cetaceans with virtually no neck (see cetacean cervical vertebrae specimen). The
constancy and apparent evolutionary stability of this number is a well-known example of an evolutionary
constraint—in other words, there is something that seems to prevent an evolutionary change (+/-) in this
number. Recent work suggests that the constraint arises from high cancer rates associated with mutations
that create more or fewer cervical vertebrae. Nevertheless, there are a few species that have either 6, 8 or
9. Can you imagine which mammals these might be and why/how they seem to have circumvented the
constraint?
II. PECTORAL GIRDLE, PELVIC GIRDLE, AND APPENDAGES
1) Fishes (H & W, pp. 92-95, Figs. 6-1, 6-2)
Material: (preserved shark skeletons, bony fish skeletons)
Compare the pelvic and pectoral girdles of the shark and the actinopterygian fishes, Amia and Perca.
Note especially that the girdles of the shark are composed of a single cartilage while those of the bony
fishes are paired, with several bones composing each half. Compare also the breadth of connection
between the fin radials and the girdles. In what ways would these differences affect the mobility of the
fins?
Identify:
PECTORAL GIRDLE
Dermal Bones
cleithrum
PELVIC GIRDLE
puboischiadic bar
iliac process
Endochondral Bones
coracoid
scapula/scapular process
EEB 3273, Lab 2, Page 3
PECT. AND PELVIC FINS
basal pterygiophores
radial pterygiophores
fin rays:
lepidotrichia in bony fishes
(dermal bone)
ceratotrichia in sharks
(keratinous)
2) Non-Mammalian Tetrapod Condition (H & W, pp. 98-99, Figs. 6-5, 6-6, 6-7)
Material: salamander (Necturus), lizard, alligator and bird skeletons
Amphibians and many living reptiles retain ancestral features of both the pectoral and pelvic girdles.
Note the partial ossification of the girdles in Necturus versus the heavy ossification seen in reptiles. Why
might this be?
In birds, both girdles are highly modified for flight. The furcula (“wishbone”) is a new element in the
pectoral girdle representing the fusion of the dermal clavicles and probably the interclavicle, as well.
What is the function of the furcula?
Identify:
PECTORAL GIRDLE
scapula (endochondral)
coracoid (endochondral)
clavicle (dermal)
interclavicle (dermal)
PELVIC GIRDLE
ilium
ischium
pubis
1
synsacrum (birds)
FORELIMB
humerus
ulna
radius
carpals (wrist)
metacarpals (palm)
phalanges (digits)
HIND LIMB
femur
tibia
fibula
2
tarsals (ankle)
metatarsals (instep/foot)
phalanges (digits)
HIND LIMB OF BIRDS
femur
3
tibiotarsus
4
tarsometatarsus
1
The synsacrum of birds consists of the sacrum + additional fused vertebrae from the trunk and
tail. In addition, some or most of the pelvic girdle bones are fused to the synsacrum, creating a
very rigid structure to resist the stresses of flight, landing and take-off
2
The calcaneus or ‘heel bone’ is a modified tarsal)
3
4
The tibiotarsus is a fusion of the tibia and some of the proximal tarsal bones
The tarsometatarsus is a fusion of the distal tarsal bones and the metatarsals. Like the cannon
bone of some hoofed mammals, it adds an additional segment to the limb
3) Mammals—Monotremes (Handout; Figs. 6-5, 6-6)
Material: monotreme skeleton (echidna/spiny anteater, Tachyglossus sp.)
In the synapsid stem lineage leading to mammals, a new endochondral bone developed in the pectoral
girdle just posterior to the existing coracoid. Unfortunately, this bone is also called a ‘coracoid’,
specifically, the posterior coracoid. Among living mammals, this condition is seen only in the
monotremes, which retain both coracoids,—the "new" posterior coracoid and the "old" anterior coracoid.
In addition, late synapsids evolved new bones projecting anteriorly from the pelvic girdle called the
epipubic bones. These are retained in monotremes and marsupials and are even found in extinct, fossil
members of the eutherian mammals. These rod-like bones serve as attachment sites for muscles and
tendons that help to stiffen the abdomen during locomotion (you might read that these bones serve to
support the ‘marsupium’, or pouch, but this is no longer thought to be the case).
In non-mammalian bony vertebrates, the pelvic girdle on each side consists of three, separate bones: a
dorsal ilium, an anterior pubis and a posterior ischium. In mammals, however, these three ossifications
EEB 3273, Lab 2, Page 4
fuse into a single large bone called the innominate bone. Although a single bone in adults, in embryos
and juveniles one can observe the original, three separate bones. The three bones meet in the
acetabulum—the socket for the femur. The two pubic bones meet anteriorly in the midline where they
are tightly fused with a fibrous cartilage joint called the pubic symphysis. The iliac crest (the ilia bones)
on each side attaches to the sacrum forming the sacro-iliac joint—also a tight, fibrocartilage joint.
Identify:
PECTORAL GIRDLE/LIMB
PELVIC GIRDLE/LIMB
scapula
anterior coracoid
posterior coracoid
clavicle
interclavicle
humerus
ulna
radius
carpals
metacarpals
phalanges
ilium
ischium
pubis
innominate bone
epipubic bone
acetabulum
femur
tibia
fibula
tarsals
metatarsals
phalanges
4) Mammals—Therians (Marsupials and Eutherians) (Figs. 6-11, 6-12, 6-13, 6-16, 6-17, 6-18)
Material: marsupial and eutherian mammal skeletons/limbs and girdls
In marsupial and eutherian mammals, the pectoral girdle is highly simplified: only a single element
remains, the large endochondral scapula. The ancestral anterior coracoid is lost and the "new" posterior
coracoid remains only as the coracoid process, now fused to the scapula. The dermal bones are lost
except for the clavicle, and even this is reduced or lost in many cursorial (running) mammals.
The pelvic girdle of The epipubic bones of the monotreme/marsupial pelvic girdle are lost in eutherian
mammals.
Identify:
PECTORAL GIRDLE/LIMB
PELVIC GIRDLE/LIMB
scapula
coracoid process
scapular spine
clavicle
humerus
ulna
radius
carpals
metacarpals/cannon bone*
phalanges
ilium
ischium
pubis
acetabulum
femur
tibia
fibula
epipubic bone (marsupials only)
tarsals
metatarsals/cannon bone*
phalanges
*in some hoofed animals the metacarpals and metatarsals are reduced in number and fused
into a cannon bone that forms an additional, vertical element of the leg, with the animal
walking on the nails of its toe tips or hooves)
EEB 3273, Lab 2, Page 5
DERIVED BONY FISH (teleost) SKELETON
BONY FISH (basal/primitive)
NOTE THE CONTINUOUS NOTOCHORD PRESENT EVEN IN THIS AND
SOME OTHER BONY FISHES—The vertebrae ossify around the
notochord, but do not replace it during embryonic development.
BONY FISH (derived)
These are trunk vertebrae with attached ribs, therefore
there are no hemal arches. Note that most derived
bony fishes (teleosts) have two sets of ribs—dorsal
and ventral. In this species the dorsal ribs are tiny.
Regardless, it is the dorsal, not the ventral ribs, that are
homologous with tetrapod ribs.
1 = PUBOISCHIADIC BAR
2 = BASAL PTERYGIOPHORE
3. Claspers are found in males only; used as intromittent
organ to introduce sperm into female cloaca
4 = RADIAL PTERYGIOPHORES; beyond these would be
the CERATOTRICHIA that support the rest of the fin
1-2 = SCAPULOCORACOID BAR
3-7 = BASAL PTERYGIOPHORES
8 = RADIAL PTERYGIOPHORES (CERATOTRICHIA not shown)
DERMAL BONES:
pt = post-temporal bone of skull
s.cl = supracleithrum
cl = CLEITHRUM
p.cl. = post-cleithrum
ENDOCHONDRAL BONES:
c = CORACOID
sc. = SCAPULA
br.c. = BASAL PTERYGIOPHORES (BASALS)
BONY FISH (TELEOST) LEFT PECTORAL GIRDLE
(lateral view)
BIRD PECTORAL GIRDLES (NOTE: fused clavicles + interclavicle =
FURCULA or ‘wishbone’)
1 = scapula
2 = coracoid
3 = interclavicle
4 = glenoid fossa (socket for humerus)
BUDGIE (PARROT)—note curvature of cervical vertebrae at rest
ALLIGATOR PECTORAL GIRDLE
LIZARD PECTORAL GIRDLE
(ventral view)
DERMAL BONES
1 = INTERCLAVICLE
2 = CLAVICLE
ENDOCHONDRAL BONES
3 = SCAPULA
4 = CORACOID
OTHER
6 = GLENOID FOSSA
7 = STERNUM
8-9 = RIBS
BASIC (ANCESTRAL)
TETRAPOD PECTORAL GIRDLE
(front view)
DIFFERENT LIZARD PECTORAL GIRDLS
(lateral view, anterior to left))
1 = suprascapula
2 = SCAPULA
3 = GLENOID FOSSA
4 = CORACOID
5 = CLAVICLE
6 = INTERCLAVICLE
PLESIOSAUR (extinct marine reptile) PELVIC GIRDLE SHOWING BASIC TETRAPOD
CONDITION
top = dorsal view
bottom = lateral view
p = PUBIS
isc = ISCHIUM
il = ILIUM (these attach to sacral vertebrae dorsally)
acet = ACETABULUM (= socket for femur; always found at junction of all 3 bones)
BIRD SYNSACRUM: dorsal (left) and ventral (right) views
HORSE FORELIMB
‘PASTERNS’ = DIGIT BONES (PHALANGES)
‘CANNON BONE’ = FUSED METATARSALS
FIBULA IS LOST OR FUSED WITH TIBIA
BIRD HIND LIMB
LIZARD LIMBS
TETRAPOD FORELIMB MODIFICATIONS
NOTE THAT WING BONES ARE
HOMOLOGOUS, BUT NOT ALL
OTHER PARTS OF THE WINGS
ARE
(e.g., bat and pterosaur wing
surfaces are stretched skin,
whereas in birds they are made
of feathers)
Transverse process
Transverse process
Tibia
Fibula
NUCHAL LIGAMENT IN UNTULATES
MONTOTREME PECTORAL GIRDLE
(platypus)
HUMAN (EUTHERIAN) PECTORAL GIRDLE (scapula with coracoid process) + clavicle (not shown)
INNOMINATE BONE (half pelvis) OF DOG (Eutherian)
ADULT (bones fused) AND JUVENINLE (bones unfused)
A and B EXTINCT SYNAPSIDS; C = MARSUPIAL; D = CAT/EUTHERIAN (left lateral view; anterior to left)
HUMAN EMBRYO—UNFUSED BONES OF PELVIS
FEMALE vs. MALE HUMAN PELVIS
(not all pelves fit into this neat dichotomy)
A and B EXTINCT SYNAPSIDS; C = MARSUPIAL; D = CAT/EUTHERIAN (left lateral view; anterior to left)
HUMAN EMBRYO—UNFUSED BONES OF PELVIS
FEMALE vs. MALE HUMAN PELVIS
(not all pelves fit into this neat dichotomy)