Comparative Vertebrate Anatomy

BIO 342
Comparative Vertebrate Anatomy
Lecture Notes 5 - Skeletal System IV
Appendicular Skeleton

Appendicular skeleton

consists of pectoral & pelvic girdles plus skeleton of fins & limbs
Some vertebrates have no appendicular skeleton (e.g., agnathans, apodans, snakes, & some lizards) & in others it is much reduced.

Pectoral girdles:

1 - brace for anterior appendages

3 - Early fishes had 3 replacement bones (coracoid, scapula, & suprascapula) and a series of dermal bones (clavicle, cleithrum, supracleithrum, and post-temporal)

4 - Later bony fishes (ganoid fish) - tendency for reduction in number and size of replacement bones

5 - Tetrapods - tendency for reduction in number of dermal bones

Source: http://www.uta.edu/biology/restricted/3452gird.htm

Bony fishes - pectoral girdles of living bony fishes have reduced coracoid & scapula (replacement bone) but large cleithrum & supracleithrum (dermal bone). A posttemporal bone (dermal) connects the supracleithrum to the skull.

Source: http://www.uta.edu/biology/restricted/3452gird.htm

Cartilaginous fishes - no dermal bone

Source: http://www.uta.edu/biology/restricted/3452gird.htm

Tetrapods - early ones had pectoral girdle similar to those of early bony fishes, but lost posttemporal & acquired interclavicle (which still occurs in several amniotes, e.g., alligator, birds, & monotremes)

Clavicle & coracoid - one or both typically brace scapula against sternum (as in birds; below)

Scapula - present in all tetrapods with even vestiges of anterior limbs, e.g., turtles & birds & mammals

Pelvic girdles

brace posterior paired appendages
no dermal components (unlike pectoral girdle)

Fishes - pelvic girdle consists of 2 cartilaginous or bony plates (ischiopubic plates) that articulate with the pelvic fins

Source: http://www.uta.edu/biology/restricted/3452gird.htm

Tetrapods - pair of cartilaginous plates form in embryos & each ossifies at 2 centers to form: pubis & ischium. An additional blastema gives rise to the ilium.

Frogs & toads

ilia elongated & extend from sacral vertebra to urostyle
joint between ilium & sacral vertebra (sacroiliac) is freely moveable (& moves when a frog or toad jumps)

Birds:

ilium & ischium expanded to accommodate musculature needed for bipedal locomotion
girdle is braced against lumbar & sacral vertebrae
pubic bones are typically reduced (long but thin); the limited pubic symphysis provides a larger outlet for eggs

Mammals - ilium, ischium, and pubis unite to form the innominate bone (the 2 innominates = pelvic girdle)

FINS

All jawed fishes (except eels) have pectoral & pelvic fins
Fins are used primarily for steering ('rudders')
Types of fins:

lobed fins - found in sarcopterygians
fin fold fins (see diagram below)

found in cartilaginous fish
consist of 1 to 5 basal cartilages plus several rows of radials

ray fin - tendency to lose proximal components of fin skeleton (see diagram below)

Limbs

Starting with amphibians, vertebrates typically have 4 limbs. However, some have lost one or both pairs &, in others, one pair is modified as arms, wings, or paddles
typically have 5 segments:

Anterior limb

brachium (upper arm) - consists of humerus
antebrachium (forearm) - consists of radius & ulna
carpus (wrist) - consists of carpals
metacarpus (palm) - consists of metacarpals
digits - consist of phalanges

Posterior limb

femur (thigh) - consists of femur
crus (shank) - consists of tibia & fibula
tarsus (ankle) - consists of tarsals
metatarsus (instep) - consists of metatarsals
digits - consist of phalanges

Source: http://www.uta.edu/biology/restricted/3452gird.htm

Some vertebrates lack both pairs of limbs

caecilians (apodans)
most snakes
snake-like lizards

Some vertebrates have forelimbs only:

manatees & dugongs
dolphins (see diagram below)
cetaceans (vestigial elements may be embedded in body wall)
sirens (salamander)

Early tetrapods - limbs were short & first segment extended straight out from the body . This posture persists among lower tetrapods (e.g, see the alligator below), but, in birds & mammals, there has been a rotation of the appendages so that the long axis of the humerus & femur more nearly parallels the vertebral column.

Arm & forearm

Upper arm = humerus
Forearm = radius & ulna
Manus (or hand)

Wrist - 3 rows of carpal bones:

proximal row = radiale, ulnare, intermedium, & pisiform
middle row = 3 central carpals (centralia)
distal row = 5 distal carplas numbered 1 through 5 (starting on thumb, or radial, side)

Palm - metacarpals
Digits

each consists of a series of phalanges
general ‘formula' (starting at thumb) = 2,3,4,5,3

Adaptive modifications of the manus
1 - Flight

Birds - loss of digits & bones plus fusion of some bones
Bats - 5 digits; elongated metacarpals (II through V) & phalanges support the patagium
Pterosaurs - 4th digit elongated to support patagium

2 - Swimming - increase in number, & size, of phalanges

3 - Terrestrial locomotion (walking & running):

Plantigrade

flat-footed
all bones of manus and/or pes on the ground
amphibians, most reptiles (see alligator photo above), & some mammals (insectivores, monkeys, apes, humans, & bears)

Digitigrade

1st digit is reduced or lost
manus & pes are elevated
rabbits, rodents, & many carnivores

Unguligrade

reduced number of digits
walk on tips of remaining digits
claws become hooves

As the fastest North American mammal, pronghorn antelope (unguligrades) can reach speeds of 60 miles per hour.
At high speed they cover the ground in strides of 14 to 24 feet, and are known to run for long distances at speeds of 30 to 40 miles per hour.

opposable thumb

saddle joint at base of thumb where it meets palm
thumb at wider angle from index finger
strong thumb muscles

Posterior limbs - bones are comparable to those of forelimbs except that a patella (‘kneecap') develops in birds & mammals

Origin of fins:

1 - Fin fold hypothesis - fins derived from continuous fold of body wall

Ammoecetes

2 - Gill arch hypothesis - fins derived from the last 2 gill arches (very unlikely)

3 - Fin spine hypothesis - fins derived from tissue attached to spines (that may have evolved to provide protection from predators)

Acanthodiian

Origin of limbs - derived from paired fins of ancient fishes:

Used by permission of John W. Kimble

The first tetrapods, Labyrinthodont amphibians (right), probably evolved from a Crossopterygian ancestor (left).
When the fresh water pools in which these fish lived became stagnant, they may have crawled up the bank to breath air using
primitive lungs. As the lobed fins of these fish evolved into stronger limbs, the first tetrapods appeared.

Comparison of paired anterior fins of lobe-finned fishes (A-D) and limbs of early tetrapods (E, F)

A. Sterropterygion, B. Sauripterus,
C. Panderichthys, D. Eusthenopteron,
E. Ichthyostega, F. Acanthostega

Here we have a possible explanation for the formation of a new morphological feature - limbs with digits - from the paired fins of sarcopterygian (lobe finned) fishes. Some of these fish (notably Eusthenopteron) have bones in their paired fins that are very similar to the bones of tetrapod limbs. Specifically, they have a single bone (similar to the humerus or femur) followed by paired bones (similar to the radius and ulna or fibula and tibia of tetrapods). Scientists think Eusthenopterons used their limbs to walk on the sea (or lake or river) bed.
What Eusthenopteron lacks are digits - having fin rays instead (although Sauripterus is a closely related fish that does have 8 digits just like the earliest amphibians - see the illustration). However, the fossil record supplies us with examples of tetrapods that are quite similar to Eusthenopteron - Acanthostega and Icthyostega.