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

Appendicular skeleton

Pectoral girdles:

 Cartilaginous fishes - no dermal bone  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

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


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


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




Some vertebrates lack both pairs of limbs

Some vertebrates have forelimbs only:

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.

Adaptive modifications of the manus
  1 - Flight

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

3 - Terrestrial locomotion (walking & running):

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.

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


 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)


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. 

Related Links:

Fins to Limbs

Legs, feet, & cursorial locomotion

Bat wings & tails

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