THE APPENDICULAR SKELETON

The appendicular skeleton includes the paired fins or limbs and the girdles, the braces within the body that support them

The anterior girdle is the shoulder or pectoral girdle, to which dermal and endochondral skeletal elements contribute, and that support a pectoral fin or limb

The posterior girdle is the hip or pelvic girdle, consisting of endochondral skeletal elements that support the pelvic fin or limb

Because the appendages of fishes and tetrapods differ, the development and morphology of the pectoral and pelvic girdles also differs strongly.

Fishes

Pectoral girdles of fishes includes cartilage or cartilage replacement bone, as well as dermal bones derived from bony plates
In cartilagenous fishes, the girdle halves are fused producing the scapulocoracoid, that is joined to the other half of the pectoral girdle by the coracoid bar
In telelosts, the basic element is called the cleithrum, which supports the supracleithrum and, in turn, the posttemporal, through which the girdle is attached directly to the skull
The pelvic girdle of fishes is based on a single element, the puboischiac bar, that is embedded in the musculature of the trunk

Tetrapods

In tetrapods, the pectoral and pelvic girdles are each composed of three basic units that are similar in structure:

Pectoral                       Pelvic
     Procoracoid                Pubis
     Coracoid                     Ischium
     Scapula                       Ilium

The degree to which each are developed depends upon whether the animal is a quadruped or a biped
Primitive Tetrapoda have an interclavicle in addition to the clavicle and cleithrum retained from fishes

- endochondral elements of early tetrapod shoulder develops from two centers of ossification, giving rise to a scapula and "coracoid"
- "coracoid" formed from the anterior procoracoid and posterior coracoid
- the anterior coracoid (precoracoid or procoracoid) appeared as a synapomorphy of Amniota, and monotreme mammals have both coracoid bones. The loss of the anterior coracoid is a synapomorphy of Theria
- in Theria, the posterior coracoid is greatly reduced to the coracoid process of the scapula
- the clavicle and cleithrum are variously present/absent in clades of tetrapods
- the clavicles are fused into a furcula in Aves; are reduced (in carnivores) or lost (in ungulates) in some groups of cursorial mammals
- Caecilians lack pectoral girdles entirely, as do all snakes and some legless lizards
The tetrapod pelvic girdle has the largest foramen in the body - obturator foramen - that allows for passage of nerves and muscle tissue - among humans the male and female pelvic girdles show differences, such as a difference in the angle of the pubic arch and the depth of the pelvis - results from the female’s requirement for a wider shallower pelvis to accommodate childbirth Appendages

The appendages are the primary locomotor appendages of vertebrates and may be subdivided into fins and tetrapod limbs

Fins

The function of fins is to prevent the body from pitching and rolling, and to brake forward motion

Composed of:

- basal pterygiophores that articulate the fin to the pectoral girdle
- radial pterygiophores
- ceratotrichia and/or fin rays that make up the bulk of the fin (Fig. 9.1 p. 306). • Chondrichthyes have ceratotrichia (keratinous) and Osteichthyes have lepidotrichia (bony) - modified rows of bony scales
• actinotrichia are keratinous rays in the distal parts of the fins of some bony fish
Median Fins: include dorsal, anal, and caudal fins - the heterocercal caudal fin is a synapomorphy of Gnathostomata
- the diphycercal type evolved independently in Dipnoi, Latimeria, and Brachiopterygii
- the homocercal tail is a synapomorphy of Neopterygii (gars, bowfins, and teleosts).
Paired fins: the paired pectoral and pelvic fins of gnathostome fishes are homologous with the limbs of tetrapods

The three types of paired fins are:

- fin-fold fins (Chondrichthyes) - fins which evolved from endoskeletal pterygiophore elements added to ventrolateral folds in the body wall followed by addition of basals and fusion across the midline
- ray fins (Actinopterygii)
- lobed (fleshy) fins (Sarcopterygii) • Dipnoi have a biserial fin, with a preaxial and postaxial series of radials
• Crossopterygian fishes, regarded as "ancestral" to tetrapods, are uniserial with only the preaxial series
All fins are supported by basals and radials, which may be cartilage or bone - basals are enlarged radials that are nearest to the girdle
- in Chondrichthyes, a basal on the pelvic fin is modified into a clasper (male copulatory organ)
Tetrapod Limbs

Tetrapod fore- and hindlimbs show similarities that are also shown in the pectoral and pelvic girdles (Fig. 9.24-26, pp. 324-325)

Tetrapod limbs have five segments: propodium, epipodium, mesopodium, metapodium, and phalanges. The manus and pes are composed of the last three of these.
 
General name Forelimb Hind limb
Propodium humerus femur
Epipodium  radius, ulna tibia, fibula
Manus Pes
Mesopodium carpals tarsals
     proximal series      radiale
     intermedium
     ulnare
     tibiale
     intermedium
     fibulare
     medial series      centralia (4)      centralia (4)
     distal series      carpalia (5)      tarsalia (5)
Metapodium metacarpals (5) metatarsals (5)
Phalanges phalanges (5) phalanges (5)

It was long believed that the first land-dwelling had 5 digits on the hands and feet

Recent finds of fossils show that Ichthyostega had 7 digits on the pes, and its close relative Acanthostega had 8 digits on the manus

The first Amniota had 5 digits on manus and pes, but living Amphibia and their fossil relatives have only 4 or fewer digits on the hand

A common formula among early Tetrapoda excluding Ichthyostega is 2-3-4-5-4(or 3).

Mammals tend to modify their limb structure associated with various locomotor activities:

- plantigrade: walking on the soles of the feet; rodents and primates
- digitigrade: walking with the ankle and heel off the ground; carnivores
- unguligrade: walking on the ends of the digits; ungulates - the terminal digits form the bony basis of the hoof • in perissodactyls the axis of weight-bearing runs through the third toe
• in artiodactyls it runs between the third and fourth toes, which are equal in size
• metapodials fused in "advanced" artiodactyls such as camels and horses to form a cannon bone
Marine tetrapods have secondarily returned to water and converted the limbs to flippers, generally by hyperphalangy, as in plesiosaurs and whales, and hyperdactyly, as in ichthyosaurs (which are also hyperphalangious).

In some species these basic units become highly modified, as in the case of species such as bats and birds that have derived pectoral appendages into wings (Fig. 9.42, p. 337)

There were three appearances of wings among tetrapods:

1. The wing of Pterosauria was a large membrane attached to the side of the body and supported by the arm and a greatly elongated 4th finger. The terminal phalanx of the 4th digit, and the entire 5th digit were lost.

2. The bat (Chiroptera) wing is a membrane supported by the last four elongated fingers. Digit 1 (thumb) is free of the wing and forms a hook. A recent hypothesis put forth by Jack Pettigrew, a neurobiologist, is that bats are polyphyletic, and that the Megachiroptera and Microchiroptera evolved wings independently.

3. The bird wing surface is formed from feathers. Only three digits are present; these are most likely 1, 2, and 3, based on comparisons with Archaeopteryx, the first bird. The phalangeal formula of this Jurassic fossil is 2-3-4. The only distinct carpals present are the radiale and ulnare; the others are fused with the metacarpals into the carpometacarpus.

Limb Evolution and Dinosaur Phylogeny

Among living taxa, Crocodylia and Aves are sister-groups that form the Archosauria

Much of the phylogeny of archosaurs is based on the morphology of the appendicular skeleton. There are four major types of ankle joint:

* Primitively in the archosaurs there is a PM (primitive mesotarsal) ankle in which the plane of bendingis along a simple hinge between the astragalus and calcaneum unit and the rest of the foot.

* The clade Crocodylotarsi had a CN (crocodile-normal) ankle, in which a peg on the astragalus fits into a socket on the calcaneum. The plane of bending runs between the astragalus and calcaneum.

* The CR (crocodile-reversed) ankle is found in Ornithosuchus and close relatives.

* The AM (Advanced mesotarsal) ankle is a synapomorphy for the Ornithodira, including dinosaurs (plus Aves) and pterosaurs. In this ankle the astragalus and calcaneum are firmly attached to the tibia and fibula, and the plane of bending is between the astragalus-calcaneum and the rest of the foot.

Synapomorphies
 
Chordata * Presence of notochord
Vertebrata * Presence of vertebrae
Gnathostomata * Presence of paired appendages
* Presence of neural arches and hemal arches
* Heterocercal caudal fin
Chondrichthyes * Presence of claspers in males
Osteichthyes * Presence of lepidotrichia
Neopterygii * Absence of clavicle
* Homocercal caudal fin
Sarcopterygii * Fleshy fins
Amniota * Presence of atlas-axis complex
* Presence of anterior coracoid
Tetrapoda * Loss of fin rays
Tetrapoda + Ichthyostega * Shoulder girdle not attached to skull
Amphibia * Loss of "rib cage"
* Loss of interclavicle
Gymnophiona * Loss of pectoral and pelvic girdles and limbs
Theria * Loss of interclavicle
* Loss of anterior coracoid
* Posterior coracoid reduced to coracoid process
* Presence of scapular spine
Aves * Presence of furcula
Ornithodira * Advanced mesotarsal ankle
Dinosauria * Three or more sacral vertebrae
* Greatly reduced fibula
* Three or fewer phalanges in the fourth digit of the hand
Ornithischia * * At least five sacral vertebrae
* Opisthopubis pelvis
Sauropodomorpha ** * At least 10 cervical vertebrae
* Enormous pollex with an enlarged claw
* Femur longer than tibiai
Theropoda *** * Digit 1 of hand reduced or absent
* Claws: unguals enlarged, compressed, pointed
* Elongate, narrow metatarsus
* Metatarsal 1 reduced, not contacting tarsus, attached to side of metatarsal 2
* Metatarsal 5 reduced to splint

* includes ankylosaurs, stegosaurs, hadrosaurs, ceratopsians

** includes the saurischian herbivores, such as Diplodocus, and Apatosaurus

*** including carnosaurs, the largest carnivorous land animals, such as Tyrannosaurus and Allosaurus; and smaller, but nonetheless vicious forms such as Deinonychus, Struthiomimus. and Aves