Tetrapod limbs are believed to have evolved from the pectoral and pelvic fins of lobed fin fishes. Limbs are much stronger than fins since on land body weight has to be lifted against gravity for locomotion, whereas in water body is supported by water and limbs have to just propel the body forward.
A TYPICAL TETRAPOD LIMB
All tetrapod limbs have the same basic pattern and arrangement of bones, which are modified for different types of locomotion.
The bone that attaches the limb to the girdle is called stylopodium, which is called humerus in fore limb and femur in hind limb. Attached distally to this are two bones, radius and ulna in fore limb and tibia and fibula in hind limb. Each bone has a small attached to it distally called radiale and ulnare in hand and tibiale and fibulare in leg. In wrist region there are three centrales and a series of 5 distal carpals or distal tarsals. This makes the movable joint of the hand and feet. Next to these to make the fingers and toes, there are 5 metacarpals in hand while 5 metatarsals in foot. Distal to these are phalanges which are variable in numbers and make 5 digits of tetrapods.
MODIFICATIONS IN TETRAPOD LIMB
Tetrapod limb bones change their shape, size and function depending on the type of locomotion adopted by the animal, such as swimming, running, climbing, jumping or flying.
ARBOREAL ADAPTATION
Vertebrates that live on trees such as perching birds and primates must have strong limbs opposable pollex in primates and hallux in birds for grasping branches firmly. Flexibility of joints is a necessity and arms in some primates are longer than legs for swinging.
SCANSORIAL ADAPTATION
Not all animals living on trees are climbers but some species can climb from the ground to the trees and jump on branches. Primates, rodents, some birds and reptiles are good climbers. In primates hand and feet have opposable thumb and toes and fingers can be wrapped around the branches. Woodpeckers and parrots have zygodactyly that helps them to climb on the tree trunks. Chamaleon exhibits syndactyly, in which five digits are fused into two groups for efficient grasp on the branches.
CURSORIAL ADAPTATION
For fast running animals require not only strong and long legs but also they must run on toes, without touching the heel to the ground. Slow moving animals have plantigrade locomotion in which heel or calcaneum bone touches the ground and entire foot carries the body weight. In carnivores such as dogs and cats, heel is lifted up and even metacarpals and metatarsals do not touch the ground. Hence the body weight is supported by digits or phalanges only. This type of locomotion is called digitigrade. The highest form of cursorial adaptation is seen in ungulates in which the animal walks on the tip of toe, which is modified into a broad convex hoof. Even phalanges do not touch the ground. In order to provide strength to the bones digits fuse to form stronger bone. For example there are only three toes in rhinoceros, two in deer, cow goat etc and only one in horses.
VOLANT ADAPTATION
Flight evolved in bats and birds in which fore limb is modified to form a wing for flying. In birds ulna is longer and stronger than radius for the attachment of remiges. Radiale and ulnare are present but carpals and metacarpals are fused to form carpo-metacarpus for the attachment of feathers. Phalanges are few and digits are reduced to one to three.
In bats four of the digits are thin and exceedingly long to support the modified skin called patagium, while the first digit bears a large curved claw for clinging on to the branches. Radius is thin and long and ulna is reduced to vestiges.
AQUATIC ADAPTATION
In aquatic animals such as dolphins and whales, limb is shortened to form a flipper. Humerus as well as radius and ulna are short and block like. Carpals and metacarpals are reduced in size but the number of phalanges is increased, called hyperphalangy. Digits are covered by flap of skin and not visible outside.
FOSSORIAL ADAPTATION
For digging limb must be short, stocky, strong and broad. Humerus is broad for attachment of large hand muscles. Olecrenon process of ulna is elongated to stop reverse flexing of the limb while digging. Metacarpals and phalanges are short and broad and claws are wide spade like for digging.
SALTATORIAL ADAPTATION
Frogs hind legs are modified for jumping. Leg is elongated supplied with strong thigh muscles that are attached to broadened ischium and pubic bones of the pelvic girdle. Tibia and fibula are fused to form tibio-fibula to provide strength to the leg. Tibiale and fibulare become elongated and connected on both ends to form astragalus-calcaneum, which adds to the length of the legs. Tarsals are fused together and pentadactyle digits form a broad foot.
GRAVIPORTAL ADAPATATION
Animals having body weight of one ton and above must possess strong pillar like limbs and broad feet as found in elephants. Humerus is broad and straight aligned with a straight scapula of pectoral girdle. Radius and ulna are large and free and olecrenon process is deflexed to keep the joint straight so that the centre of gravity passes through the middle of the leg. Feet are broad and five toes radiate over a cushiony pad of tissue.