fourlimb

Fourlimb is a term sometimes used to refer to the four limbs of a tetrapod vertebrate, comprising two forelimbs and two hindlimbs. In formal anatomy, the terms forelimb and hindlimb are preferred, and “four limbs” is more common in descriptive or comparative contexts. The word is not a distinct anatomical unit but a way of signaling a quartet of limbs in discussions of locomotion, posture, or evolution.

Typically, forelimbs and hindlimbs share a parallel plan of bones, with proximal elements (humorus in the forelimb; femur in the hindlimb) followed by radius and ulna or tibia and fibula, then the carpus and tarsus, and finally the metacarpals or metatarsals and the phalanges. Across tetrapods, the arrangement is homologous though specialized for different functions, such as grasping claws in some species or weight-bearing structures in others.

Fourlimbs enable various locomotor modes. Quadrupedal gait patterns include walking, trotting, pacing, and galloping, with limb coordination tuned to speed and terrain. Forelimbs may also contribute to manipulation, climbing, or stabilization, while hindlimbs often provide propulsion.

Four limbs originated in early tetrapods during the transition from aquatic to terrestrial life. Fossil and comparative evidence supports a common origin of forelimb and hindlimb bones, with diversification across amphibians, reptiles, birds, and mammals.

Some tetrapod lineages reduce or lose limbs, such as snakes and caecilians, or whales which retain small hindlimb remnants. In humans, the four-limb body plan remains but locomotion typically uses two legs with arms serving manipulation.

See also: tetrapod, forelimb, hindlimb, quadruped, limb evolution