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Phylogenetic analyses distribute the features that developed along the tetrapod stem and display a stepwise process of character acquisition, rather than abrupt. [1] The complete transition occurred over a period of 30 million years beginning with the tetrapodomorph diversification in the Middle Devonian (380 myr).
In typical early tetrapod posture, the upper arm and upper leg extended nearly straight horizontal from its body, and the forearm and the lower leg extended downward from the upper segment at a near right angle. The body weight was not centered over the limbs, but was rather transferred 90 degrees outward and down through the lower limbs, which ...
The evolution of tetrapods began about 400 million years ago in the Devonian Period with the earliest tetrapods evolved from lobe-finned fishes. [1] Tetrapods (under the apomorphy-based definition used on this page) are categorized as animals in the biological superclass Tetrapoda, which includes all living and extinct amphibians, reptiles, birds, and mammals.
A forelimb or front limb is one of the paired articulated appendages attached on the cranial end of a terrestrial tetrapod vertebrate's torso. With reference to quadrupeds, the term foreleg or front leg is often used instead. In bipedal animals with an upright posture (e.g. humans and some other primates), the term upper limb is often used.
Polydactyly in early tetrapod aquatic animals, such as in Acanthostega gunnari (Jarvik 1952), one of an increasing number of genera of stem-tetrapods known from the Upper Devonian, which are providing insights into the appearance of tetrapods and the origin of limbs with digits. It also occurs secondarily in some later tetrapods, such as ...
The distalmost portion or extremity of the limb, i.e. the hand or foot, is known as the autopodium (plural: autopodia). Hands are technically known as the manus, and feet as the pes. The proximal part of the autopodium, i.e. the wrist or ankle region, has many small nodular bones, collectively termed the mesopodium (plural: mesopodia).
The upper limbs or upper extremities are the forelimbs of an upright-postured tetrapod vertebrate, extending from the scapulae and clavicles down to and including the digits, including all the musculatures and ligaments involved with the shoulder, elbow, wrist and knuckle joints. [1]
Limb mesenchymal cells, when dissociated and grown in culture or reintroduced within ectodermal "hulls" can recapitulate essential aspects of pattern formation, morphogenesis and differentiation. [36] [37] Peculiarities of the limb skeletal pattern in the mouse Doublefoot mutant are predicted outcomes of a Turing-type mechanism. [38]