User:Rauisuchian/Tetrapod limb

Tetrapod limbs or the limbs of terrestrial vertebrates have adapted in a number of forms since the emergence of Tetrapodomorpha in the Early Devonian. Tetrapod limbs include forelimbs or hindlimbs. Limbs have been modified into flippers, grasping arms, hooves, wings, or even reduced to being absent in the case of snakes and legless lizards.

All vertebrate limbs are homologous, meaning that they all evolved from the same structures, ultimately from the fins of lobe-finned fish. They then specialized into a number of body plans. Specific adaptations of the limbs, such as the case of bird and bat wings, may be analogous.

Several postures exist for tetrapods, from the sprawling posture of lizards, to the erect posture of mammals and dinosaurs (including birds), to the pillar-erect posture of some extinct crocodylomorphs. Unique bipedal postures are present among humans and kangaroos.

Walking strategies include plantigrade, digitigrade, unguligrade, knuckle-walking, and others.

Locomotion
Tetrapod limbs have been adapted for several purposes.


 * Legged – Walking by using appendages
 * Limbless locomotion – moving without legs, primarily using the body itself as a propulsive structure
 * Flight - Movement by flapping wings or soaring from thermal air currents
 * Swimming - Use of limbs as flippers, which in cetaceans has occurred only in the forelimbs, but in other aquatic tetrapods uses all four limbs.

Terminology
The skeletons of all mammals are based on a common pentadactyl ("five-fingered") template but optimized for different functions. While many mammals can perform other tasks using their forelimbs, their primary use in most terrestrial mammals is one of three main modes of locomotion: unguligrade (hoof walkers), digitigrade (toe walkers), and plantigrade (sole walkers). Generally, the forelimbs are optimized for speed and stamina, but in some mammals some of the locomotion optimisation have been sacrificed for other functions, such as digging and grasping.

Posture
Appendages can be used for movement in a lot of ways: the posture, the way the body is supported by the legs, is an important aspect. There are three main ways in which vertebrates support themselves with their legs – sprawling, semi-erect, and fully erect. Some animals may use different postures in different circumstances, depending on the posture's mechanical advantages. There is no detectable difference in energetic cost between stances.

The "sprawling" posture is the most primitive, and is the original limb posture from which the others evolved. The upper limbs are typically held horizontally, while the lower limbs are vertical, though upper limb angle may be substantially increased in large animals. The body may drag along the ground, as in salamanders, or may be substantially elevated, as in monitor lizards. This posture is typically associated with trotting gaits, and the body flexes from side-to-side during movement to increase step length. All limbed reptiles and salamanders use this posture, as does the platypus and several species of frogs that walk. Unusual examples can be found among amphibious fish, such as the mudskipper, which drag themselves across land on their sturdy fins, though these are not tetrapods. The semi-erect posture is more accurately interpreted as an extremely elevated sprawling posture. This mode of locomotion is typically found in large lizards such as monitor lizards and tegus.

Mammals and birds typically have a fully erect posture, though each evolved it independently. In these groups the legs are placed beneath the body. This is often linked with the evolution of endothermy, as it avoids Carrier's constraint and thus allows prolonged periods of activity. The fully erect stance is not necessarily the "most-evolved" stance; evidence suggests that crocodilians evolved a semi-erect stance in their forelimbs from ancestors with fully erect stance as a result of adapting to a mostly aquatic lifestyle, though their hindlimbs are still held fully erect. For example, the mesozoic prehistoric crocodilian Erpetosuchus is believed to have had a fully erect stance and been terrestrial.

Evolution
Evolution of the limb may be characterized by many trends. The number of digits, their characteristics, as well as the shape and alignment of radius, ulna, and humerus, have had major evolutionary implications.

Changes in body size, foot posture, habitat, and substrate are frequently found to influence one another (and to connect to broader potential drivers, such as changing climate).

Polydactyly
Tetrapods were initially understood to have first developed five digits as an ancestral characteristic, which were then reduced or specialized into a number of uses. Certain animals retained 'primitive' forelimbs, such as pentadactylous (five-fingered) reptiles and primates. This has mostly held true, but the earliest tetrapod or "fishapod" ancestors may have had more than five digits. This was notably challenged by Stephen Jay Gould in his 1991 essay "Eight (Or Fewer) Little Piggies".

Polydactyly in early tetrapods should be understood as having more than five digits to the finger or foot, a condition that was the natural state of affairs in the very first tetrapods. Early groups like Acanthostega had eight digits, while the more derived Ichthyostega had seven digits, the yet-more derived Tulerpeton had six toes.

Tetrapods evolved from animals with fins such as found in lobe-finned fishes. From this condition a new pattern of limb formation evolved, where the development axis of the limb rotated to sprout secondary axes along the lower margin, giving rise to a variable number of very stout skeletal supports for a paddle-like foot.

Digit specialization
Digits may be specialized for different forms of locomotion. A classic example is the horse's development of a single toe (monodactyly). Other hooves, like those of even-toed and odd-toed ungulates, and even the hoof-like foot of extinct hadrosaurs, may be regarded as similar specializations.

To bear their immense weight, sauropods, the most derived being titanosaurs, developed a tubular manus (front foot) and gradually lost their digits, standing on their metacarpals. The stegosaurian forelimb has evidence for a sauropod−like metacarpal configuration This was a different evolutionary strategy than megafaunal mammals such as modern elephants.

Therapsids started evolving diverse and specialized forelimbs 270 million years ago, during the Permian.

Opposable thumbs
Modern humans are unique in the musculature of the forearm and hand, though opposable thumbs or structures like them have arisen in a few animals.

In dinosaurs, a primitive autonomization of the first carpometacarpal joint (CMC) may have occurred. In primates, a real differentiation appeared perhaps 70 mya, while the shape of the human thumb CMC finally appears about 5 mya.


 * Primates fall into one of four groups:
 * Nonopposable thumbs: tarsiers and marmosets
 * Pseudo-opposable thumbs: all strepsirrhines and Cebidae
 * Opposable thumbs: Old World monkeys and all great apes
 * Opposable with comparatively long thumbs: gibbons (or lesser apes)

Pandas have evolved pseudo-opposable thumbs by extension of the sesamoid bone, which is not a true digit.

Pronation and supination
The ability to pronate the manus (hand) and forearm in therian mammals is achieved by a rounded head of the radius, which allows it to swivel across the ulna. Supination requires a dorsal glide of the distal radius and pronation a palmar glide in relation to the distal ulna.

Pronation has evolved multiple times, among mammals, chameleons, and varanids. However, the more basal condition is to be unable to pronate. Dinosaurs were not capable of more than semi-pronation of the wrist, though bipedal origins of all quadrupedal dinosaur clades could have allowed for greater disparity in forelimb posture than often considered. Monotremes have forearms that are not as dexterous as therians. Monotremes have a sprawling posture, and multiple elements in their pectoral girdles, which are ancestral traits for mammals.

In birds, the forearm muscles supinate, pronate, flex and extend the distal wing.

Wings
All tetrapod forelimbs are homologous, evolving from the same initial structures in lobe-finned fish. However, another distinct process may be identified, analog y, by which the wings of birds, bats, and extinct pterosaurs evolved the same purpose in drastically different ways. These structures have similar form or function but were not present in the last common ancestor of those groups.

Bat wings are composed largely of a thin membrane of skin supported on the five fingers, whereas bird wings are composed largely of feathers supported on much reduced fingers, with finger 2 supporting the alula and finger 4 the primary feathers of the wing; there are only distant homologies between birds and bats, with much closer homologies between any pair of bird species, or any pair of bat species.