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A forelimb or front limb is one of the paired joint (limbs) attached on the cranial (anterior) end of a terrestrial tetrapod vertebrate's torso. With reference to , the term foreleg or front leg is often used instead. In bipedal animals with an upright posture (e.g. humans and some other ), the term upper limb is often used.
A forelimb is not to be confused with a forearm, which is a distal portion of the human upper limb between the elbow and the wrist.
All vertebrate forelimbs are homologous, meaning that they all evolved from the same structures. For example, the flipper of a turtle or of a dolphin, the arm of a human, the foreleg of a horse, and the Bird wing of both bats and birds are ultimately homologous, despite the large differences between them.
Specific uses of the forelimbs may be analogous if they evolved from different sub-structures of the forelimb, such as the flippers of turtles and dolphins, and the wings of birds and bats.
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).
Mustelidae carnivorans that have an arboreal lifestyle tend to have long and slender forelimb long bones, which allow for improved movement and flexibility. Semi-fossorial and aquatic musteloid species tend to have short and robust forelimb long bones to deal with the strain from digging and swimming.
In the order Carnivora, felids, which usually ambush and grapple with their prey, have shorter and more robust limbs. Their forelimbs are used for both short sprints and grappling, which means that they need to be flexible and durable. In contrast, canids, which often pursue their prey over greater distances, have longer, more gracile limbs. Running is pretty much the only use for their forelimbs, so they do not need to be adapted for anything else and can be less flexible.
Predators hunting prey that is half their body weight or greater evolved shorter and more sturdy radii, ulnas, and humeri to decrease the likelihood of the bone breaking or fracturing while hunting. Predators hunting prey less than half their body weight tended to have longer and more slender forelimb long bones to improve energetic efficiency.
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 . 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.
To bear their immense weight, Sauropoda, the most derived being Titanosauria, 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.
started evolving diverse and specialized forelimbs 270 million years ago, during the Permian.
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.
Pandas have evolved pseudo-opposable thumbs by extension of the sesamoid bone, which is not a true digit.
Pronation has evolved multiple times, among , , and Varanidae. However, the more basal condition is to be unable to pronate. 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. 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.
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.
When comparing cetaceans to pinnipeds to sirenians, 133 parallel amino acid substitutions occur. Comparing and contrasting cetaceans-pinnipeds, cetaceans-sirenians, and pinnipeds-sirenians, 2,351, 7,684, and 2,579 substitutions occur, respectively.
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