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87-474: Toe bones or phalanges of the foot. Note the big toe has no middle phalanx. People vary; sometimes the smallest toe also has none (not shown). The phalanges are the bones that make up the fingers of the hand and the toes of the foot. There are 56 phalanges in the human body, with fourteen on each hand and foot. Three phalanges are present on each finger and toe, with the exception of the thumb and big toe , which possess only two. The middle and far phalanges of

174-430: A 2-3-3-3-3 formula in both the hands (or paws ) and feet . Primitive reptiles usually had the formula 2-3-4-4-5, and this pattern, with some modification, remained in many later reptiles and in the mammal-like reptiles . The phalangeal formula in the flippers of cetaceans (marine mammals) varies widely due to hyperphalangy (the increase in number of phalanx bones in the digits). In humpback whales , for example,

261-552: A clade that is closely related to the perissodactyls. The desmostylians were large amphibious quadrupeds with massive limbs and a short tail. They grew to 1.8 metres (6 ft) in length and were thought to have weighed more than 200 kilograms (440 lb). Their fossils were known from the northern Pacific Rim , from southern Japan through Russia , the Aleutian Islands and the Pacific coast of North America to

348-439: A distant clade Afrotheria . Living ungulates are divided into two orders: Perissodactyla including equines , rhinoceroses , and tapirs ; and Artiodactyla including cattle , antelope , pigs , giraffes , camels , sheep , deer , and hippopotamuses , among others. Cetaceans such as whales , dolphins , and porpoises are also classified as artiodactyls, although they do not have hooves. Most terrestrial ungulates use

435-473: A flagpole; tension from these muscular guy-wires must be provided in all directions to maintain stability in the articulated column formed by the bones of the thumb. Because this stability is actively maintained by muscles rather than by articular constraints, most muscles attached to the thumb tend to be active during most thumb motions. The muscles acting on the thumb can be divided into two groups: The extrinsic hand muscles, with their muscle bellies located in

522-524: A forerunner of Homo sapiens . This, however, is the suggested result of evolution from Homo erectus (around 1 mya ) via a series of intermediate anthropoid stages, and is therefore a much more complicated link. Modern humans are unique in the musculature of their forearm and hand. Yet, they remain autapomorphic, meaning each muscle is found in one or more non-human primates. The extensor pollicis brevis and flexor pollicis longus allow modern humans to have great manipulative skills and strong flexion in

609-439: A rotary movement is referred to as circumduction . Primatologists and hand research pioneers John and Prudence Napier defined opposition as: "A movement by which the pulp surface of the thumb is placed squarely in contact with – or diametrically opposite to – the terminal pads of one or all of the remaining fingers." For this true , pulp-to-pulp opposition to be possible, the thumb must rotate about its long axis (at

696-425: A terrestrial lifestyle has led to reduced finger length and thus hand proportions similar to those of humans. Consequently, these primates have dexterous hands and are able to grasp objects using a pad-to-pad grip. It can thus be difficult to identify hand adaptations to manipulation-related tasks based solely on thumb proportions. The evolution of the fully opposable thumb is usually associated with Homo habilis ,

783-458: Is a cladistic (evolution-based) group, or merely a phenetic group ( form taxon ) or folk taxon (similar, but not necessarily related). Some studies have indeed found the mesaxonian ungulates and paraxonian ungulates to form a monophyletic lineage, closely related to either the Ferae (the carnivorans and the pangolins ) in the clade Fereuungulata or to the bats . Other studies found

870-547: Is a human characteristic. Darwinius masillae , an Eocene primate transitional fossil between prosimian and simian , had hands and feet with highly flexible digits featuring opposable thumbs and halluces. Additionally, in many polydactyl cats , both the innermost toe and outermost toe ( pinky ) may become opposable, allowing the cat to perform more complex tasks. In addition to these, some other dinosaurs may have had partially or completely opposed toes in order to manipulate food and/or grasp prey. The skeleton of

957-527: Is an increase in the number of phalanges beyond the plesiomorphic mammal condition of three phalanges-per-digit. Hyperphalangy was present among extinct marine reptiles -- ichthyosaurs , plesiosaurs , and mosasaurs -- but not other marine mammals, leaving whales as the only marine mammals to develop this characteristic. The evolutionary process continued over time, and a very derived form of hyperphalangy, with six or more phalanges per digit, evolved convergently in rorqual whales and oceanic dolphins , and

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1044-430: Is concave, and the head presents a trochlear surface for articulation with the second phalanx. The middle are remarkably small and short, but rather broader than the proximal. The distal phalanges, as compared with the distal phalanges of the finger, are smaller and are flattened from above downward; each presents a broad base for articulation with the corresponding bone of the second row, and an expanded distal extremity for

1131-399: Is largest proximally and tapers to the distal end. The proximal part of the phalanx presents a broad base for articulation with the middle phalanx, and an expanded distal extremity for the support of the nail and end of the toe. The phalanx ends in a crescent-shaped rough cap of bone epiphysis  — the apical tuft (or ungual tuberosity/process) which covers a larger portion of the phalanx on

1218-457: Is the first digit of the hand , next to the index finger . When a person is standing in the medical anatomical position (where the palm is facing to the front), the thumb is the outermost digit. The Medical Latin English noun for thumb is pollex (compare hallux for big toe), and the corresponding adjective for thumb is pollical . The English word finger has two senses , even in

1305-624: The Great American Interchange . Perissodactyls were the dominant group of large terrestrial browsers right through the Oligocene. However, the rise of grasses in the Miocene (about 20 Mya) saw a major change: the artiodactyl species with their more complex stomachs were better able to adapt to a coarse, low-nutrition diet, and soon rose to prominence. Nevertheless, many perissodactyl species survived and prospered until

1392-484: The ankle or foot that allow humans to walk on two legs". Evidence to date shows that of the 110,000 gene enhancer sequences identified in the human genome , HACNS1 has undergone the most change during the human evolution since the chimpanzee–human last common ancestor . Ungulate Ungulates ( / ˈ ʌ ŋ ɡ j ʊ l eɪ t s , - ɡ j ə -, - l ɪ t s , - l ə t s / UNG -gyuu-layts, -⁠gyə-, -⁠lits, -⁠ləts ) are members of

1479-406: The bi-modality seen in other recessive genetic traits. Other variations of the thumb include brachydactyly type D (which is a thumb with a congenitally short distal phalanx), a triphalangeal thumb (which is a thumb which has 3 phalanges instead of the usual two), and polysyndactyly (which is a combination of radial polydactyly and syndactyly). One of the earlier significant contributors to

1566-414: The carpometacarpal joint ). Arguably, this definition was chosen to underline what is unique to the human thumb. The spider monkey compensates for being virtually thumbless by using the hairless part of its long, prehensile tail for grabbing objects. In apes and Old World monkeys , the thumb can be rotated around its axis, but the extensive area of contact between the pulps of the thumb and index finger

1653-405: The deep branch of the radial nerve (C7-T1). The tendons of the extensor pollicis longus and extensor pollicis brevis form what is known as the anatomical snuff box (an indentation on the lateral aspect of the thumb at its base) The radial artery can be palpated anteriorly at the wrist (not in the snuffbox). There are three thenar muscles : The abductor pollicis brevis (APB) originates on

1740-424: The flexor pollicis longus (asymmetric towards the radial side), an ungual fossa, and a pair of unequal ungual spines (the ulnar being more prominent). This asymmetry is necessary to ensure that the thumb pulp is always facing the pulps of the other digits, an osteological configuration which provides the maximum contact surface with held objects. In the foot, the distal phalanges are flat on their dorsal surface. It

1827-469: The fourth and fifth toes are often fused together (symphalangism). The phalanges of the hand are commonly known as the finger bones. The phalanges of the foot differ from the hand in that they are often shorter and more compressed, especially in the proximal phalanges, those closest to the torso. A phalanx is named according to whether it is proximal , middle, or distal and its associated finger or toe. The proximal phalanges are those that are closest to

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1914-519: The mesonychians and the artiodactyls. The first artiodactyls looked like today's chevrotains or pigs: small, short-legged creatures that ate leaves and the soft parts of plants . By the Late Eocene (46 million years ago), the three modern suborders had already developed: Suina (the pig group); Tylopoda (the camel group); and Ruminantia (the goat and cattle group). Nevertheless, artiodactyls were far from dominant at that time:

2001-415: The scaphoid tubercle and the flexor retinaculum . It inserts to the radial sesamoid bone and the proximal phalanx of the thumb. It is innervated by the median nerve (C8-T1). The flexor pollicis brevis (FPB) has two heads. The superficial head arises on the flexor retinaculum, while the deep head originates on three carpal bones: the trapezium , trapezoid , and capitate . The muscle is inserted onto

2088-400: The "hands" — the metacarpal and phalangeal bones — are elongated to the extent that they serve little use beyond locomotion. The giraffe , the largest even-toed ungulate, has large terminal phalanges and fused metacarpal bones able to absorb the stress from running. The sloth spends its life hanging upside-down from branches, and has highly specialized third and fourth digits for

2175-588: The anatomical basis of hand movement. Napier proposed two primary prehensile grips: the precision grip and the power grip . The precision and power grip are defined by the position of the thumb and fingers where: Opposability of the thumb should not be confused with a precision grip as some animals possess semi-opposable thumbs yet are known to have extensive precision grips ( Tufted Capuchins for example). Nevertheless, precision grips are usually only found in higher apes, and only in degrees significantly more restricted than in humans. The pad-to-pad pinch between

2262-513: The anthracotheres, except that which evolved into Hippopotamidae , became extinct during the Pliocene without leaving any descendants. The family Raoellidae is said to be the closest artiodactyl family to the cetaceans. Consequentially, new theories in cetacean evolution hypothesize that whales and their ancestors escaped predation, not competition, by slowly adapting to the ocean. Mesonychians were depicted as "wolves on hooves" and were

2349-433: The apical tufts vary in size, but they are never larger than in humans. Enlarged apical tufts, to the extent they actually reflect expanded digital pulps, may have played a significant role in enhancing friction between the hand and held objects during Neolithic toolmaking. Among non-human primates phylogenesis and style of locomotion appear to play a role in apical tuft size. Suspensory primates and New World monkeys have

2436-441: The base of the distal phalanx of the thumb. It is innervated by the anterior interosseus branch of the median nerve (C7-C8) It is a persistence of one of the former contrahentes muscles that pulled the fingers or toes together. Three dorsal forearm muscles act on the thumb: The abductor pollicis longus (APL) originates on the dorsal sides of both the ulna and the radius, and from the interosseous membrane. Passing through

2523-418: The bones at the tips of the fingers or toes. The proximal, intermediate, and distal phalanges articulate with one another through interphalangeal joints of hand and interphalangeal joints of the foot . Each phalanx consists of a central part, called the body , and two extremities. In the foot, the proximal phalanges have a body that is compressed from side to side, convex above, and concave below. The base

2610-1153: The case with the relationship between the pecoran families and the baleen whale families. See each family for the relationships of the species as well as the controversies in their respective articles. Below is the general consensus of the phylogeny of the ungulate families. Equidae [REDACTED] Tapiridae [REDACTED] Rhinocerotidae [REDACTED] Camelidae [REDACTED] Tayassuidae [REDACTED] Suidae [REDACTED] Tragulidae [REDACTED] Antilocapridae [REDACTED] Giraffidae [REDACTED] Cervidae [REDACTED] Moschidae [REDACTED] Bovidae [REDACTED] Hippopotamidae [REDACTED] Balaenidae [REDACTED] Cetotheriidae [REDACTED] Balaenopteridae [REDACTED] Physeteridae [REDACTED] Kogiidae [REDACTED] Platanistidae [REDACTED] Ziphiidae [REDACTED] † Lipotidae [REDACTED] Pontoporiidae [REDACTED] Iniidae [REDACTED] Delphinidae [REDACTED] Phocoenidae [REDACTED] Monodontidae [REDACTED] Perissodactyla and Artiodactyla include

2697-683: The close of the Eocene epoch, with only one genus, Mongolestes , surviving into the Early Oligocene epoch, as the climate changed and fierce competition arose from the better adapted creodonts . Ungulates were in high diversity in response to sexual selection and ecological events; most ungulates lack a collar bone . Terrestrial ungulates were for the most part herbivores, with some of them being grazers . However, there were exceptions to this as pigs, peccaries, hippos and duikers were known to have an omnivorous diet. Some cetaceans were

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2784-606: The context of appendages of a single typical human hand: 1) Any of the five terminal members of the hand. 2) Any of the four terminal members of the hand, other than the thumb. Linguistically, it appears that the original sense was the first of these two: * penkwe-ros (also rendered as * penqrós ) was, in the inferred Proto-Indo-European language , a suffixed form of * penkwe (or * penqe ), which has given rise to many Indo-European -family words (tens of them defined in English dictionaries) that involve, or stem from, concepts of fiveness. The thumb shares

2871-562: The derived elongated hand pattern and poorly developed thumb musculature of other extant hominoids . In Neanderthals , the apical tufts were expanded and more robust than in modern and early upper Paleolithic humans. A proposal that Neanderthal distal phalanges was an adaptation to colder climate (than in Africa) is not supported by a recent comparison showing that in hominins , cold-adapted populations possessed smaller apical tufts than do warm-adapted populations. In non-human, living primates

2958-454: The distal phalanges of human thumbs closely reflects an adaptation for a refined precision grip with pad-to-pad contact. This has traditionally been associated with the advent of stone tool-making. However, the intrinsic hand proportions of australopiths and the resemblance between human hands and the short hands of Miocene apes, suggest that human hand proportions are largely plesiomorphic (as found in ancestral species) — in contrast to

3045-465: The diverse clade Euungulata ("true ungulates"), which primarily consists of large mammals with hooves . Once part of the clade "Ungulata" along with the clade Paenungulata , "Ungulata" has since been determined to be a polyphyletic and thereby invalid clade based on molecular data. As a result, true ungulates had since been reclassified to the newer clade Euungulata in 2001 within the clade Laurasiatheria while Paenungulata has been reclassified to

3132-446: The dorsal side of the ulna and the interosseous membrane. Passing through the third tendon compartment, it is inserted onto the base of the distal phalanx of the thumb. It uses the dorsal tubercle on the lower extremity of the radius as a fulcrum to extend the thumb and also dorsiflexes and abducts the hand at the wrist. It is innervated by the deep branch of the radial nerve (C7-C8). The extensor pollicis brevis (EPB) originates on

3219-579: The early Eocene. They appeared very similar to modern forms, but were about half the size, and lacked the proboscis. The first true tapirs appeared in the Oligocene . By the Miocene , such genera as Miotapirus were almost indistinguishable from the extant species. Asian and American tapirs were believed to have diverged around 20 to 30 million years ago; and tapirs migrated from North America to South America around 3 million years ago, as part of

3306-588: The early Eocene. Fossils of Hyrachyus eximus found in North America date to this period. This small hornless ancestor resembled a tapir or small horse more than a rhino. Three families, sometimes grouped together as the superfamily Rhinocerotoidea, evolved in the late Eocene: Hyracodontidae , Amynodontidae and Rhinocerotidae , thus creating an explosion of diversity unmatched for a while until environmental changes drastically eliminated several species. The first tapirids, such as Heptodon , appeared in

3393-437: The enormous brontotheres and the bizarre chalicotheres . The largest perissodactyl, an Asian rhinoceros called Paraceratherium , reached 15 tonnes (17 tons), more than twice the weight of an elephant . It has been found in a cladistic study that the anthracobunids and the desmostylians – two lineages that have been previously classified as Afrotherians (more specifically closer to elephants) – have been classified as

3480-642: The extinct mesonychians are carnivorous. Ungulate is from the Late Latin adjective ungulatus ' hoofed ' . Ungulatus is a diminutive form of Latin unguis ' nail ' (finger nail; toe nail). Euungulata is a clade (or in some taxonomies, a grand order) of mammals. The two extant orders of ungulates are the Perissodactyla (odd-toed ungulates) and Artiodactyla (even-toed ungulates) . Hyracoidea (hyraxes) , Sirenia (sea cows, dugongs and manatees) and Proboscidea (elephants) were in

3567-434: The first and second (proximal and distal) phalanges varies between 0° and almost 90° when the thumb is in a thumbs-up gesture. It has been suggested that the variation is an autosomal recessive trait , called a hitchhiker's thumb , with homozygous carriers having an angle close to 90°. However this theory has been disputed, since the variation in thumb angle is known to fall on a continuum and shows little evidence of

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3654-565: The first major mammalian predators, appearing in the Paleocene. Early mesonychians had five digits on their feet, which probably rested flat on the ground during walking ( plantigrade locomotion), but later mesonychians had four digits that ended in tiny hooves on all of their toes and were increasingly well adapted to running. Like running members of the even-toed ungulates, mesonychians ( Pachyaena , for example) walked on their digits ( digitigrade locomotion). Mesonychians fared very poorly at

3741-400: The first metacarpal. It opposes the thumb and assists in adduction. It is innervated by the median nerve . Other muscles involved are: The adductor pollicis also has two heads. The transversal head originates along the entire third metacarpal bone, while the oblique head originates on the carpal bones proximal to the third metacarpal. The muscle is inserted onto the ulnar sesamoid bone of

3828-444: The first tendon compartment, it inserts to the base of the first metacarpal bone . A part of the tendon reaches the trapezium, while another fuses with the tendons of the extensor pollicis brevis and the abductor pollicis brevis. Except for abducting the hand, it flexes the hand towards the palm and abducts it radially. It is innervated by the deep branch of the radial nerve (C7-C8). The extensor pollicis longus (EPL) originates on

3915-455: The following with each of the other four fingers: The thumb contrasts with each of the other four fingers by being the only one that: and hence the etymology of the word: * tum is Proto-Indo-European for 'swelling' (cf 'tumor' and 'thigh') since the thumb is the stoutest of the fingers. Anatomists and other researchers focused on human anatomy have hundreds of definitions of opposition . Some anatomists restrict opposition to when

4002-447: The forearm, and the intrinsic hand muscles, with their muscle bellies located in the hand proper. A ventral forearm muscle, the flexor pollicis longus (FPL) originates on the anterior side of the radius distal to the radial tuberosity and from the interosseous membrane . It passes through the carpal tunnel in a separate tendon sheath , after which it lies between the heads of the flexor pollicis brevis. It finally attaches onto

4089-422: The grasping hand, the distal pads can therefore conform to uneven surfaces while pressure is distributed more evenly in the finger tips. The distal pad of the human thumb is divided into a proximal and a distal compartment, the former more deformable than the latter, which allows the thumb pad to mold around an object. In robotics, almost all robotic hands have a long and strong opposable thumb. Like human hands,

4176-429: The hand or foot. In the hand, the prominent, knobby ends of the phalanges are known as knuckles . The proximal phalanges join with the metacarpals of the hand or metatarsals of the foot at the metacarpophalangeal joint or metatarsophalangeal joint . The intermediate phalanx is not only intermediate in location, but usually also in size. The thumb and large toe do not possess a middle phalanx. The distal phalanges are

4263-597: The hand the centres for the bodies appear at the distal extremities of the phalanges, instead of at the middle of the bodies, as in the other phalanges. Moreover, of all the bones of the hand, the distal phalanges are the first to ossify. The distal phalanges of ungulates carry and shape nails and claws and these in primates are referred to as the ungual phalanges . The term phalanx or phalanges refers to an ancient Greek army formation in which soldiers stand side by side, several rows deep, like an arrangement of fingers or toes. Most land mammals including humans have

4350-413: The hind feet, and four on the front feet. They were herbivorous browsers on relatively soft plants, and were already adapted for running. The complexity of their brains suggest that they already were alert and intelligent animals. Later species reduced the number of toes, and developed teeth more suited for grinding up grass and other tough plant food. Rhinocerotoids diverged from other perissodactyls by

4437-464: The hoofed tips of their toes to support their body weight while standing or moving. Two other orders of ungulates, Notoungulata and Litopterna , both native to South America, became extinct at the end of the Pleistocene, around 12,000 years ago. The term means, roughly, "being hoofed" or "hoofed animal". As a descriptive term, "ungulate" normally excludes cetaceans as they do not possess most of

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4524-869: The late Pleistocene (about 10,000 years ago) when they faced the pressure of human hunting and habitat change. The artiodactyls were thought to have evolved from a small group of condylarths, Arctocyonidae , which were unspecialized, superficially raccoon-like to bear-like omnivores from the Early Paleocene (about 65 to 60 million years ago). They had relatively short limbs lacking specializations associated with their relatives (e.g. reduced side digits, fused bones, and hooves), and long, heavy tails. Their primitive anatomy makes it unlikely that they were able to run down prey, but with their powerful proportions, claws, and long canines, they may have been able to overpower smaller animals in surprise attacks. Evidently these mammals soon evolved into two separate lineages:

4611-400: The majority of large land mammals. These two groups first appeared during the late Paleocene , rapidly spreading to a wide variety of species on numerous continents, and have developed in parallel since that time. Some scientists believed that modern ungulates were descended from an evolutionary grade of mammals known as the condylarths . The earliest known member of this group may have been

4698-410: The metacarpophalangeal joint. It adducts the thumb, and assists in opposition and flexion. It is innervated by the deep branch of the ulnar nerve (C8-T1). The first dorsal interosseous , one of the central muscles of the hand, extends from the base of the thumb metacarpal to the radial side of the proximal phalanx of the index finger. There is a variation of the human thumb where the angle between

4785-591: The most recent study recovers them as within the true ungulate assemblage, closest to Carodnia . In Australia, the recently-extinct marsupial Chaeropus ("pig-footed bandicoot") also developed hooves similar to those of artiodactyls, an example of convergent evolution . Perissodactyls were thought to have evolved from the Phenacodontidae , small, sheep-sized animals that were already showing signs of anatomical features that their descendants would inherit (the reduction of digit I and V for example). By

4872-446: The motor sequencing of the thumb. A primitive autonomization of the first carpometacarpal joint (CMC) may have occurred in dinosaurs. A real differentiation appeared an estimated 70 mya in early primates, while the shape of the human thumb CMC finally appears about 5 mya. The result of this evolutionary process is a human CMC joint positioned at 80° of pronation, 40 of abduction, and 50° of flexion in relation to an axis passing through

4959-437: The notoungulates were closely related to the perissodactyls. The oldest known fossils assigned to Equidae date from the early Eocene , 54 million years ago. They had been assigned to the genus Hyracotherium , but the type species of that genus is now considered not a member of this family, but the other species have been split off into different genera. These early Equidae were fox-sized animals with three toes on

5046-403: The only modern ungulates that were carnivores; baleen whales consume significantly smaller animals in relation to their body size, such as small species of fish and krill ; toothed whales, depending on the species, can consume a wide range of species: squid , fish, sharks , and other species of mammals such as seals and other whales. In terms of ecosystem ungulates have colonized all corners of

5133-589: The past grouped within the clade "Ungulata", later found to be a polyphyletic and now invalid clade. The three orders of Paenungulata are now considered a clade and grouped in the Afrotheria clade, while Euungulata is now grouped under the Laurasiatheria clade. In 2009, morphological and molecular work found that aardvarks, hyraxes, sea cows, and elephants were more closely related to each other and to sengis , tenrecs , and golden moles than to

5220-413: The perissodactyls and artiodactyls, and form the clade Afrotheria . Elephants, sea cows, and hyraxes were grouped together in the clade Paenungulata , while the aardvark has been considered as either a close relative to them or a close relative to sengis in the clade Afroinsectiphilia . This is a striking example of convergent evolution . There is now some dispute as to whether this smaller Euungulata

5307-437: The perissodactyls were much more successful and far more numerous. Artiodactyls survived in niche roles, usually occupying marginal habitats , and it is presumably at that time that they developed their complex digestive systems , which allowed them to survive on lower-grade food. While most artiodactyls were taking over the niches left behind by several extinct perissodactyls, one lineage of artiodactyls began to venture out into

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5394-534: The phalangeal formula is 0/2/7/7/3; in pilot whales the formula is 1/10/7/2/1. In vertebrates, proximal phalanges have a similar placement in the corresponding limbs, be they paw , wing or fin . In many species, they are the longest and thickest phalanx ("finger" bone). The middle phalanx also has a corresponding place in their limbs, whether they be paw , wing , hoof or fin . The distal phalanges are cone-shaped in most mammals, including most primates, but relatively wide and flat in humans. The morphology of

5481-419: The proto-whale Pakicetus and other early cetacean ancestors collectively known as Archaeoceti , which eventually underwent aquatic adaptation into the completely aquatic cetaceans . The other branch became the anthracotheres , a large family of four-legged beasts, the earliest of whom in the late Eocene would have resembled skinny hippopotamuses with comparatively small and narrow heads. All branches of

5568-404: The purpose. They have short and squat proximal phalanges with much longer terminal phalanges. They have vestigial second and fifth metacarpals, and their palm extends to the distal interphalangeal joints . The arboreal specialization of these terminal phalanges makes it impossible for the sloth to walk on the ground where the animal has to drag its body with its claws. Thumb The thumb

5655-413: The radial sesamoid bone of the metacarpophalangeal joint. It acts to flex, adduct, and abduct the thumb, and is therefore also able to oppose the thumb. The superficial head is innervated by the median nerve , while the deep head is innervated by the ulnar nerve (C8-T1). The opponens pollicis originates on the tubercle of the trapezium and the flexor retinaculum. It is inserted onto the radial side of

5742-587: The seas. The traditional theory of cetacean evolution was that cetaceans were related to the mesonychian . These animals had unusual triangular teeth very similar to those of primitive cetaceans. This is why scientists long believed that cetaceans evolved from a form of mesonychian. Today, many scientists believe cetaceans evolved from the same stock that gave rise to hippopotamuses. This hypothesized ancestral group likely split into two branches around 54 million years ago . One branch would evolve into cetaceans , possibly beginning about 52 million years ago with

5829-434: The second and third CMC joints. Opposable thumbs are shared by some primates , including most catarrhines . The climbing and suspensory behaviour in orthograde apes, such as chimpanzees , has resulted in elongated hands while the thumb has remained short. As a result, these primates are unable to perform the pad-to-pad grip associated with opposability. However, in pronograde monkeys such as baboons , an adaptation to

5916-594: The smallest apical tufts, while terrestrial quadrupeds and Strepsirrhines have the largest. A study of the fingertip morphology of four small-bodied New World monkey species, indicated a correlation between increasing small-branch foraging and reduced flexor and extensor tubercles in distal phalanges and broadened distal parts of distal phalanges, coupled with expanded apical pads and developed epidermal ridges. This suggests that widened distal phalanges were developed in arboreal primates, rather than in quadrupedal terrestrial primates. Whales exhibit hyperphalangy. Hyperphalangy

6003-625: The somewhat tapir-like pyrotheres and astrapotheres , the mesaxonic litopterns and the diverse notoungulates . As a whole, meridiungulates were said to have evolved from animals like Hyopsodus . For a while their relationships with other ungulates were a mystery. Some paleontologists have even challenged the monophyly of Meridiungulata by suggesting that the pyrotheres may be more closely related to other mammals, such as Embrithopoda (an African order that were related to elephants ) than to other South American ungulates. A recent study based on bone collagen has found that at least litopterns and

6090-469: The southern tip of Baja California . Their dental and skeletal form suggests desmostylians were aquatic herbivores dependent on littoral habitats. Their name refers to their highly distinctive molars, in which each cusp was modified into hollow columns, so that a typical molar would have resembled a cluster of pipes, or in the case of worn molars, volcanoes. They were the only marine mammals to have gone extinct. The South American meridiungulates contain

6177-645: The start of the Eocene , 55 million years ago (Mya), they had diversified and spread out to occupy several continents. Horses and tapirs both evolved in North America; rhinoceroses appear to have developed in Asia from tapir-like animals and then colonised the Americas during the middle Eocene (about 45 Mya). Of the approximately 15 families, only three survive (McKenna and Bell, 1997; Hooker, 2005). These families were very diverse in form and size; they included

6264-412: The study of hand grips was orthopedic primatologist and paleoanthropologist John Napier , who proposed organizing the movements of the hand by their anatomical basis as opposed to work done earlier that had only used arbitrary classification. Most of this early work on hand grips had a pragmatic basis as it was intended to narrowly define compensable injuries to the hand, which required an understanding of

6351-420: The support of the nail and end of the toe. In the hand, the distal phalanges are flat on their palmar surface, small, and with a roughened, elevated surface of horseshoe form on the palmar surface, supporting the finger pulp. The flat, wide expansions found at the tips of the distal phalanges are called apical tufts. They support the fingertip pads and nails. The phalanx of the thumb has a pronounced insertion for

6438-417: The thumb and index finger is made possible because of the human ability to passively hyperextend the distal phalanx of the index finger. Most non-human primates have to flex their long fingers in order for the small thumb to reach them. In humans, the distal pads are wider than in other primates because the soft tissues of the finger tip are attached to a horseshoe-shaped edge on the underlying bone, and, in

6525-443: The thumb consists of the first metacarpal bone which articulates proximally with the carpus at the carpometacarpal joint and distally with the proximal phalanx at the metacarpophalangeal joint . This latter bone articulates with the distal phalanx at the interphalangeal joint . Additionally, there are two sesamoid bones at the metacarpophalangeal joint. The muscles of the thumb can be compared to guy-wires supporting

6612-399: The thumb is approximated to the fifth finger (little finger) and refer to other approximations between the thumb and other fingers as apposition . To anatomists, this makes sense as two intrinsic hand muscles are named for this specific movement (the opponens pollicis and opponens digiti minimi respectively). Other researchers use another definition, referring to opposition-apposition as

6699-467: The thumb of a robotic hand also plays a key role in gripping an object. One inspiring approach of robotic grip planning is to mimic human thumb placement. In a sense, human thumb placement indicates which surface or part of the object is good for grip. Then the robot places its thumb to the same location and plans the other fingers based on the thumb placement. The function of the thumb declines physiologically with aging. This can be demonstrated by assessing

6786-466: The thumb. However, a more likely scenario may be that the specialized precision gripping hand (equipped with opposable thumb) of Homo habilis preceded walking, with the specialized adaptation of the spine, pelvis, and lower extremities preceding a more advanced hand. And, it is logical that a conservative, highly functional adaptation be followed by a series of more complex ones that complement it. With Homo habilis , an advanced grasping-capable hand

6873-447: The tiny Protungulatum , a mammal that co-existed with the last of non-avian dinosaurs 66 million years ago. However, many authorities do not consider it a true placental, let alone an ungulate. The enigmatic dinoceratans were among the first large herbivorous mammals, although their exact relationship with other mammals is still debated with one of the theories being that they might just be distant relatives to living ungulates;

6960-403: The transition between flexion-abduction and extension-adduction; the side of the distal thumb phalanx thus approximated to the palm or the hand's radial side (side of index finger) during apposition and the pulp or "palmar" side of the distal thumb phalanx approximated to either the palm or other fingers during opposition . Moving a limb back to its neutral position is called reposition and

7047-472: The two orders not that closely related, as some place the perissodactyls as close relatives to bats and Ferae in Pegasoferae and others place the artiodactyls as close relatives to bats. Below is a simplified taxonomy (assuming that ungulates do indeed form a natural grouping) with the extant families, in order of the relationships. Keep in mind that there were still some grey areas of conflict, such as

7134-403: The typical morphological characteristics of other ungulates, but recent discoveries indicate that they were also descended from early artiodactyls . Ungulates are typically herbivorous and many employ specialized gut bacteria to enable them to digest cellulose, though some members may deviate from this: several species of pigs and the extinct entelodonts are omnivorous, while cetaceans and

7221-399: The ulna distal to the abductor pollicis longus, from the interosseus membrane, and from the dorsal side of the radius. Passing through the first tendon compartment together with the abductor pollicis longus, it is attached to the base of the proximal phalanx of the thumb. It extends the thumb and, because of its close relationship to the long abductor, also abducts the thumb. It is innervated by

7308-421: The ungual fossa distally and the proximopalmar fossa proximally. The number of phalanges in animals is often expressed as a "phalangeal formula" that indicates the numbers of phalanges in digits, beginning from the innermost medial or proximal. For example, humans have a 2-3-3-3-3 formula for the hand , meaning that the thumb has two phalanges, whilst the other fingers each have three. In the distal phalanges of

7395-405: The volar side than on the dorsal side. Two lateral ungual spines project proximally from the apical tuft. Near the base of the shaft are two lateral tubercles. Between these a V-shaped ridge extending proximally serves for the insertion of the flexor pollicis longus . Another ridge at the base serves for the insertion of the extensor aponeurosis . The flexor insertion is sided by two fossae  —

7482-465: Was accompanied by facultative bipedalism , possibly implying, assuming a co-opted evolutionary relationship exists, that the latter resulted from the former as obligate bipedalism was yet to follow. Walking may have been a by-product of busy hands and not vice versa. HACNS1 (also known as Human Accelerated Region 2) is a gene enhancer "that may have contributed to the evolution of the uniquely opposable human thumb, and possibly also modifications in

7569-440: Was likely associated with another wave of signaling within the interdigital tissues. In ungulates (hoofed mammals) the forelimb is optimized for speed and endurance by a combination of length of stride and rapid step; the proximal forelimb segments are short with large muscles, while the distal segments are elongated with less musculature. In two of the major groups of ungulates, odd-toed and even-toed ungulates, what remain of

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