How does vestigial structures support the theory of evolution

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Another example would be how human ancestors had tails before and since we longer needed them, over time, while in the fetus, the tail is absorbed by the body yes we have tails when we are a fetus. How do vestigial structures relate to evolution? May 12, Explanation: Vestigial structures are structures that are no longer useful for the current organism but it was of some use for its ancestors. Even if fetal blood from the right atrium escapes passage through the foramen ovale into the left atrium and travels, as in postnatal mammals, to the right ventricle, it cannot go to the lungs because they are not yet inflated.

The ductus arteriosus AKA ductus Botalli , a bypass from the pulmonary trunk to the aorta i. This remnant, which connects the pulmonary trunk and aorta, can be found in all mammals. The ductus venosus of fetal mammals shunts oxygenated blood returning from the placenta via the umbilical vein away from the liver and instead directly into the inferior vena cava.

This oxygenated blood serves the developing brain and other organs via the systemic circulation. The ductus venosus closes shortly after birth functionally, with full structural closure occurring days later ; its fibrous remnant is known as the ligamentum venosum. This is often continuous with the round ligament of the liver, also known as the ligamentum teres hepatis, described below.

As noted, the umbilical vein carries oxygenated blood from the placenta. It is open at birth but closes within a week. During the time it is open it can be catheterized, allowing for delivery of drugs or for blood transfusion. Within days after closure the umbilical vein is mostly obliterated except for a fibrous portion on the abdominal wall that remains as the ligamentum teres hepatis. Curiously, this may reopen in adults with extreme hypertension although this is debatable; Lafortune et al.

Umbilical arteries carry deoxygenated fetal blood back to the placenta, and as such are unnecessary after birth. Umbilical arteries give way to a functional section of the internal iliac arteries and superior vesicular artery that delivers blood to the dorsal portion of the urinary bladder, as well as a non-functional, obliterated portion which led to the umbilical cord that becomes the medial umbilical ligament, not to be confused with the median umbilical ligament, a remnant of the urinary urachus, itself an embryonic remnant of the allantois.

The carotid duct or ductus caroticus remains in some postnatal vertebrates lungfishes and some amphibians and reptiles as a remnant of the original embryonic connection between the third and fourth aortic arches. This occurs because although the sex of a developing embryo is determined at conception in humans, as in other mammals, due to the presence of an X or Y chromosome in the sperm , this sex does not become phenotypically manifested until much later in development.

Often, leftovers of earlier stages can be seen in adults. Homologies of shared urogenital structures occur because sexual development typically occurs along a default female track that, in the absence of any outside influence—namely the testis determining factor TDF produced by the sex-determining region of the Y chromosome SRY —turns the indifferent stage into a female embryo.

The mesonephric AKA archinephric or Wolffian and paramesonephric AKA Mullerian ducts of embryogenesis begin developing in both male and female vertebrates, including humans. In males, the Wolffian duct becomes the efferent or deferent duct including the epididymis, vas deferens, and seminal vesicle that stores and transports sperm and seminal fluids.

In females, the Wolffian duct degenerates and remains solely as a withered rudiment. In females, the Mullerian duct becomes the oviduct, including the Fallopian tubes, uterus, cervix, and upper portion of the vagina, whereas in males the Mullerian duct regresses completely or is seen as a small remnant. Many exceptions can found among vertebrate clades in the fate of both Wolffian and Mullerian ducts, particularly teleost fishes, which have novel testicular and ovarian ducts—the story is much abbreviated here for simplicity—but the basic account of one tubule being used as a functional duct in males and the other in females holds true; retention in other sexes occurs solely as non-functional residues.

Sections of the Wolffian archinephric duct normally remain in human females. The Wolffian duct develops in males into the vas deferens. In mammals testes normally descend permanently or temporarily into a scrotal sac for better spermatogenesis, the cooler temperatures of the scrotum allowing for improved production of viable sperm.

This weak spot in the abdominal wall is the cause of inguinal hernias in men. The vas deferens efferent duct takes a long, looping path over the ureter, a developmental holdover of the original descent of the testes into the scrotal pouch. The odd looping pathway the ureter takes is reminiscent of the recurrent laryngeal nerve, a branch of the vagus nerve cranial nerve X , whose unusual circuitous detour on the left side, as it loops under the aorta, was noted in giraffes by anatomist Richard Owen [ ].

These are the first and most important difference in the indifferent gonad. In females, the primary medullary sex cords degenerate, where they may remain as ontogenetic vestiges, and instead secondary sex cords invade the cortex of the gonad and become ovarian follicles.

In all mammals, nipples develop along the milk line of the mammary ridge. Cats, dogs, and other familiar pet or barnyard mammals have abdominal mammaries that develop much more posteriorly inferiorly than in humans. Ungulates have inguinal mammary glands; rodents and pigs develop them along the entire trunk, including both the thorax and abdomen.

In humans the milk lines or mammary ridges develop around the sixth week, and although two nipples usually form, extra supernumerary nipples occur not infrequently in males and females. But why should male mammals have nipples at all? Interestingly, Darwin [ ] perpetuated the myth that male nipples are true evolutionary vestiges that stem from lactation in male as well as female ancestral mammals.

This mistaken notion was based on rumors that circulated with the first, largely erroneous European scientific descriptions of the duck-billed platypus. We now know male monotremes do not produce milk, so male nipples are not in any way an evolutionary holdover. They are, however, a developmental holdover from the early, indifferent stage of embryonic development, when the milk line arises but before the embryo has developed along the default female track or divergent male pathway.

After this switch has occurred, nipples are already present and are not resorbed. Evidently, there is little or no cost to retaining nipples and associated mammary tissue in males, aside, perhaps, from a slight risk of breast cancer.

If there was a true cost to retention of male nipples or selection pressure against their presence, they would likely not be retained. Like the urogenital ducts described above, male nipples are default structures formed earlier in development than the differentiation between males and females. Just as footprints reveal traces of a person or animal that is no longer present, numerous features of organisms reveal historical information about phylogeny and ontogeny. In many cases there are features of organisms that may unlike vestiges retain an original function but nonetheless reveal glimpses of history via comparative cladistic analysis of a species and its lineage Table 1.

Again these can be divided into traces from the past that indicate prior states of evolution or development. On the evolutionary side, these include shared, derived characters synapomorphies like the stereoscopic binocular vision and fingernails of primates, which yield information about the ancestry and evolutionary relationships of this mammalian order when compared to other mammals.

Bilaterally symmetric embryos of echinoderms, which as adults display pentamerous radial symmetry, likewise are relics that disclose secrets of echinoderm origins. Even the possession of a head region at one end rather than in the center of cephalized animals is a form of historical remain: still obviously functional yet a reminder of the past, when heads first arose. In this sense they are indeed faint footprints, but not true vestiges in the proper evolutionary sense.

Other functional remains indicate developmental history. Examples include cranial and other bony sutures that reveal formerly distinct ossification centers. During formation of endochondral cartilage replacement long bones, a growth zone forms between the bony shaft diaphysis and end plate epiphysis.

These elements eventually fuse when growth ends, leaving a tell-tale epiphyseal line where elements were formerly separated. This too is an artifact of ontogeny. Strictly speaking this is a scar: although the umbilical cord has a function, the umbilicus itself never had one, making this a nonfunctional embryonic holdover, unlike the formerly functional circulatory and urogenital remains described earlier.

For example, the vestigial coccyx, a remnant of a now-missing tail, shows how developmental remnants reflect evolutionary history, revealing the complex association that leads to such structures being expressed or not , which is itself an informative aspect of organismal history. To take another example of compound historical clues, apoptosis programmed cell death typically transforms a paddle-like vertebrate limb bud into a hand or foot with distinct, discrete digits.

The webbing between digits of a foot, as in waterfowl like the flightless cormorant, generally reflects the lack of such cell death and thus reveals the largely unaltered embryonic limb bud as a sort of simple ontogenetic artifact, yet one that is still obviously functional as a webbed foot and that bears too on the evolution of the taxon when viewed in comparison with related taxa e.

Consider too that these traits can apparently be shifted on and off—i. What atavisms tell us is that what evolution has put away, development can quickly restore. In many cases it is not easy to disentangle the evolutionary and developmental processes that interact to produce wholesale phenotypic alteration.

The slits themselves are transformed during development, as they have been transformed during evolution, into portions of the inner ear, tonsils, and thymus, all with distinct innervation by various cranial nerves Sadler [ ].

In humans, the first pouch develops into the pharyngotympanic Eustachian tube; the second pouch contributes to the palatine tonsils and middle ear supplied by the facial nerve ; the third into the inferior parathyroid glands and cells of the thymus supplied by the glossopharyngeal nerve ; the fourth into laryngeal musculature and cartilage, as well as the superior parathyroid glands and calcitonin-producing ultimobranchial bodies, which form the parafollicular portion of the thyroid gland; the fifth into part of the thyroid; and the sixth pouch into musculature and cartilage of the larynx.

In short, it is not possible to disconnect ontogenetic from phylogenetic remnants in this instance. Homologies of these jointly conserved evolutionary-developmental structures constitute another type of historical residues.

Similarly, the endostyle—another of the six diagnostic chordate features—is known from its metabolic and iodine-concentrating activities to be homologous to the thyroid gland. This represents another relic of shared ontogenetic and phylogenetic transformations. Likewise all chordates possess, again at some stage of the life cycle, a postanal tail.

Remains of the notochord are retained as the gel-like nucleus pulposus at the center of intervertebral discs. In Ateles spider monkeys , Brachyteles woolly monkeys , and Colobus colobus the thumb is vestigial. Its metacarpal is reduced in diameter and length Fig.

Some specimens retain a single phalanx that is reduced to a nub, while in others the thumb lacks phalanges Tague, ; Tague, In Perodicticus potto potto and Arctocebus angwantibos , the African members of the primate family Lorisidae, the second finger is vestigial, although the thumb is fully expressed. The second metacarpal is reduced in length, its proximal phalanx is reduced in length and diameter, its middle phalanx is reduced to a nub, and its distal phalanx is lost Fig.

In the fleshed-out animal, the second finger is reduced to a short stump in Arctocebus. Reduction of the thumb Fig. In many cases it is miniscule in comparison to the other fingers Figs. This is common in the Muroidea rats, mice, and kin , Dipodidae jerboas, jumping mice, and kin , Gliridae dormice , Heteromyidae kangaroo rats and kin , Octodontidae degus and kin , and Sciuridae squirrels Kingdon, ; Kingdon, ; Garbutt, ; and P Senter, pers.

It is also the case in Chinchilla chinchillas. Because the terminal phalanx is therefore an ungual, the digit does not satisfy the second criterion for vestigiality. Also, in such cases the tip of the thumb is used in opposition to the second finger, to grasp objects, as P Senter has personally observed pers. Thumb of certain rodents A—C , thumb of certain carnivores dewclaw D—F , fibula that is fused to the tibia G—I , second and fifth toes of deer J , and syndactylous second and third toes of marsupials K—L.

Black arrows indicate thumbs. White arrows indicate the distal end of the fibula. In several other rodent families are a plethora of cases in which the thumb is lost or is so reduced that it does not protrude externally and satisfies all three criteria for vestigiality. In the squirrel genus Sciurus is an ambiguous case: the thumb is tipped with a claw, but only the claw protrudes externally. Future studies will be necessary to determine what function, if any, this thumb claw serves.

The thumb is lost in the Caviidae cavies, capybaras, and kin , and the metacarpal is vestigial. It remains only as a tiny, ovoid bone no larger than a distal carpal and usually much smaller Figs. In the Chinchillidae the thumb is lost in Lagidium mountain viscachas and Lagostomus trichodactylus plains viscacha. In the latter two the first metacarpal is vestigial; it is a tiny, transversely flattened ovoid Fig.

In Erethizon dorsatum North American porcupine and Coendou prehensile-tailed porcupines the thumb is vestigial. Its metacarpal is highly reduced, and it retains only one highly reduced phalanx with a variable shape Fig. The genus Hystrix , a member of Hystricidae Old World porcupines is unusual in that different species of one genus exhibit different degrees of thumb reduction Fig.

It retains only one phalanx, which is no larger than the highly reduced metacarpal that is typical for the genus. In the Canidae and Felidae the thumb Figs. It is shorter than the other digits and does not contact the ground. Some authors consider it vestigial e. Also, it retains a claw and therefore does not satisfy the second criterion. Nor does it satisfy the third criterion, because it retains the typical function of a finger with a sharp, curved ungual and claw: prehension.

Its use is an important part of prey capture in felids Londei, , and we have personally observed that domestic dogs use the dewclaw to snag and maintain a grip on objects. Its shortening is therefore not the reduction of an unused organ. We suggest that the functional advantage of the shortening is to keep the claw sharp by preventing wear that would result from contact with the ground.

Indeed, according to P Senter pers. An analogy can therefore be made between the dewclaw and the second toe of dromaeosaurid and troodontid dinosaurs, which was also held clear of the ground Senter, , was used to puncture Fowler et al. A vestigial dewclaw is present in Hyaena striped hyena and brown hyena and Crocuta spotted hyena. In both, the metacarpal is reduced to a small block, the distal phalanx is lost, and the proximal phalanx is reduced.

The proximal phalanx is a small, shapeless lump in Hyaena and a tiny spike in Crocuta Figs. The hyaenid Proteles cristata aardwolf has an unreduced dewclaw Fig.

In the Perissodactyla the first finger and its metacarpal are lost. In Equus horses the second, fourth, and fifth fingers are also lost, as are the first and fifth metacarpals. The second and fourth metacarpals are vestigial. They remain as thin splints that taper to a point without reaching the distal end of the metacarpus Fig.

In the Cetartidoactyla the first finger is lost, and in ruminant cetartiodactyls the second and fifth fingers are reduced Tragulidae [chevrotains] and Cervidae [deer] or lost Antilocapridae [pronghorn], and Giraffidae [giraffes and okapi].

The reduced second and fifth fingers of deer are called dewclaws McBride, ; Elbroch, , and some authors consider them vestigial e. However, they bear hooves and therefore do not meet the second criterion for vestigiality. Nor do they meet the third criterion for vestigiality, because during fast locomotion they make sufficient contact with the ground Elbroch, to exhibit a major function of digits: bodily support.

In the Cervidae deer the second and fifth fingers are not vestigial according to our criteria. However, the second and fifth metacarpals of Cervinae Old World deer are vestigial; they are reduced to proximal splints that resemble the vestigial metacarpals of horses Geist, In the second and fifth metacarpals of Capreolinae New World deer the proximal end is lost, leaving only the distal end, which articulates with the proximal phalanx.

The shaft of each of these metacarpals is vestigial and is reduced to a small splint Geist, Fig. In the Bovidae cattle, antelope, sheep, and goats the second and fifth fingers have only one or two phalanges apiece, and these do not articulate with the rest of the skeleton.

When present, the fifth metacarpal is vestigial. It remains only as a tiny, proximal splint Nickel et al. Vestigial fingers are common in the Odontoceti toothed whales.

In odontocetes other than Physeteroidea sperm whales and kin the thumb is vestigial Figs. In a few species there are some individuals that have two thumb phalanges, but their conspecifics have only one phalanx or none Cooper et al. In several odontocete clades, the fifth finger is also vestigial, retaining one or two pebble-like phalanges or just a reduced metacarpal.

In the latter two clades the fourth finger is also reduced enough to consider vestigial. In addition to the first finger and its metacarpal, the fifth finger and its metacarpal are also lost in the Rhinocerotidae rhinoceroses.

Flower identified a small bone in the wrist of Dicerorhinus sumatrensis Sumatran rhinoceros as a vestigial fifth metacarpal, but it is more likely a sesamoid. It does not articulate with the lateral surface of the fourth metacarpal or the lateral surface of the hamate carpal, as would be expected of a fifth metacarpal.

Rather, it is on the palmar surface of the hamate. We found the homologous bone in the wrist of a specimen of Rhinoceros sondaicus Javan rhinoceros and in photos, supplied by the Museum of Comparative Zoology, of articulated hands of two specimens of R. The bone is small and rounded, is on the palmar side of the hamate, and does not articulate with the lateral surface of the hamate or the fourth metacarpal. These are characteristics that are consistent with a sesamoid but not with a vestigial fifth metacarpal.

It was lost independently four times: in the Antilocapridae, Giraffidae, Camelidae, and Equidae. The fourth finger became vestigial in Chaeropus , and is not vestigial in any other Recent mammal.

It was independently lost twice: in Choloepus and Equus. The fifth finger became vestigial at least seven times: in Priodontes , Dasypus , Tamandua , Globicephalinae, Delphininae, Phocoena , and Inia. Although Choloepus is the closest living relative to Bradypus , it must have lost its fifth finger independently, because the two genera are in different families, and some extinct members of the Megalonychidae which includes Choloepus retained a vestige of the fifth finger P Senter, pers.

The first metacarpal was independently lost four times: in Tubulidentata, Perissodactyla, Camelidae, and Ruminantia. The second metacarpal became vestigial independently at least twice: in Equus and Cervidae. It was lost independently lost four times: in Camelidae, Antilocapridae, Giraffidae, and Bovidae.

The fourth metacarpal became vestigial independently three times: in Chaeropus , Choloepus , and Equus. Ancestrally, the mammalian pelvic girdle consists of three bones: the ilium, ischium, and pubis Figs. All three bones contribute to the acetabulum hip socket. The ilium is attached to the vertebral column, and via this attachment the hindlimb propels the entire vertebral column during locomotion. The vertebrae that contact the ilium are fused to a few more vertebrae posterior to them.

Together this series of fused vertebrae is called the sacrum Figs. The left and right pubes are ventral in location and meet in the midline at a symphysis. The left and right ischia extend posteriorly and do not meet each other.

The pubis and ischium surround an opening called the obturator foramen. In most mammals, by adulthood the three bones of the pelvic girdle have fused together to form a single bone called the coxal bone or innominate.

The pelvic girdle is vestigial in Sirenia. It is extremely reduced in size and has lost contact with the vertebral column Figs. Therefore, none of the three bones is lost. In the dugong the pubis is highly reduced, and the pelvic girdle consists mainly of the ilium and ischium Abel, According to Abel , in Trichechus manatees , the pubis and ilium are both lost or reduced almost unto loss, leaving only the ischium, which retains its original shape.

The pelvic girdle is vestigial in Cetacea whales. It is extremely reduced in size, has lost contact with the vertebral column, and has lost a distinct acetabulum Figs. However, the presence of the acetabulum in mysticetes Struthers, ; Fordyce et al.

Apparently, then, in the evolution of the cetacean pelvis, reduction of the ilium and ischium occurred first, followed by the reduction of the pubis and ilium and subsequently their loss in odontocetes.

Ancestrally, the mammalian hindlimb includes a single bone in the thigh the femur and two in the shank or crus the tibia and fibula, with the fibula the more lateral of the two. The proximal end of the tibia articulates with the femur, and the proximal end of the fibula articulates with a lateral shelf of the tibia. Both the tibia and the fibula articulate distally with the tarsus Figs.

The fibula functions as a strut between the tarsus and the proximal tibia. The shaft of the fibula is fused to the tibia in many small mammals, including Macroscelidea elephant shrews , Tarsiidae tarsiers , Eulipotyphla shrews, moles, hedgehogs, and kin , Lagomorpha rabbits and pikas , and many rodents. In some cases only the distal half of the fibular shaft is fused to the tibia, but in others only a small, proximal portion of the fibula is free of the tibia Figs.

This yields the illusion that most of the fibula has been lost, in which case the remaining portion could be considered vestigial. However, close inspection shows that even in cases with extreme amounts of fusion, the fibula is present for its full length and is a strut between the tarsus and the proximal tibia.

It therefore does not satisfy the first or third criterion for vestigiality. An unambiguously vestigial fibula is present in three extant ungulate taxa: Camelidae camels and kin , Pecora ruminants other than chevrotains , and Equus horses. In Camelidae the shaft and proximal end of the fibula are lost. All that remains is a distal vestige: a block of bone called the malleolar bone or os malleolare, which fits into a cleft in the tibia and articulates with the two proximal tarsal bones Flower, In Pecora the shaft of the fibula is replaced by a ligament, and its proximal and distal extremities remain as vestiges.

The proximal vestige, all that remains of the head of the fibula, is a small spike that is fused to the lateral condyle of the tibia. The distal vestige is a malleolar bone resembling that of camelids Nickel et al.

In Equus the distal half of the fibular shaft is lost. The proximal vestige of the tibia includes the head and a thin rod that represents the remainder of the fibular shaft.

The distal vestige is similar to that of camelids and pecorans but is fused to the tibia Nickel et al. Parts of vestigial fibulae are circled with broken line. Note that the distal fibula is missing from the right crus, on which an arrow indicates the socket for the distal fibula.

A Anterior view B Posterior view. Typically, the bowhead whale Balaena mysticetus retains the femur and tibia; the humpback Megaptera novaeangliae and fin whale Balaenoptera physalus retain only the femur; the minke whale Balaenoptera bonaerensis retains the femur in about one-third of individuals; and the hindlimb is absent in the sei whale Balaenoptera borealis Struthers, ; Hosokawa, ; Omura, Occasional atavistic specimens retain the more distal elements.

For example, Andrews described a humpback whale with an ossified tibia and metatarsal and a cartilaginous femur and tarsus. In the extant Sirenia there is usually no hindlimb. However, an example of an atavistic, diminutive femur has been described in an example of Trichechus manatus West Indian manatee Abel, It is tiny enough to consider vestigial.

The crus was independently lost three times, in the Sirenia, Odontoceti, and Balaenopteridae. The fibula became vestigial independently at least three times: in the Camelidae, Pecora, and Equus. Ancestrally, the mammalian foot has five digits with two phalanges in the first toe and three phalanges in each other toe, and a metatarsus in which all five metatarsals are of similar diameter Fig. The metatarsals function as struts between the phalanges and the tarsus.

In numerous mammalian taxa one or more toes have become vestigial Fig. In numerous others, one or more toes are lost and the associated metatarsals are reduced to a vestigial state.

We did not find examples of identifiably vestigial tarsal bones. In marsupials of the orders Peramelemorphia bandicoots and bilbies and Diprotodontia wombats, possums, kangaroos, and kin , the second and third toes are syndactylous.

The skeletons of the two digits share a common sheath of soft tissue, so that the two toes are separate only at the last phalanx, which bears the claw. The two toes are therefore functionally a single toe with two claws. The metatarsals and phalanges of the two toes are usually half or less the diameter of those of the fourth toe, and their reduction in size makes them appear vestigial Figs.

However, the two digits do not meet the second criterion for vestigiality, because they bear claws. They also fail to meet the third criterion for vestigiality, because together they functionally constitute a single toe that is used as a toe.

Chaeropus ecaudatus pig-footed bandicoot is an exception to the above rule. Its second, third, and fifth toes fit all three criteria for vestigiality Flower, The foot of C. The first toe is reduced in some members of the marsupial family Dasyuridae. It is reduced enough to fit all three criteria for vestigiality in Antechinomys laniger the kultarr Szalay, , Dasyuroides byrnei the kowari , and Sarcophilus Tasmanian devils.

In all three cases it is represented only by a very short metatarsal and a single phalanx shaped like a small spike Fig. In Thylacinus cynocephalus , the recently-extinct thylacine or Tasmanian wolf, the first toe is lost, and the first metatarsal is vestigial. It is a flattened oval, not much longer than the transverse width of one of the other metatarsals Fig.

In the Hyracoidea the first and fifth toes are lost, as is the first metatarsal. The fifth metatarsal is vestigial. It is tiny and transversely flattened Fig. In Bradypus three-toed sloths and Choloepus two-toed sloths the first and fifth toes are lost, and the first and fifth metatarsals are reduced to a vestigial state. In Bradypus these two metatarsals remain only as small, hook-shaped bones Humphry, that are coossified with the neighboring metatarsals Fig.

In Choloepus the first and fifth metatarsal are each little more than half the length of the neighboring metatarsal Flower, ; these vestigial metatarsals are transversely flattened and lack a distal articulating surface Fig. In the Lagomorpha the first toe is lost.

Its metatarsal is vestigial in Leporidae rabbits Fig. In the Caviidae the first and fifth toes are lost, and their metatarsals are vestigial Figs. Previous authors e. However, several lines of evidence show that these two bones are not sesamoids but are the first and fifth metatarsals.

Secondly, the two bones are in the locations of the proximal ends of the first and fifth metatarsals of other rodents, and they exhibit the articulations with neighboring bones that the first and fifth metatarsals of other rodents do. In all caviid genera, the vestigial first and fifth metatarsals are transversely flattened. The first metatarsal is a proximodistally elongate ovoid, and the fifth is near-circular in shape in lateral view.

In the Dasyproctidae agoutis and acouchis the first and fifth toes are lost, as is the fifth metatarsal. The first metatarsal is vestigial Fig. In the Chinchillidae chinchillas and viscachas the first toe is lost and its metatarsal is vestigial. It is a transversely flattened, proximodistally elongate ovoid Figs.

Of the three genera in this family, the fifth toe is present and fully expressed in two: Chinchilla chinchillas and Lagidium mountain viscachas. In the remaining species, Lagostomus trichodactylus the plains viscacha , the fifth toe is absent and its metatarsal is vestigial; it resembles the first metatarsal but is proximodistally shorter Fig.

In the Pedetidae springhares the first toe is lost, and its metatarsal is vestigial. As in other rodents with a vestigial first metatarsal, it is transversely flattened and proximodistally elongate, and its distal margin is rounded. However, unlike the case in other rodents, it tapers to a point proximally Fig.

In the Thryonomyidae cane rats the first toe is vestigial. It retains only one phalanx, which is less than half the size of the highly reduced metatarsal and is shaped like a small spike Fig. In Dipodomys kangaroo rats and Jaculus African jerboas , members of the Dipodidae, the first toe is lost, and its metatarsal is vestigial; it is reduced to a tiny, proximal sliver Howell, In other members of the Dipodidae all five toes are fully expressed.

Most members of the Carnivora retain all five toes. Exceptions are the families Canidae, Felidae, and Hyaenidae. In the Canidae the first toe is vestigial.