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(of an organ or part of the body) degenerate, rudimentary, or atrophied, having become functionless in the course of evolution.
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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.

Vestigial Organs

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.

Related groups of terms

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.

Vestigial organs: Remnants of evolution

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.

Vestigial organs: Remnants of evolution | New Scientist

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.

EVOLUTION NEWS

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. Its metatarsal is tiny, and its toe has only one phalanx, which is reduced to a small spike or nubbin Figs. In the Felidae the first toe is lost and its metatarsal is vestigial Flower, Figs.

In the Hyaenidae, as in the Canidae, the first toe is vestigial. In peccaries Tayassuidae the fifth toe is lost, and its metatarsal is retained but vestigial. It is a transversely flattened, proximal splint that is shorter than the other metatarsals Figs. The second and fifth toes are reduced in the Tragulidae chevrotains , Bovidae cattle, antelope, sheep, and goats , and Cervidae deer , but they do not meet our criteria for vestigiality, because they bear hooves Fig.

The second and fifth metatarsals are fully expressed in the Tragulidae. It was once thought that these two metatarsals are absent in other ruminants Flower, , but they are often present and fused to the rest of the metatarsus. When present, each remains only as a proximal sliver that fits the criteria for vestigiality.

Vestigial second metatarsals are common in the Bovidae cattle and kin , Capreolinae New World deer , and Giraffidae giraffe and okapi ; they are uncommon in Cervinae Old World deer and unknown in pronghorn Antilocapra americana. In Equus the first and fifth metatarsals are lost, as are the phalanges of all but the third toe. The second and fourth metatarsals are vestigial Figs. They remain as narrow splints that taper to a point distally without reaching the distal end of the third metatarsal Nickel et al.

In the Rhinocerotidae rhinos and Tapiridae tapirs the first toe is lost and the first metatarsal is vestigial. In tapirs the first metatarsal is a small, blocky bone, almost cube-shaped in some specimens, at the tip of the ectocuneiform the tarsal bone proximal to it. It is medially displaced onto the palmar surface of the foot Fig.

In rhinos the first metatarsal is a tiny nub that is fused to the end of the ectocuneiform Radinsky, The entire foot is lost in the Sirenia and Cetacea. For other extant mammal taxa, the phylogenetic distribution of character states Fig. Uncertainty in rodent phylogeny makes it difficult to tell how many times the fist toe was lost in rodents. If those two pairs of possible sister taxa are indeed pairs of sister taxa, then the toe was lost independently at least three times in the Rodentia.

The second toe became vestigial independently at least twice: in Chaeropus and the Cervidae. It was lost independently four times: in the Camelidae, Antilocapridae, Giraffidae, and Bovidae. The third toe is vestigial in only one Recent mammalian taxon: Chaeropus. Among extant mammals, the fourth toe has been lost only in Equus and is not vestigial in any taxon. The fifth toe became vestigial at least twice: in Chaeropus and the Cervidae.