Many

vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...

s have evolved limbless, limb-reduced, or apodous forms.

Reptile Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephalians ( ...

s have on a number of occasions evolved into limbless forms –

snake Snakes are elongated, Limbless vertebrate, limbless, carnivore, carnivorous reptiles of the suborder Serpentes . Like all other Squamata, squamates, snakes are ectothermic, amniote vertebrates covered in overlapping Scale (zoology), scales. Ma ...

amphisbaenia Amphisbaenia (called amphisbaenians or worm lizards) is a group of usually legless squamates, comprising over 200 extant species. Amphisbaenians are characterized by their long bodies, the reduction or loss of the limbs, and rudimentary eyes. As ...

, and

legless lizard Legless lizard may refer to any of several groups of lizards that have independently lost limbs or reduced them to the point of being of no use in locomotion.Pough ''et al.'' 1992. Herpetology: Third Edition. Pearson Prentice Hall:Pearson Education ...

s (limb loss in lizards has evolved independently several times, examples include the families

Pygopodidae Pygopodidae, commonly known as legless lizard, legless lizards, snake-lizards, or flap-footed lizards, is a Family (biology), family of squamates with reduced or absent limbs, and are a type of gecko. At least 35 species are placed in two subfami ...

and

Dibamidae Dibamidae or blind skinks is a family of lizards characterized by their elongated cylindrical body and an apparent lack of limbs. Female dibamids are entirely limbless and the males retain small flap-like hind limbs, which they use to grip their ...

and species of ''

Isopachys ''Isopachys'' is a genus of skinks endemic to Asia. Geographic range Species in the genus ''Isopachys'' are found in Thailand and Myanmar.. Species There are four species in this genus: *'' Isopachys anguinoides'' - Thai snake skink, Heyer's ...

'', '' Anguis'', and ''

Ophisaurus ''Ophisaurus'' (from the Greek 'snake-lizard') is a genus of superficially snake-like lizards in the family Anguidae. Known as joint snakes, glass snakes, or glass lizards, they are so-named because their tails are easily broken; like many liz ...

)''. The same is true of

amphibian Amphibians are tetrapod, four-limbed and ectothermic vertebrates of the Class (biology), class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terres ...

s –

caecilian Caecilians (; ) are a group of limbless, vermiform or serpentine amphibians. They mostly live hidden in the ground and in stream substrates, making them the least familiar order of amphibians. Caecilians are mostly distributed in the tropics of ...

Sirenidae Sirenidae, the sirens, are a family of neotenic aquatic salamanders. Family members have very small fore limbs and lack hind limbs altogether. In one species, the skeleton in their fore limbs is made of only cartilage. In contrast to most other ...

(a clade of

salamander Salamanders are a group of amphibians typically characterized by their lizard-like appearance, with slender bodies, blunt snouts, short limbs projecting at right angles to the body, and the presence of a tail in both larvae and adults. All ten ...

s that are limbless except for atrophied front limbs),

Amphiumidae ''Amphiuma'' is a genus of aquatic salamanders from the United States, the only extant genus within the family Amphiumidae . They are colloquially known as amphiumas. They are also known to fishermen as "conger eels" or "Congo snakes", which are ...

(a clade of salamander with extremely atrophied limbs that appear non-functional) and at least three extinct groups (

Aïstopoda Aistopoda (Greek for " avingnot-visible feet") is an order of highly specialised snake-like stegocephalians known from the Carboniferous and Early Permian of Europe and North America, ranging from tiny forms only , to nearly in length. They fir ...

Lysorophia Lysorophia is an order of fossorial Carboniferous and Permian tetrapods within the Recumbirostra. Lysorophians resembled small snakes, as their bodies are extremely elongate. There is a single family, the Molgophidae (previously known as Lysoro ...

, and

Adelospondyli Adelospondyli is an order of elongated, presumably aquatic, Carboniferous amphibians (''sensu lato''). They have a robust skull roofed with solid bone, and orbits located towards the front of the skull. The limbs were almost certainly absent, ...

). Larval amphibians,

tadpole A tadpole is the larval stage in the biological life cycle of an amphibian. Most tadpoles are fully aquatic, though some species of amphibians have tadpoles that are terrestrial. Tadpoles have some fish-like features that may not be found i ...

s, are also often limbless. Legless forms of reptiles and amphibians probably evolved so as to be able to move underground or in water more easily. Some analyses suggest that elongation and

undulatory locomotion Undulatory locomotion is the type of motion characterized by wave-like movement patterns that act to propel an animal forward. Examples of this type of gait include crawling in snakes, or swimming in the lamprey. Although this is typically the ...

(slithering) evolved first, before limb loss. The debate about the origin of limblessness led to a temporary hypothesis about a marine origin for snakes, which is no longer favored since the discovery of snake fossils with hindlimbs. Phlegethontia

In the case of limb loss during evolution, vestigial structures testify to this change (remains of the pelvis, rudimentary femur or spurs in boas,

pythons The Pythonidae, commonly known as pythons, are a family of nonvenomous snakes found in Africa, Asia, and Australia. Among its members are some of the largest snakes in the world. Ten genera and 42 species are currently recognized. Distribution ...

and

Typhlops ''Typhlops'' is a genus of blind snakes in the family Typhlopidae. The genus is endemic to the West Indies. Some species which were formerly placed in the genus ''Typhlops'' have been moved to the genera ''Afrotyphlops, Amerotyphlops, Anilios, ...

). The evolutionary process of transforming quadrupedal lizards into legless forms results in three main characteristics: the regression of the limbs is carried out gradually, via the reduction in their size and the reduction in the number of phalanges or fingers; the multiplication of the vertebrae (up to 600 in some snakes) induces a lengthening and a gain in flexibility of the trunk; and the vertebral axis is homogenized from the neck to the cloaca, evoking an interminable ribcage. There are also a number of fish with elongated bodies that have no or reduced appendage-like fins, for example

eel Eels are ray-finned fish belonging to the order Anguilliformes (), which consists of eight suborders, 19 families, 111 genera, and about 800 species. Eels undergo considerable development from the early larval stage to the eventual adult stage ...

s and

swamp eel The swamp eels (also written "swamp-eels") are a family (Synbranchidae) of freshwater eel-like fishes of the tropics and subtropics. Most species are able to breathe air and typically live in marshes, ponds and damp places, sometimes burying the ...

s. While

hagfish Hagfish, of the class Myxini (also known as Hyperotreti) and order Myxiniformes , are eel-shaped, slime-producing marine fish (occasionally called slime eels). They are the only known living animals that have a skull but no vertebral column, a ...

and

lamprey Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of jawless fish of the order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by a toothed, funnel-like s ...

also do not have appendage-like fins, they may not have lost them, but simply retained the form that vertebrates had before they evolved limbs. There are no known limbless species of mammal or bird, although partial limb-loss and reduction has occurred in several groups, including whales and dolphins,

sirenians The Sirenia (), commonly referred to as sea-cows or sirenians, are an order of fully aquatic, herbivorous mammals that inhabit swamps, rivers, estuaries, marine wetlands, and coastal marine waters. The Sirenia currently comprise two distinct f ...

kiwi Kiwi most commonly refers to: * Kiwi (bird), a flightless bird native to New Zealand * Kiwi (nickname), a nickname for New Zealanders * Kiwifruit, an edible berry * Kiwi dollar or New Zealand dollar, a unit of currency Kiwi or KIWI may also ref ...

s, and the extinct

moa Moa are extinct giant flightless birds native to New Zealand. The term has also come to be used for chicken in many Polynesian cultures and is found in the names of many chicken recipes, such as Kale moa and Moa Samoa. Moa or MOA may also refe ...

and

elephant birds Elephant birds are members of the Extinction, extinct ratite family (biology), family Aepyornithidae, made up of flightless birds that once lived on the island of Madagascar. They are thought to have become extinct around 1000-1200 CE, probably ...

. The moa in particular are notable for having completely lost their wings, without even vestigial wings remaining outside their bodies. Despite its name, the

finless porpoise ''Neophocaena'' is a genus of porpoise native to the Indian and Pacific oceans, as well as the freshwater habitats of the Yangtze River basin in China China, officially the People's Republic of China (PRC), is a country in East Asia. ...

has two fins, and several species of

s have tiny useless legs, such as pygopodids which retain rudimentary flaps. Contrarily, the worm lizard '' Bipes'' as its scientific name suggests has two stubby forelimbs which actually assist in digging similar to a mole. All other amphisbaenians have reduced or absent forelimb girdles.

References

{{DEFAULTSORT:Limbless Vertebrates Vertebrate anatomy

See also

References