(
Anguis fragilis). '', fossil amphibians belonging to the limbless
aistopods. The jawless fish (
hagfish and
lamprey) do not have appendage-like fins. They may not have lost them, but rather, simply retained the form that vertebrates had before the evolution of limbs. There are also a number of fish with elongated bodies that have no fins or reduced appendage-like fins, for example
eels and
swamp eels. Reptiles have on a number of occasions evolved into limbless forms –
snakes,
amphisbaenians, and
legless lizards (limb loss in lizards has evolved independently over 26 times, examples include the families
Pygopodidae and
Dibamidae, the subfamily
Anguinae and the species
Isopachys). Several species of
legless lizards 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. Larval amphibians, tadpoles, are often limbless. Adult amphibians have also evolved limblessness multiple times –
caecilians,
Sirenidae (a clade of
salamanders that are limbless except for atrophied front limbs),
Amphiumidae (a clade of salamander with extremely atrophied limbs that appear non-functional) and at least three extinct groups (
Aïstopoda,
Lysorophia, and
Adelospondyli). 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,
kiwi, and the extinct
moa and
elephant birds. 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 has two fins (the name refers to the lack of a dorsal fin ridge). == Evolution ==