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Caecilians (; ) are a group of limbless, vermiform or serpentine
amphibian Amphibians are four-limbed and ectothermic vertebrates of the class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial, fossorial, arborea ...
s. 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 South and Central America, Africa, and southern Asia. Their diet consists of small subterranean creatures such as earthworms. All modern caecilians and their closest fossil relatives are grouped as a clade, Apoda , within the larger group Gymnophiona , which also includes more primitive extinct caecilian-like amphibians. The name derives from the Greek words γυμνος (''gymnos'', naked) and οφις (''ophis'', snake), as the caecilians were originally thought to be related to snakes. The body is cylindrical dark brown or bluish black in colour. The skin is slimy and bears grooves or ringlike markings.


Description

Caecilians completely lack limbs, making the smaller species resemble worms, while the larger species, with lengths up to , resemble snakes. Their tails are short or absent, and their
cloaca In animal anatomy, a cloaca ( ), plural cloacae ( or ), is the posterior orifice that serves as the only opening for the digestive, reproductive, and urinary tracts (if present) of many vertebrate animals. All amphibians, reptiles and birds, an ...
e are near the ends of their bodies. Their skin is smooth and usually dark, but some species have colourful skins. Inside the skin are
calcite Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on the Mohs scale of mineral hardness, based on scratc ...
scales Scale or scales may refer to: Mathematics * Scale (descriptive set theory), an object defined on a set of points * Scale (ratio), the ratio of a linear dimension of a model to the corresponding dimension of the original * Scale factor, a number w ...
. Because of these scales, the caecilians were once thought to be related to the fossil
Stegocephalia Stegocephali (often spelled Stegocephalia) is a group containing all four-limbed vertebrates. It is equivalent to a broad definition of Tetrapoda: under this broad definition, the term "tetrapod" applies to any animal descended from the first ve ...
, but they are now believed to be a secondary development, and the two groups are most likely unrelated. Scales are absent in the families
Scolecomorphidae The Scolecomorphidae are the family of caecilians, also known as tropical caecilians, buried-eyed caecilians, or African caecilians. They are found in Cameroon in West Africa, and Malawi and Tanzania in East Africa. Caecilians are legless amphibi ...
and
Typhlonectidae Typhlonectidae, also known as aquatic caecilians or rubber eels, are a family of caecilians found east of the Andes in South America. They are viviparous animals, giving birth to young that possess external gills. Of the five genera in the famil ...
, except the species
Typhlonectes compressicauda ''Typhlonectes compressicauda'', the Cayenne caecilian, is a species of amphibian in the family Typhlonectidae that lives in water. It is found in Amazonian Brazil, Peru, and Colombia as well as in Guyana and French Guiana, and likely Suriname, ...
where minute scales have been found in the hinder region of the body. The skin also has numerous ring-shaped folds, or annuli, that partially encircle the body, giving them a segmented appearance. Like some other living amphibians, the skin contains glands that secrete a toxin to deter predators. The skin secretions of '' Siphonops paulensis'' have been shown to have hemolytic properties. Caecilians' vision is limited to dark-light perception, and their anatomy is highly adapted for a burrowing lifestyle. They have a strong skull, with a pointed snout used to force their way through soil or mud. In most species, the bones in the skull are reduced in number and fused together, and the mouth is recessed under the head. Their muscles are adapted to pushing their way through the ground, with the skeleton and deep muscles acting as a piston inside the skin and outer muscles. This allows the animal to anchor its hind end in position, and force the head forwards, and then pull the rest of the body up to reach it in waves. In water or very loose mud, caecilians instead swim in an eel-like fashion. Caecilians in the family
Typhlonectidae Typhlonectidae, also known as aquatic caecilians or rubber eels, are a family of caecilians found east of the Andes in South America. They are viviparous animals, giving birth to young that possess external gills. Of the five genera in the famil ...
are aquatic, and the largest of their kind. The representatives of this family have a fleshy fin running along the rear section of their bodies, which enhances propulsion in water. All but the most primitive caecilians have two sets of muscles for closing the jaw, compared with the single pair found in other creatures. These are more highly developed in the most efficient burrowers among the caecilians, and appear to help keep the skull and jaw rigid. All caecilians possess a pair of
tentacle In zoology, a tentacle is a flexible, mobile, and elongated organ present in some species of animals, most of them invertebrates. In animal anatomy, tentacles usually occur in one or more pairs. Anatomically, the tentacles of animals work main ...
s located between their eyes and nostrils. These are probably used for a second olfactory capability, in addition to the normal sense of smell based in the nose. The ringed caecilian (''
Siphonops annulatus ''Siphonops annulatus'', the ringed caecilian, is a species of caecilian in the family Siphonopidae from South America. It might have the broadest known distribution among terrestrial caecilian species. Description Ringed caecilian measures in ...
'') has dental glands that may be homologous to the venom glands of some
snakes Snakes are elongated, limbless, carnivorous reptiles of the suborder Serpentes . Like all other squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales. Many species of snakes have skulls with several more joints ...
and
lizard Lizards are a widespread group of squamate reptiles, with over 7,000 species, ranging across all continents except Antarctica, as well as most oceanic island chains. The group is paraphyletic since it excludes the snakes and Amphisbaenia althou ...
s. The function of these glands is unknown. The middle ear consists of only the stapes and the
oval window The oval window (or ''fenestra vestibuli'' or ''fenestra ovalis'') is a membrane-covered opening from the middle ear to the cochlea of the inner ear. Vibrations that contact the tympanic membrane travel through the three ossicles and into the in ...
, which transfer vibration to the inner ear through a reentrant fluid circuit as seen in some reptiles. The species within the
Scolecomorphidae The Scolecomorphidae are the family of caecilians, also known as tropical caecilians, buried-eyed caecilians, or African caecilians. They are found in Cameroon in West Africa, and Malawi and Tanzania in East Africa. Caecilians are legless amphibi ...
lack both stapes and an oval window, making them the only known amphibians missing all the components of a middle ear apparatus. Except for one lungless species, ''
Atretochoana eiselti ''Atretochoana eiselti'' is a species of caecilian originally known only from two preserved specimens discovered by Sir Graham Hales in the Brazilian rainforest, while on an expedition with Sir Brian Doll in the late 1800s, but rediscovered in 2 ...
'', all caecilians have lungs, but also use their skin or mouths for oxygen absorption. Often, the left lung is much smaller than the right one, an adaptation to body shape that is also found in snakes.


Distribution

Caecilians are native to wet, tropical regions of Southeast Asia, India,
Bangladesh Bangladesh (}, ), officially the People's Republic of Bangladesh, is a country in South Asia. It is the eighth-most populous country in the world, with a population exceeding 165 million people in an area of . Bangladesh is among the mos ...
, Nepal and Sri Lanka, parts of East and West Africa, the
Seychelles Seychelles (, ; ), officially the Republic of Seychelles (french: link=no, République des Seychelles; Creole: ''La Repiblik Sesel''), is an archipelagic state consisting of 115 islands in the Indian Ocean. Its capital and largest city, ...
Islands in the Indian Ocean,
Central America Central America ( es, América Central or ) is a subregion of the Americas. Its boundaries are defined as bordering the United States to the north, Colombia to the south, the Caribbean Sea to the east, and the Pacific Ocean to the west. C ...
, and in northern and eastern South America. In Africa, caecilians are found from
Guinea-Bissau Guinea-Bissau ( ; pt, Guiné-Bissau; ff, italic=no, 𞤘𞤭𞤲𞤫 𞤄𞤭𞤧𞤢𞥄𞤱𞤮, Gine-Bisaawo, script=Adlm; Mandinka: ''Gine-Bisawo''), officially the Republic of Guinea-Bissau ( pt, República da Guiné-Bissau, links=no ) ...
('' Geotrypetes'') to southern Malawi (''
Scolecomorphus The Scolecomorphidae are the family of caecilians, also known as tropical caecilians, buried-eyed caecilians, or African caecilians. They are found in Cameroon in West Africa, and Malawi and Tanzania in East Africa. Caecilians are legless amphib ...
''), with an unconfirmed record from eastern Zimbabwe. They have not been recorded from the extensive areas of tropical forest in central Africa. In South America, they extend through subtropical eastern Brazil well into temperate northern
Argentina Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country in the southern half of South America. Argentina covers an area of , making it the second-largest country in South America after Brazil, th ...
. They can be seen as far south as
Buenos Aires Buenos Aires ( or ; ), officially the Autonomous City of Buenos Aires ( es, link=no, Ciudad Autónoma de Buenos Aires), is the capital and primate city of Argentina. The city is located on the western shore of the Río de la Plata, on South Am ...
, when they are carried by the flood waters of the Paraná River coming from farther north. Their American range extends north to southern Mexico. The northernmost distribution is of the species '' Ichthyophis sikkimensis'' of northern India. ''
Ichthyophis ''Ichthyophis'' is a genus of caecilians (limbless amphibians, sometimes called the Asian caecilians) found in Southeast Asia, the southern Philippines The Philippines (; fil, Pilipinas, links=no), officially the Republic of the Philippi ...
'' is also found in
South China South China () is a geographical and cultural region that covers the southernmost part of China. Its precise meaning varies with context. A notable feature of South China in comparison to the rest of China is that most of its citizens are not n ...
and
Northern Vietnam Northern Vietnam ( vi, Bắc Bộ) is one of three geographical regions within Vietnam. It consists of three administrative regions: the Northwest (Vùng Tây Bắc), the Northeast (Vùng Đông Bắc), and the Red River Delta (Đồng Bằng ...
. In Southeast Asia, they are found as far east as Java,
Borneo Borneo (; id, Kalimantan) is the third-largest island in the world and the largest in Asia. At the geographic centre of Maritime Southeast Asia, in relation to major Indonesian islands, it is located north of Java, west of Sulawesi, and ea ...
, and the southern Philippines, but they have not crossed
Wallace's line The Wallace Line or Wallace's Line is a faunal boundary line drawn in 1859 by the British naturalist Alfred Russel Wallace and named by English biologist Thomas Henry Huxley that separates the biogeographical realms of Asia and Wallacea, a tra ...
and are not present in Australia or nearby islands. There are no known caecilians in Madagascar, but their presence in the Seychelles and India has led to speculation on the presence of undiscovered extinct or extant caecilians there. In 2021, a live specimen of ''
Typhlonectes natans ''Typhlonectes natans'', also incorrectly called the rubber eel, is a species of caecilian in the family Typhlonectidae found in Colombia, Venezuela, and possibly Trinidad and Tobago. Its natural habitats are dry savanna, subtropical or tropical ...
'', a caecilian native to Colombia and Venezuela, was collected from a drainage canal in South Florida. It was the only caecilian ever reported in the wild in the United States, and is considered to be an
introduction Introduction, The Introduction, Intro, or The Intro may refer to: General use * Introduction (music), an opening section of a piece of music * Introduction (writing), a beginning section to a book, article or essay which states its purpose and ...
, perhaps from the wildlife trade. Whether a breeding population has been established in the area is unknown.


Taxonomy

The name ''caecilian'' derives from the Latin word '' caecus'', meaning "blind", referring to the small or sometimes nonexistent eyes. The name dates back to the taxonomic name of the first species described by Carl Linnaeus, which he named ''Caecilia tentaculata''. There has historically been disagreement over the use of the two primary scientific names for caecilians, Apoda and Gymnophiona. Some specialists prefer to use the name Gymnophiona to refer to the "crown group", that is, the group containing all modern caecilians and extinct members of these modern lineages. They sometimes use the name Apoda to refer to the total group, that is, all caecilians and caecilian-like amphibians that are more closely related to modern groups than to frogs or salamanders. However, many scientists have advocated for the reverse arrangement, where Apoda is used as the name for modern caecilian groups. Some have argued that this use makes more sense, because the name "Apoda" means "without feet", and this is a feature associated mainly with modern species (some stem-group caecilian-like amphibians, such as ''Eocaecilia'', had legs). The most recent classification of caecilians, by Wilkinson et al. (2011), divided the caecilians into 9 families containing nearly 200 species. Since then, a tenth caecilian family has been discovered,
Chikilidae Chikilidae is a family of Indian caecilians, the 10th and most recent (2012) family of caecilians (legless amphibians) to be identified, although the type species, '' Chikila fulleri'' (formerly '' Herpele fulleri'') was first described in 1904. ...
. This classification is based on a thorough definition of monophyly based on morphological and molecular evidence, and it solves the longstanding problems of
paraphyly In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and most of its descendants, excluding a few monophyletic subgroups. The group is said to be paraphyletic ''with respect to'' the excluded subgroups. In c ...
of the
Caeciliidae Caeciliidae is the family of common caecilians. They are found in Central and South America. Like other caecilians, they superficially resemble worms or snakes. Although they are the most diverse of the caecilian families, the caeciliids do ha ...
in previous classifications without an exclusive reliance upon synonymy. It contains 256 species in 56 genera. *
Rhinatrematidae Rhinatrematidae is a family of caecilians, also known as the Neotropical tailed caecilians, American tailed caecilians. or beaked caecilians. They are found in the equatorial countries of South America. They are usually regarded as the most bas ...
– 3 genera, 14 species; South America *
Ichthyophiidae The Ichthyophiidae are the family of Asiatic tailed caecilians or fish caecilians found in South and Southeast Asia as well as southernmost China. They are primitive caecilians, lacking many of the derived characters found in the other families ...
– 2 genera, 57 species; South and Southeast Asia *
Scolecomorphidae The Scolecomorphidae are the family of caecilians, also known as tropical caecilians, buried-eyed caecilians, or African caecilians. They are found in Cameroon in West Africa, and Malawi and Tanzania in East Africa. Caecilians are legless amphibi ...
– 2 genera, 6 species; Africa *
Herpelidae Herpelidae are a family of caecilians, sometimes known as the African caecilians. They are found in Sub-Saharan Africa. Like other caecilians, they superficially resemble worms or snakes. They are the sister group to the newly discovered Chikili ...
– 2 genera, 10 species; Africa *
Chikilidae Chikilidae is a family of Indian caecilians, the 10th and most recent (2012) family of caecilians (legless amphibians) to be identified, although the type species, '' Chikila fulleri'' (formerly '' Herpele fulleri'') was first described in 1904. ...
– 1 genus, 4 species; India *
Caeciliidae Caeciliidae is the family of common caecilians. They are found in Central and South America. Like other caecilians, they superficially resemble worms or snakes. Although they are the most diverse of the caecilian families, the caeciliids do ha ...
– 2 genera, 43 species; South and Central America *
Typhlonectidae Typhlonectidae, also known as aquatic caecilians or rubber eels, are a family of caecilians found east of the Andes in South America. They are viviparous animals, giving birth to young that possess external gills. Of the five genera in the famil ...
– 5 genera, 14 species; South America * Grandisoniidae (formerly
Indotyphlidae The Grandisoniidae are a family of common caecilians found in Africa, Seychelles and India. Like other caecilians, they superficially resemble worms or snakes. The family was formerly known as Indotyphlidae. Taxonomy The genera in this fami ...
) – 7 genera, 24 species; Seychelles, India, Africa *
Siphonopidae The Siphonopidae are the family of common caecilians. They are found in Central and South America. Like other caecilian Caecilians (; ) are a group of limbless, vermiform or serpentine amphibians. They mostly live hidden in the ground and ...
– 5 genera, 28 species; South America *
Dermophiidae The Dermophiidae are a family of common caecilians. They are found in Central and South America, and Africa. Like other caecilian Caecilians (; ) are a group of limbless, vermiform or serpentine amphibians. They mostly live hidden in the ...
– 4 genera, 14 species; Africa, Central and South America The most recent phylogeny of caecilians is based on molecular mitogenomic evidence examined by San Mauro et al. (2014), and modified to include some more recently described genera such as ''
Amazops ''Amazops'' is a monotypic genus of caecilian in the family Rhinatrematidae. It contains only one species, ''Amazops amazops''. It is endemic to Ecuador and is only known from a single specimen collected around 1990 from the Virgen La Dolores Far ...
''.


Evolution

Little is known of the evolutionary history of the caecilians, which have left a very sparse fossil record. The first fossil, a vertebra dated to the Paleocene, was not discovered until 1972. Other vertebrae, which have characteristic features unique to modern species, were later found in Paleocene and Late Cretaceous ( Cenomanian) sediments. The earliest fossil attributed to a stem-caecilian (a species closer to caecilians than to frogs or salamanders but not a member of the extant lineage) comes from the Jurassic period. This primitive genus, '' Eocaecilia'', had small limbs and well-developed eyes. In their 2008 description of the fossil
batrachian A frog is any member of a diverse and largely carnivorous group of short-bodied, tailless amphibians composing the order Anura (ανοὐρά, literally ''without tail'' in Ancient Greek). The oldest fossil "proto-frog" ''Triadobatrachus'' is ...
''
Gerobatrachus ''Gerobatrachus'' is an extinct genus of amphibamid temnospondyl (represented by the type species ''Gerobatrachus hottoni'') that lived in the Early Permian, approximately 290 million years ago (Ma), in the area that is now Baylor County, Texas. ...
'', Anderson and co-authors suggested that caecilians arose from the
Lepospondyl Lepospondyli is a diverse taxon of early tetrapods. With the exception of one late-surviving lepospondyl from the Late Permian of Morocco (''Diplocaulus minumus''), lepospondyls lived from the Early Carboniferous ( Mississippian) to the Early Per ...
group of ancestral tetrapods, and may be more closely related to
amniotes Amniotes are a clade of tetrapod vertebrates that comprises sauropsids (including all reptiles and birds, and extinct parareptiles and non-avian dinosaurs) and synapsids (including pelycosaurs and therapsids such as mammals). They are distingu ...
than to frogs and salamanders, which arose from
Temnospondyl Temnospondyli (from Greek τέμνειν, ''temnein'' 'to cut' and σπόνδυλος, ''spondylos'' 'vertebra') is a diverse order of small to giant tetrapods—often considered primitive amphibians—that flourished worldwide during the Carb ...
ancestors. Numerous groups of lepospondyls evolved reduced limbs, elongated bodies, and burrowing behaviors, and morphological studies on Permian and Carboniferous lepospondyls have placed the early caecilian (''Eocaecilia'') among these groups. Divergent origins of caecilians and other extant amphibians may help explain the slight discrepancy between fossil dates for the origins of modern amphibia, which suggest Permian origins, and the earlier dates, in the
Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carboniferous ...
, predicted by some molecular clock studies of DNA sequences. Most morphological and molecular studies of extant amphibians, however, support monophyly for caecilians, frogs, and salamanders, and the most recent molecular study based on multi-locus data suggest a Late
Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carboniferous ...
Early Permian 01 or '01 may refer to: * The year 2001, or any year ending with 01 * The month of January * 1 (number) Music * 01'' (Richard Müller album), 2001 * ''01'' (Son of Dave album), 2000 * ''01'' (Urban Zakapa album), 2011 * ''O1'' (Hiroyuki Sawano ...
origin of extant amphibians. The Late Triassic stem-caecilian ''
Chinlestegophis ''Chinlestegophis'' is a diminutive Late Triassic stereospondyl that has been interpreted as a putative stem caecilian, a living group of legless burrowing amphibians. If ''Chinlestegophis'' is indeed both an advanced stereospondyl and a relative ...
'' from the
Chinle Formation The Chinle Formation is an Upper Triassic continental geological formation of fluvial, lacustrine, and palustrine to eolian deposits spread across the U.S. states of Nevada, Utah, northern Arizona, western New Mexico, and western Colorado. In N ...
of Colorado bolsters the proposed pre-Triassic origin of
Lissamphibia The Lissamphibia is a group of tetrapods that includes all modern amphibians. Lissamphibians consist of three living groups: the Salientia (frogs, toads, and their extinct relatives), the Caudata (salamanders, newts, and their extinct relatives), ...
suggested by molecular clocks by filling a gap in the fossil record of early caecilians and suggesting that groups of stereospondyls, including
Metoposauridae Metoposauridae is an extinct family of trematosaurian temnospondyls. The family is known from the Triassic period. Most members are large, approximately long and could reach 3 m long.Brusatte, S. L., Butler R. J., Mateus O., & Steyer S. J. (201 ...
, are closely related to caecilians. However, affinities between ''Chinlestegophis'' and gymnophionans have been disputed both because of the choice of characters and because reanalysis of the original data matrix supports other equally parsimonious positions of ''
Chinlestegophis ''Chinlestegophis'' is a diminutive Late Triassic stereospondyl that has been interpreted as a putative stem caecilian, a living group of legless burrowing amphibians. If ''Chinlestegophis'' is indeed both an advanced stereospondyl and a relative ...
'' and gymnophionans among tetrapods.


Behavior


Reproduction

Caecilians are the only order of amphibians to use internal insemination exclusively (although most salamanders have internal fertilization and the
tailed frog The tailed frogs are two species of frogs in the genus ''Ascaphus'', the only taxon in the family Ascaphidae . The "tail" in the name is actually an extension of the male cloaca. The tail is one of two distinctive anatomical features adapting the ...
in the US uses a tail-like appendage for internal insemination in its fast-flowing water environment). The male caecilians have a long tube-like intromittent organ, the phallodeum, which is inserted into the cloaca of the female for two to three hours. About 25% of the species are
oviparous Oviparous animals are animals that lay their eggs, with little or no other embryonic development within the mother. This is the reproductive method of most fish, amphibians, most reptiles, and all pterosaurs, dinosaurs (including birds), and ...
(egg-laying); the eggs are guarded by the female. For some species, the young caecilians are already
metamorphosed Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causi ...
when they hatch; others hatch as larvae. The larvae are not fully aquatic, but spend the daytime in the soil near the water. About 75% of caecilians are viviparous, meaning they give birth to already-developed offspring. The foetus is fed inside the female with cells lining the
oviduct The oviduct in mammals, is the passageway from an ovary. In human females this is more usually known as the Fallopian tube or uterine tube. The eggs travel along the oviduct. These eggs will either be fertilized by spermatozoa to become a zygote, o ...
, which they eat with special scraping teeth. The egg-laying species '' Boulengerula taitana'' feeds its young by developing an outer layer of skin, high in fat and other nutrients, which the young peel off with modified teeth. This allows them to grow by up to 10 times their own weight in a week. The skin is consumed every three days, the time it takes for a new layer to grow, and the young have only been observed to eat it at night. It was formerly thought that the juveniles subsisted only on a liquid secretion from their mothers. Some larvae, such as those of ''
Typhlonectes ''Typhlonectes'' (from el, τῠφλός , 'blind' and el, νηκτῆς , 'swimmer') is a genus of caecilians in the family Typhlonectidae. These fully aquatic amphibians are found in the Amazon Basin The Amazon basin is the part of S ...
'', are born with enormous external
gills A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they are ...
which are shed almost immediately.


Diet

The diets of caecilians are not well known. Mature caecilians seem to feed mostly on insects and other invertebrates found in the habitat of the respective species. The stomach contents of 14 specimens of '' Boulengerula taitana'' consisted of mostly unidentifiable organic material and plant remains. Where identifiable remains were most abundant, they were found to be termite heads. While the undefinable organic material may show the caecilians eat detritus, the remains may be from
earthworm An earthworm is a terrestrial invertebrate that belongs to the phylum Annelida. They exhibit a tube-within-a-tube body plan; they are externally segmented with corresponding internal segmentation; and they usually have setae on all segments. Th ...
s. Caecilians in captivity can be easily fed with earthworms, and worms are also common in the habitat of many caecilian species.


See also

* Caecilians of the Western Ghats * Minhocão – a cryptid that resembles caecilians


References


External links

* * {{Authority control Mesozoic amphibians Hettangian first appearances Extant Early Jurassic first appearances Taxa named by Johannes Peter Müller