Caecilians (; ) are a group of limbless,
vermiform
Vermiform (ˈvərməˌfôrm) describes something shaped like a worm. The expression is often employed in biology and anatomy to describe usually soft body parts or animals that are more or less tubular or cylindrical. The word root is Latin, ''ve ...
or serpentine
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. 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
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 ...
.
All modern caecilians and their closest fossil relatives are grouped as a
clade
A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...
, 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, a ...
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 minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...
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 fami ...
, 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
''Siphonops paulensis'', or Boettger's caecilian, is a species of caecilian in the family Siphonopidae. It is found in northern Argentina, Paraguay, eastern Bolivia, and southern Brazil
Brazil ( pt, Brasil; ), officially the Federative R ...
'' have been shown to have
hemolytic
Hemolysis or haemolysis (), also known by several other names, is the rupturing (lysis) of red blood cells (erythrocytes) and the release of their contents (cytoplasm) into surrounding fluid (e.g. blood plasma). Hemolysis may occur in vivo o ...
properties.
![Caecilia pulchraserrana](https://upload.wikimedia.org/wikipedia/commons/8/87/Caecilia_pulchraserrana.jpg)
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 fami ...
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'') 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 joi ...
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 alt ...
s. The function of these glands is unknown.
The middle ear consists of only the
stapes
The ''stapes'' or stirrup is a bone in the middle ear of humans and other animals which is involved in the conduction of sound vibrations to the inner ear. This bone is connected to the oval window by its annular ligament, which allows the foot ...
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'', all caecilians have
lung
The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of t ...
s, but also use their skin or mouths for
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
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
Southeast Asia, also spelled South East Asia and South-East Asia, and also known as Southeastern Asia, South-eastern Asia or SEA, is the geographical United Nations geoscheme for Asia#South-eastern Asia, south-eastern region of Asia, consistin ...
,
India
India, officially the Republic of India (Hindi: ), is a country in South Asia. It is the seventh-largest country by area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the so ...
,
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
Nepal (; ne, नेपाल ), formerly the Federal Democratic Republic of Nepal ( ne,
सङ्घीय लोकतान्त्रिक गणतन्त्र नेपाल ), is a landlocked country in South Asia. It is mai ...
and
Sri Lanka
Sri Lanka (, ; si, ශ්රී ලංකා, Śrī Laṅkā, translit-std=ISO (); ta, இலங்கை, Ilaṅkai, translit-std=ISO ()), formerly known as Ceylon and officially the Democratic Socialist Republic of Sri Lanka, is an ...
, parts of East and
West Africa
West Africa or Western Africa is the westernmost region of Africa. The United Nations defines Western Africa as the 16 countries of Benin, Burkina Faso, Cape Verde, The Gambia, Ghana, Guinea, Guinea-Bissau, Ivory Coast, Liberia, Mali, Maurit ...
, 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, V ...
Islands in the
Indian Ocean
The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or ~19.8% of the water on Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia to the east. To the south it is bounded by th ...
,
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. ...
, and in northern and eastern
South America
South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the southe ...
. 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
Malawi (; or aláwi Tumbuka: ''Malaŵi''), officially the Republic of Malawi, is a landlocked country in Southeastern Africa that was formerly known as Nyasaland. It is bordered by Zambia to the west, Tanzania to the north and northeast ...
(''
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 amphi ...
''), with an unconfirmed record from eastern
Zimbabwe
Zimbabwe (), officially the Republic of Zimbabwe, is a landlocked country located in Southeast Africa, between the Zambezi and Limpopo Rivers, bordered by South Africa to the south, Botswana to the south-west, Zambia to the north, and Mozam ...
. They have not been recorded from the extensive areas of tropical forest in central Africa. In South America, they extend through subtropical eastern
Brazil
Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...
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 ...
, when they are carried by the flood waters of the
Paraná River
The Paraná River ( es, Río Paraná, links=no , pt, Rio Paraná, gn, Ysyry Parana) is a river in south-central South America, running through Brazil, Paraguay, and Argentina for some ."Parana River". Encyclopædia Britannica. Encyclopædia Br ...
coming from farther north. Their American range extends north to southern
Mexico
Mexico (Spanish: México), officially the United Mexican States, is a country in the southern portion of North America. It is bordered to the north by the United States; to the south and west by the Pacific Ocean; to the southeast by Guatema ...
. The northernmost distribution is of the species ''
Ichthyophis sikkimensis
''Ichthyophis sikkimensis'', the Sikkimese caecilian or Darjeeling caecilian, is a species of caecilian found in India (Sikkim and West Bengal), Nepal and possibly Bhutan. It was described by Edward Harrison Taylor
Edward Harrison Taylor (Apri ...
'' 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 S ...
. In Southeast Asia, they are found as far east as
Java
Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's List ...
,
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 eas ...
, and the southern
Philippines
The Philippines (; fil, Pilipinas, links=no), officially the Republic of the Philippines ( fil, Republika ng Pilipinas, links=no),
* bik, Republika kan Filipinas
* ceb, Republika sa Pilipinas
* cbk, República de Filipinas
* hil, Republ ...
, 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 tran ...
and are not present in
Australia
Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...
or nearby islands. There are no known caecilians in
Madagascar
Madagascar (; mg, Madagasikara, ), officially the Republic of Madagascar ( mg, Repoblikan'i Madagasikara, links=no, ; french: République de Madagascar), is an island country in the Indian Ocean, approximately off the coast of East Africa ...
, 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
Colombia (, ; ), officially the Republic of Colombia, is a country in South America with insular regions in North America—near Nicaragua's Caribbean coast—as well as in the Pacific Ocean. The Colombian mainland is bordered by the Car ...
and
Venezuela
Venezuela (; ), officially the Bolivarian Republic of Venezuela ( es, link=no, República Bolivariana de Venezuela), is a country on the northern coast of South America, consisting of a continental landmass and many islands and islets in th ...
, was collected from a drainage canal in South
Florida
Florida is a state located in the Southeastern region of the United States. Florida is bordered to the west by the Gulf of Mexico, to the northwest by Alabama, to the north by Georgia, to the east by the Bahamas and Atlantic Ocean, and to ...
. 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 g ...
, perhaps from the
wildlife trade
Wildlife trade refers to the of products that are derived from non-domesticated animals or plants usually extracted from their natural environment or raised under controlled conditions. It can involve the trade of living or dead individuals, ti ...
. 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
Carl Linnaeus (; 23 May 1707 – 10 January 1778), also known after his ennoblement in 1761 as Carl von Linné Blunt (2004), p. 171. (), was a Swedish botanist, zoologist, taxonomist, and physician who formalised binomial nomenclature, the ...
, 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, ''Fuller's caecilian, Chikila fulleri'' (formerly ''Herpele fulleri'') was first d ...
.
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 co ...
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
South America is a continent entir ...
– 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, ''Fuller's caecilian, Chikila fulleri'' (formerly ''Herpele fulleri'') was first d ...
– 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 fami ...
– 5 genera, 14 species; South America
*
Grandisoniidae
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 ...
(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 an ...
– 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 caecilians, they superficially resemble worms or snakes.
They are the only viviparous caecilians (species that give birth ...
– 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''.
Evolution
![Eocaecilia BW](https://upload.wikimedia.org/wikipedia/commons/2/27/Eocaecilia_BW.jpg)
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
The Paleocene, ( ) or Palaeocene, is a geological epoch (geology), epoch that lasted from about 66 to 56 mya (unit), million years ago (mya). It is the first epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), E ...
, was not discovered until 1972. Other vertebrae, which have characteristic features unique to modern species, were later found in Paleocene and Late Cretaceous (
Cenomanian
The Cenomanian is, in the ICS' geological timescale, the oldest or earliest age of the Late Cretaceous Epoch or the lowest stage of the Upper Cretaceous Series. An age is a unit of geochronology; it is a unit of time; the stage is a unit in the s ...
) 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
The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The J ...
period. This primitive genus, ''
Eocaecilia
''Eocaecilia'' is an extinct genus of gymnophionan amphibian from the early Jurassic Kayenta Formation of Arizona, United States. One species is described, ''Eocaecilia micropodia''.
''Eocaecilia'' shared some characteristics with salamanders a ...
'', 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 Carnivore, carnivorous group of short-bodied, tailless amphibians composing the order (biology), order Anura (ανοὐρά, literally ''without tail'' in Ancient Greek). The oldest fossil "proto-f ...
''
Gerobatrachus
''Gerobatrachus'' is an extinct genus of Amphibamidae, 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 ...
'', 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
Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids (reptiles, including dinosaurs and therefore birds) and synapsids (pelycosaurs, extinct therapsids ...
, 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 language, Greek τέμνειν, ''temnein'' 'to cut' and σπόνδυλος, ''spondylos'' 'vertebra') is a diverse order (biology), order of small to giant tetrapods—often considered Labyrinthodontia, primitive amphi ...
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
The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Paleoz ...
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 ''Carbonifero ...
, predicted by some molecular clock studies of DNA sequences. Most morphological and molecular studies of extant amphibians, however, support
monophyly
In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic grou ...
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 ''Carbonifero ...
–
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), 01'' (Richard Müller album), 2001
* 01 (Son of Dave album), ''01'' (Son of Dave album), 2000
* 01 (Urban ...
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 Ne ...
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
![CaecilianNHM](https://upload.wikimedia.org/wikipedia/commons/a/ad/CaecilianNHM.png)
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
An intromittent organ is any external organ of a male organism that is specialized to deliver sperm during copulation. Intromittent organs are found most often in terrestrial species, as most non-mammalian aquatic species fertilize their eggs e ...
, 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, causin ...
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
Among animals, viviparity is development of the embryo inside the body of the parent. This is opposed to oviparity which is a reproductive mode in which females lay developing eggs that complete their development and hatch externally from the m ...
, 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
''Boulengerula taitana'' (common names: Taita African caecilian, Taita Hills caecilian, Taita Mountains caecilian) is a species of caecilian. It is endemic to the Taita Hills region of southeast Kenya. ''Boulengerula taitana'' are unique caecilia ...
'' 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 and Northern South America, and typically ...
'', 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
''Boulengerula taitana'' (common names: Taita African caecilian, Taita Hills caecilian, Taita Mountains caecilian) is a species of caecilian. It is endemic to the Taita Hills region of southeast Kenya. ''Boulengerula taitana'' are unique caecilia ...
'' 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
In biology, detritus () is dead particulate organic material, as distinguished from dissolved organic material. Detritus typically includes the bodies or fragments of bodies of dead organisms, and fecal material. Detritus typically hosts commun ...
, 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