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Caecilians (; ) are a group of limbless, vermiform (worm-shaped) or serpentine (snake-shaped) with small or sometimes nonexistent eyes. They mostly live hidden in soil or in streambeds, and this cryptic lifestyle renders caecilians among the least familiar amphibians. Modern caecilians live in the tropics of South America and Central America, Africa, and southern Asia. Caecilians feed on small subterranean creatures, such as . The body is cylindrical and often darkly coloured, and the skull is bullet-shaped and strongly built. Caecilian heads have several unique adaptations, including fused cranial and jaw bones, a two-part system of jaw muscles, and a Chemoreceptor tentacle in front of the eye. The skin is slimy and bears ringlike markings or grooves and may contain scales.
Modern caecilians are a clade, the order Gymnophiona (or Apoda ), one of the three living amphibian groups alongside Anura () and Urodela (). Gymnophiona is a crown group, encompassing all modern caecilians and all descendants of their last common ancestor. There are more than 220 living species of caecilian classified in 10 families. Gymnophionomorpha is a recently coined name for the corresponding total group which includes Gymnophiona as well as a few extinct stem-group caecilians (extinct amphibians whose closest living relatives are caecilians but are not descended from any caecilian). Some palaeontologists have used the name Gymnophiona for the total group and the old name Apoda for the crown group. However, Apoda has other even older uses, including as the name of a genus of butterfly, making its use potentially confusing and best avoided. 'Gymnophiona' derives from the Greek words γυμνος / and οφις / , as the caecilians were originally thought to be related to snakes and to lack scales.Wilkinson, M. (2012). Caecilians. Current Biology, 22(17), R668-R669.
The study of caecilian evolution is complicated by their poor fossil record and specialized anatomy. Genetic evidence and some anatomical details (such as pedicellate teeth) support the idea that frogs, salamanders, and caecilians (collectively known as ) are each other's closest relatives. Frogs and salamanders show many similarities to Dissorophoidea, a group of extinct amphibians in the order Temnospondyli. Caecilians are more controversial; many studies extend dissorophoid ancestry to caecilians. Some studies have instead argued that caecilians descend from extinct Lepospondyli or Stereospondyli amphibians, contradicting evidence for lissamphibian monophyly (common ancestry). Rare fossils of early gymnophionans, such as Eocaecilia and Funcusvermis, have helped to test the various conflicting hypotheses for the relationships between caecilians and other living and extinct amphibians.
Except for one lungless species, Atretochoana eiselti, all caecilians have , 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.
Their trunk muscles are adapted to pushing their way through the ground, with the vertebral column and its musculature acting as a piston inside the outer layer of the body wall musculature, which is closely attached to the skin. The Ecology and Behavior of Amphibians By contracting the outer layer of muscles it squeezes the coelom and generates a strong hydrostatic force that lengthens the body. Caecilians - Current Biology This muscle system 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 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.
The bones in the skull are reduced in number compared to prehistoric amphibian species. Many bones of the skull are fused together: the maxilla and Palatine bone bones have fused into a maxillopalatine in all living caecilians, and the Nasal bone and premaxilla bones fuse into a nasopremaxilla in some families. Some families can be differentiated by the presence or absence of certain skull bones, such as the , Prefrontal bone, an/or a Postfrontal bone-like bone surrounding the orbit (eye socket). The braincase is encased in a fully integrated compound bone called the os basale, which takes up most of the rear and lower parts of the skull. In skulls viewed from above, a mesethmoid bone may be visible in some species, wedging into the midline of the skull roof. All caecilians have a pair of unique sensory structures, known as , located on either side of the head between the eyes and nostrils. These are probably used for a second olfaction, 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 of some Venomous snake and . The function of these glands is unknown.
The middle ear consists of only the stapes bone and the oval window, which transfer vibrations into the inner ear through a reentrant fluid circuit as seen in some reptiles. Adults of species in the family Scolecomorphidae lack both a stapes and an oval window, making them the only known amphibians missing all the components of a middle ear apparatus. Hearing and Sound Communication in Amphibians
The lower jaw is specialized in caecilians. Gymnophionans, including extinct species, have only two components of the jaw: the pseudodentary (at the front, bearing teeth) and pseudoangular (at the back, bearing the jaw joint and muscle attachments). These two components are what remains following fusion between a larger set of bones. An additional inset tooth row with up to 20 teeth lies parallel to the main marginal tooth row of the jaw.
All but the most primitive caecilians have two sets of muscles for closing the jaw, compared with the single pair found in other amphibians. One set of muscles, the adductors, insert into the upper edge of the pseudoangular in front of the jaw joint. Adductor muscles are commonplace in vertebrates, and close the jaw by pulling upwards and forwards. A more unique set of muscles, the abductors, insert into the rear edge of the pseudoangular below and behind the jaw joint. They close the jaw by pulling backwards and downwards. Jaw muscles are more highly developed in the most efficient burrowers among the caecilians, and appear to help keep the skull and jaw rigid.
In 2021, a live specimen of Typhlonectes natans, 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, perhaps from the wildlife trade. Whether a breeding population has been established in the area is unknown.
There has historically been disagreement over the use of the two primary scientific names for caecilians, Apoda and Gymnophiona. Some palaeontologists prefer to use the name Apoda to refer to the "crown group", that is, the group containing all modern caecilians and extinct members of these modern lineages and the name Gymnophiona 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, Apoda been used for groups of fishes and of sea cucumbers and is the name of a genus of moth, and its continued use in caecilian taxonomy is potentially confusing and unhelpful.
A classification of caecilians by Wilkinson et al. (2011) divided the living caecilians into 9 families containing nearly 200 species. In 2012, a tenth caecilian family was newly described, Chikilidae. This classification is based on a thorough definition of monophyly based on morphological and molecular evidence, and it solves the longstanding problems of paraphyly of the Caeciliidae in previous classifications without an exclusive reliance upon synonymy. There are 219 species of caecilian in 33 genera and 10 families.
| Caeciliidae | Caecilia subnigricans | Rafinesque, 1814 | 2 | 47 | Common caecilians | Central America and South America (Bolivia north to Costa Rica). |
| Chikilidae | Kamei et al., 2012 | 1 | 4 | Northeast Indian caecilians | Northeast India and Bangladesh, with possible occurrences in Myanmar. | |
| Dermophiidae | Geotrypetes seraphini | Taylor, 1969 | 4 | 15 | Neotropical caecilians | Equatorial Africa (West Africa, Tanzania, Kenya), Central and South America (Colombia north to Mexico). |
| Grandisoniidae (formerly Indotyphlidae) | Grandisonia sechellensis | Lescure, Renous & Gasc, 1986 | 7 | 24 | Indo-African caecilians | Equatorial Africa (Cameroon, Ethiopia), the Seychelles, western India (Western Ghats). |
| Herpelidae | Boulengerula taitana | Raymond Laurent, 1984 | 2 | 10 | African caecilians | Equatorial Africa (Nigeria south to the Democratic Republic of the Congo, Kenya south to Malawi, possible occurrences in Angola and Zambia). |
| Ichthyophiidae | Ichthyophis kodaguensis | Taylor, 1969 | 2 | 57 | Asian tailed caecilians | South Asia and Southeast Asia (western India north to Nepal, east to the Philippines, southern China and Indonesia). |
| Rhinatrematidae | Epicrionops | Nussbaum, 1977 | 3 | 14 | American tailed caecilians | Northern South America (northernmost Brazil west to Venezuela, Colombia, Ecuador, and Peru). |
| Scolecomorphidae | Scolecomorphus kirkii | Taylor, 1969 | 2 | 6 | Buried-eyed caecilians | Equatorial Africa (Cameroon, Tanzania, Malawi, Mozambique). |
| Siphonopidae | Microcaecilia dermatophaga | Bonaparte, 1850 | 5 | 28 | South American caecilians | South America (Colombia south to northern Argentina, Paraguay, and southernmost Brazil). |
| Typhlonectidae | Typhlonectes natans | Taylor, 1968 | 5 | 14 | Aquatic caecilians | South America (Colombia and Venezuela south to northern Argentina and Uruguay). |
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.
Phylogenetic evidence suggests that the ancestors of caecilians and (including frogs and salamanders) diverged from one another during the Carboniferous. This leaves a gap of more than 70 million years between the presumed origins of caecilians and the earliest definitive fossils of stem-caecilians.
Prior to 2023, the earliest fossil attributed to a stem-caecilian (an amphibian closer to caecilians than to frogs or salamanders but not a member of the extant caecilian lineage) comes from the Jurassic period. This primitive genus, Eocaecilia, had small limbs and well-developed eyes. In their 2008 description of the Early Permian amphibian Gerobatrachus, Anderson and co-authors suggested that caecilians arose from the Lepospondyl group of ancestral tetrapods, and may be more closely related to amniotes than to frogs and salamanders, which arose from Temnospondyl 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, 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–Cisuralian origin of extant amphibians.
Chinlestegophis, a Stereospondyli temnospondyl from the Late Triassic Chinle Formation of Colorado, was proposed to be a stem-caecilian in a 2017 paper by Pardo and co-authors. If confirmed, this would bolster the proposed pre-Triassic origin of Lissamphibia suggested by molecular clocks. It would fill a gap in the fossil record of early caecilians and suggest that stereospondyls as a whole qualify as stem-group caecilians. However, affinities between Chinlestegophis and gymnophionans have been disputed along several lines of evidence. A 2020 study questioned the choice of characters supporting the relationship, and a 2019 reanalysis of the original data matrix found that other equally parsimonious positions were supported for the placement of Chinlestegophis and gymnophionans among tetrapods. In 2024, Chinlestegophis was consistently recovered as a sister taxon of Rileymillerus within various positions of Stereospondyli outside Lissamphibia based on phylogenetic analyses and revisions.
A 2023 paper by Kligman and co-authors described Funcusvermis, another amphibian from the Chinle Formation of Arizona. Funcusvermis was strongly supported as a stem group caecilian based on traits of its numerous skull and jaw fragments, the largest sample of caecilian fossils known. The paper discussed the various hypotheses for caecilian origins: the polyphyly hypothesis (caecilians as lepospondyls, and other lissamphibians as temnospondyls), the lepospondyl hypothesis (lissamphibians as lepospondyls), and the newer hypothesis supported by Chinlestegophis, where caecilians and other lissamphibians had separate origins within temnospondyls. Nevertheless, all of these ideas were refuted, and the most strongly supported hypothesis combined lissamphibians into a monophyletic group of dissorophoid temnospondyls closely related to Gerobatrachus.
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, which they eat with special scraping teeth. Some larvae, such as those of Typhlonectes, are born with enormous external gills which are shed almost immediately. The egg-laying Herpelidae 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. This form of parental care, known as maternal dermatophagy, has also been reported in two species in the family Siphonopidae: Siphonops annulatus and Microcaecilia dermatophaga. Siphonopids and herpelids are not closely related to each other, having diverged in the Cretaceous Period. The presence of maternal dermatophagy in both families suggest that it may be more widespread among caecilians than previously considered.
Herpele squalostoma caecilians vertically transmit the mother's microbiome to their offspring through maternal dermatophagy. In comparison to other amphibians, the extended parenting of caecilians can provide beneficial bacteria and fungi, but this transmission risks the spread of diseases like chytridiomycosis.
In the folklore of certain regions of India, caecilians are feared and reviled, based on the belief, mostly incorrect, that they are fatally venomous. While some species of caecillians have venomous bites, they are not thought to be fatal to adult humans. Caecilians in the Eastern Himalayas are colloquially known as "back ache snakes",Sathyabhamu, Das Biju, Rachunliu G Kamei, David Gower, & Mark Wilkinson (2009) " Conservation of Caecilians in the Eastern Himalayas Region" Critical Ecosystem Partnership Fund Project Report. pp. 1–22. while in the Western Ghats, Ichthyophis tricolor is considered to be more toxic than a king cobra.K. Ramachandran & Oommen V. Oommen (August 2008) " Deep-rooted myths and their impact on the population of gymnophionan amphibians among the inhabited areas of Kerala, India" FrogLog v. 88. pp 3–5. Despite deep cultural respect for the cobra and other dangerous animals, the caecilian is killed on sight by salt and kerosene. These myths have complicated conservation initiatives for Indian caecilians.
Crotaphatrema lamottei, a rare species native to Mount Oku in Cameroon, is classified as a Kefa-ntie (burrowing creature) by the Oku. Kefa-ntie, a term also encompassing native moles and blind snakes, are considered poisonous, causing painful sores if encountered, contacted, or killed. According to Oku tradition, the ceremony to cleanse the affliction involves a potion composed of ground herbs, palm oil, snail shells, and chicken blood applied to and licked off of the left thumb.
South American caecilians have a variable relationship to local cultures. The minhocão, a legendary worm-like beast in Brazilian folklore, may be inspired by caecilians. Colombian folklore states that the aquatic caecilian, Typhlonectes natans, can be manifested from a lock of hair sealed in a sunken bottle. In southern Mexico and Central America, Dermophis mexicanus is colloquially known as the "tapalcua", a name referencing the belief that it emerges to embed itself in the rear end of any unsuspecting person who chooses to relieve themself over its home. This may be inspired by their tendency to nest in refuse heaps.
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