Caecilians (New Latin for 'blind ones', from caecus, blind) are a group of limbless, worm-shaped or snake-shaped amphibians, with either small eyes or no eyes, comprising the order Gymnophiona. They mostly live hidden in soil or in streambeds, making them some of the least familiar amphibians. Modern caecilians live in the tropics of South and Central America, Africa, and southern Asia. Caecilians feed on small subterranean creatures, such as earthworms. The body is noodle-like and often dark in colour, and the skull is bullet-shaped and strongly built. The caecilian head has several unique adaptations, such as fused skull and jaw bones, a two-part system of jaw muscles, and chemosensory tentacles between the eyes and nostrils. The skin is slimy, with ringlike markings or grooves, and in some species hides scales underneath. Modern caecilians are a clade, the order Gymnophiona (or Apoda), one of the three living amphibian groups alongside Anura (frogs and toads) and Urodela (salamanders). 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. The clade's name 'Gymnophiona' comes from Ancient Greek γυμνος (gumnos), meaning "naked", and ὄφις (óphis), meaning "snake", as the caecilians were originally thought to be related to snakes and to lack scales. 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 lissamphibians) are each other's closest relatives. Frogs and salamanders show many similarities to dissorophoids, 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 lepospondyl or stereospondyl 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.
Description
Caecilians' anatomy is highly adapted for a burrowing lifestyle. In a couple of species, belonging to the primitive genus Ichthyophis, vestigial traces of limbs have been found, and in Typhlonectes compressicauda the presence of limb buds has been observed during embryonic development, remnants in an otherwise completely limbless body. This makes the smaller species resemble worms, while the larger species like Caecilia thompsoni, with lengths up to 1.5 m (5 ft), resemble snakes. Their tails are short or absent, and their cloacae are near the ends of their bodies. Except for one lungless species, Atretochoana eiselti, 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. 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. By contracting the outer layer of muscles it squeezes the coelom and generates a strong hydrostatic force that lengthens the body. 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 first fossil discovered belonging to an extant family of caecilians, Ymboirana acrux, is from family Typhlonectidae. The representatives of this family have a fleshy fin running along the rear section of their bodies, which enhances propulsion in water.
Skull and senses Caecilians have small or absent eyes, with only a single known class of photoreceptors, and their vision is limited to dark-light perception. Despite their restricted vision, caecilians display circadian rhythms in response to the photoperiod and tend to demonstrate more surface activity at night. Unlike other modern amphibians (frogs and salamanders), the skull is compact and solid, with few large openings between plate-like cranial bones. The snout is pointed and bullet-shaped, used to force their way through soil or mud. In most species, the mouth is recessed under the head, so that the snout overhangs the mouth. The bones in the skull are reduced in number compared to prehistoric amphibian species. Caecilians have one of two skull types, stegokrotaphic or zygokrotaphic. In stegokrotaphic skulls the squamosal covers the temporal region and the jaw closing muscles while in zygokrotaphic skulls the squamosal only partially covers the temporal region. Many bones of the skull are fused together: the maxilla and palatine bones have fused into a maxillopalatine in all living caecilians, and the nasal 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 septomaxillae, prefrontals, an/or a postfrontal-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.
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