Concept

Caecilian

Summary
(sᵻˈsɪliən) are a group of limbless, vermiform (worm-shaped) or serpentine (snake-shaped) amphibians. 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 and Central America, Africa, and southern Asia. Caecilians feed on small subterranean creatures such as earthworms. 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 chemosensory tentacle in front of the eye. The skin is slimy and bears ringlike markings or grooves, which may contain tiny scales. Modern caecilians are grouped as a clade, Apoda 'aep@d@, one of three living amphibian groups alongside Anura (frogs) and Urodela (salamanders). Apoda is a crown group, encompassing modern, entirely limbless caecilians. There are more than 200 living species of caecilian distributed across 10 families. The caecilian total group is an order known as Gymnophiona dZImn@'fai@n@, which includes Apoda as well as a few extinct stem-group caecilians (amphibians related to modern caecilians but evolving prior to the crown group). Gymnophiona as a name derives from the Greek words γυμνος / gymnos and οφις / ophis, as the caecilians were originally thought to be related to snakes. 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 others' 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).
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