Middle Jurassic – Present, 167.7–0 Ma
|Hellbender, Cryptobranchus alleganiensis|
Caudata is a group of amphibians containing the salamanders (Urodela) and all extinct species of salamander-like amphibians more closely related to salamanders than to frogs. They are typically characterized by a superficially lizard-like appearance, with slender bodies, blunt snouts, short limbs projecting at right angles to the body, and the presence of a tail in both larvae and adults.
Disagreement exists between different authorities as to the definition of the terms "Caudata" and "Urodela". Some maintain that Urodela should be restricted to the crown group, with Caudata being used for the total group. Others restrict the name Caudata to the crown group and use Urodela for the total group. The former approach seems to be most widely adopted and is used in this article.
The origins and evolutionary relationships between the three main groups of amphibians (apodans, urodeles and anurans) is a matter of debate. A 2005 molecular phylogeny, based on rDNA analysis, suggested that the first divergence between these three groups took place soon after they had branched from the lobe-finned fish in the Devonian (around 360 million years ago), and before the breakup of the supercontinent Pangaea. The briefness of this period, and the speed at which radiation took place, may help to account for the relative scarcity of amphibian fossils that appear to be closely related to lissamphibians. However, more recent studies have generally found more recent (Late Carboniferous to Permian) age for the basalmost divergence among lissamphibians.
The first known fossil salamanders are Kokartus honorarius from the Middle Jurassic of Kyrgyzstan and two species of the apparently neotenic, aquatic Marmorerpeton from England of a similar date. They looked superficially like robust modern salamanders but lacked a number of anatomical features that developed later. Karaurus sharovi from the Upper Jurassic of Kazakhstan resembled modern mole salamanders in morphology and probably had a similar burrowing lifestyle.
The Cryptobranchoidea (primitive salamanders) and the Salamandroidea, also known as Diadectosalamandroidei, (advanced salamanders) are believed to be sister groups. Both seem to have appeared before the end of the Jurassic, the former being exemplified by Chunerpeton tianyiensis, Pangerpeton sinensis, Jeholotriton paradoxus, Regalerpeton weichangensis, Liaoxitriton daohugouensis and Iridotriton hechti, and the latter by Beiyanerpeton jianpingensis. By the Upper Cretaceous, most or all of the living salamander families had probably appeared.
All known fossil salamanders and all extinct species fall under the order Caudata, while sometimes the extant species are grouped together as the Urodela. There are about 655 extant species of salamander.
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