Modern Forms

Today there are three lineages of extant amphibians, anurans (frogs and toads), urodeles (salamanders and newts), and gymnophionans (caecilians). While these three groups vary greatly in appearance they are all placed in the same clade, Lissamphibia (means smooth skin), since they share a number of characters.

There are three theories regarding the relationships among the three living amphibian groups and their origins.

Molecular studies indicate that the living amphibians form a natural group, hence casting doubt on the diphyletic theory, and the fossils strongly indicate that the 'temnospondyl theory' is most likely correct.

Anurans - the frogs and toads. These are the most successful and numerous of the three lineages with well over 4,100 species, split into 23 families, and having a global distribution. The main difference between frogs and toads is that frogs only have teeth on their upper jaw while toads have no teeth at all, although toads usually are more terrestrial and have a broader body and rougher skin.

Frogs have a reduced spinal column, usually with fewer than ten vertebrae. This is because of their method of jumping - if the spine were any longer, then it would be weakened when the frog jumped, which would cause the spine to shatter. Other frog skeletal elements include broad flat skulls with reduced cranial elements, fused tibia and fibula, posterior vertebrae are fused into a urostyle and the hind legs are highly elongated for jumping.

Pygmy rain frog - Eleutherodactylus ridens. Copyright 2003 Twan Leenders. Cane toad - Bufo marinus. Copyright 2001 Johm P. Clark.
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Urodeles - the salamanders and newts. There are at least 410 species of urodeles and they represent the most generalized land vertebrates. Unlike most amphibians many use a unique form of internal fertilization, in which the male produces a spermatophore onto stone and the female then walks over it and picks it up.

Metamorphosis in urodeles is very simple, but neoteny can occur. This is when the organism develops gonads but doesn't undergo the rest of the changes from larvae to adult. In some species neoteny is facultative (optional for each generation) while in others it is permanent and irreversible.

The body of urodeles are generally elongated, with short limbs and a flat tale for swimming. Their skulls are broad and flat with light bones and the braincase is fused with the parietal bones.

Red spotted Eastern newt - Notophthalmus viredescens viredescens. Copyright 2002 Jessica Miller. California Tiger salamander - Ambyostoma californiense. Copyright 2000 Joyce Gross.
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Gymnophiona - caecilians. About 160 species of earthworm-like amphibians. They have no legs, no girdles and only rudimentary eyes. They are tropical burrowers and live in leaf-litter and soft soil, for which their skulls are solidly built and reinforced. they have elongated bodies, with up to 200 vertebrae, and very short tails. They are often compared to snakes but they have very short trunks and long bodies.

Argentine caecilian - Chthonerpeton indistinctum. Copyright 2003 Mirco Sole.
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