Amphibians

There are three types of amphibians: 1) Frogs (Anura), 2) Salamanders (Caudata), and 3) Caecilians (Gymniophiona)

The "typical" life cycle of amphibians is a larval stage that lasts a few months followed by a brief metamorphosis and a long multiyear life on land. The adults return to the water to court and ultimately lay eggs. In the case of frogs, the development is illustrated in the two images below. They are taken from "Textbook of Vertebrate Embryology" by N.N. Majumdar.

There are some exceptions to this rule. such as frogs of the genus PRISTIMANTIS, which lay eggs on land, which develop into miniature adults, with no tadpole stage.

Until recently, it was believed that frogs, salamanders, and caecilians all shared a common ancestor. The discovery of the 290 million year old fossil life form Gerobatrachus hottoni, shown below suggests that frogs and salamanders branched out from one set of ancestors while caecilians descended from another.

The creature's combination of frog- and salamander-like features supports earlier studies suggesting that frogs and salamanders are more closely related to each other than to caecilians. (See National Geographic News, 21 May 2008.)

This extinct amphibian fits into the gap between temnospondyls and living amphibians.

(A) Anura

The order anura consists of frogs and toads. The term "frog" usually refers to species that are aquatic or semi-aquatic with smooth skins, and the term "toad" generally refers to species that tend to be terrestrial with dry, warty skins. The use of the common names "frog" and "toad" has no taxonomical justification.

Most frogs are characterised by long hind legs, a short body, webbed digits, protruding eyes, and the absence of a tail. Frogs are widely known as exceptional jumpers. Most species are found in tropical rainforests. They are among the most diverse of vertebrates, but the populations of certain species are declining significantly.

Many frogs contain mild toxins, that make them unpalatable to potential predators. All toads have large poison glands -- the parotoid glands -- located behind the eyes, on top of the head. However, some frog species are edible. (See the wikipedia articles FROG LEGS and EDIBLE FROG.)

(B) Caudata

The salamander is a tailed amphibian with soft, moist skin. Most species have four limbs, a few have only two. The largest species grows to about 5 1/2 feet, the smallest measures 1 1/2 inches. Salamanders look more like lizards than other amphibians. However, they are not as dry and scaly as lizards are, and they don't have claws.

After a salamander loses its tail, it can regenerate, or regrow, a new one. Some salamanders can regenerate legs too. Some can even regenerate parts of their spinal cord, organs, and eyes.

There are about 55 genera of salamanders grouped into the following 8 families. (See HowStuffWorks "Kinds of Salamanders" for more details.)

• Hynobiidoe
  Asiatic land salamanders. They are found only in eastern Asia.
• Cryptobranchidae
  Giant salamanders. These include the giant salamander of Japan and the hellbender of North America.
• Ambystomatidae
  Mole salamanders. They are found throughout North America and include the marbled salamander and the axolotl.
• Salamandridae
  Fire salamanders and newts.
• Amphiumidae
• Plethodontidae
  Lungless salamanders. It is the largest family, with 180 species. The red-backed salamander is mentioned in the HowStuffWorks page. It is the most common salamander in the United States.
• Proteidae
  Olm and mud puppies. The olm is the most long lived of all amphibians, with a life span of 58 years.
• Sirenidae
  Sirens. In contrast to most other salamanders, they have external gills bunched together on the neck in in both larval and adult stages. They are the only salamanders without hind limbs. Found only in the United States and Mexico.

(The tolweb page on caudata shows 10 families.)

(C) Gymnophiona

Detailed information can be found at Gymnophiona.org.

The gymnophiona are an order of limbless, worm-like amphibians known as caecilians. Caecilians typically are blind or nearly so and possess sensory tentacles between the eyes and nostrils. Along with deep sea fishes, they are some of the least studied and least understood vertebrates on the planet. They are adapted primarily for burrowing.

Elongation of the body and the reduction or loss of limbs is a common adaptation in all burrowing tetrapods and caecilians are extreme examples of this. The result is a worm-like or snake-like appearance.Some aquatic species (Typhlonectes, Atretochona, and Potomotyphlus) have a small raised "fin" along the posterior portion of the body to aid in swimming. there is no trace of pelvic or pectoral girdles left. The body is arranged in rings (annuli), giving caecilians an earthworm-like appearance.

Caecilian skulls are strongly built and heavily ossified. The head acts like a battering ram as caecilians push and prod their way through the soil. Illustrations of caecilian skulls are found at www.gymnophiona.org/morphology.

Caecilians are predominantly tropical and can be found in the tropical parts of the Americas, Africa, India, The Seychelles Archipelago, and Southeast Asia.

Links. General information can be found in the pages listed at

1. amphibiaweb.org
2. Wikipedia - Category Amphibians
3. Howstuffworks "Amphibians"
4. Flickr: Salamanders & Caecilians -- Contains a large number of photographs.

Some further links from AmphibiaWeb Biology Links are listed below:

SALAMANDERS

• Caudata.org -- Information on captive maintenance and breeding of salamanders
• Salamander Feeding Videos by Stephan Deban
• Salamanders of the Old World -- Information on the natural history of the newts and salamanders of Europe, Asia and Northern Africa

FROGS AND TADPOLES

• African Frogs at the California Academy of Sciences
• Frogs and Tadpoles of Borneo
• Frogs.org -- A great resource on frog conservation, by the Amphibian Conservation Alliance
• Frogs.org.au -- Australian Frog Conservation site
• Leptodactylus -- information bank for Leptodactylus frogs
• Tadpole Identification Key for Europe, by Friedo Berninghausen
• Tadpole Identification Key for the United States and Canada, hosted by the U.S. Geological Survey
• Virtual Frog Dissection Kit
• Western Ghats Frogs (geocities: only a small portion has been archived)
• Western Paleartic Water Frogs

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