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Amphibians - The First Terrestrial Vertebrates
Amphibians were the first step in the evolution of terrestrial vertebrates from aquatic vertebrates.  Many of their characteristics reflect this transition.  For example, most amphibians still lay eggs in water, which is a characteristic that relates to their aquatic ancestry.  At the same time, most amphibians breathe air and are freely mobile on land, key adaptations for terrestrial life.  These traits are the typical amphibian traits most people are familiar with; however, amphibians are too diverse and complex to fit into such a narrow definition. 

Amphibian Evolution
Amphibians are the first terrestrial vertebrates.  Several features of tetrapods (reptiles, mammals, fish, and birds) evolved first in amphibians.  Lungs and limbs are among the most important; however, an increase in the density of the skeleton and changes in musculature were necessary for movement on land.  The skull also changed dramatically in the transition for aquatic to terrestrial lifestyles.  Changes in the eyes and ears were associated with this transition but these changes are difficult to trace in the fossil record because the eyes and ears are soft tissue.  Soft tissue does not typically get preserved in the fossil record.  Once amphibians were in the terrestrial environment, there was a great radiation of species because of the lack of competition. 

The earliest terrestrial amphibian-like fossils date back more than 360 million years, but the evolution of each order of amphibians is a little less understood.  Salamanders are most similar to the first terrestrial vertebrates in terms of their physical appearance.  The transition from this salamander-like form to caecilians and frogs is not well understood or documented in the fossil record. 

Amphibians exhibit a wide range of characteristics.  They are ectothermic vertebrates with moist skin and a three-chambered heart.  They may reproduce by laying eggs or giving birth to live young.  Most go through a larval stage before metamorphosing into adults; however, many cases of direct development are known.  Parental care is not necessary for the survival of the young.  Respiration in amphibians may be via the skin, lungs, or gills.  Amphibian skin contains mucous and poison glands (granular glands) that vary from non-toxic to lethally toxic.  All adult amphibians are carnivorous; however, some larvae are herbivorous.  Approximately 5500 species of amphibians exist.

The class Amphibia contains three orders:  1) Gymnophiona (caecilians), 2) Caudata (salamanders and newts), and 3) Anura (toads and frogs).  Systematic biologists are still disputing the relationship between these three orders.  Skeletal morphology closely links the frogs to salamanders; yet, soft tissue morphology links salamanders and caecilians.  DNA evidence is a little ambiguous, but a combination of all evidence supports the frog and salamander link.  Each order is discussed in detail below:

Caecilians - The Order Gymnophiona
Caecilians are elongate, legless amphibians that appear to have segmented bodies much like worms.  The annular rings encircling the body give caecilians their segmented appearance. Scale-like structures can be found within these rings; however, they are not the same as reptile scales.  Caecilian scales are layers of fibers topped by mineralized nodules whereas reptile scales are keratinized folds of skin.  The eyes of caecilians are either covered by skin or bone; however, they all have a retractable tentacle that acts as a chemosensory organ.  Fertilization is internal and young may hatch from eggs and go through a larval stage before becoming adults or they may develop in the egg directly into the adult form; however, most species (75%) are viviparous.  That is, the young develop directly inside the female and are born live.  Caecilians are terrestrial, fossorial or aquatic.  Approximately 170 species have been described and they are found only in the tropics.

Salamanders and Newts - The Order Caudata
Most salamanders have at least two pairs of limbs, a tail, and no annular rings.  However, some aquatic forms may lack limbs. Fertilization may be external or internal and young may hatch from eggs and go through a larval stage before metamorphosing into adults.  However, one family, Plethodontidae – lungless salamanders, lay eggs on land that develop directly into the adult form. A small number of species are viviparous.  Salamanders may be terrestrial, fossorial, or aquatic.  Respiration may be via gills, lungs, or skin or a combination of these depending upon the life stage.  Several species actually exhibit the larval characteristic of external gills throughout their adult life.  They are referred to as neotenic salamanders.  Scientists have described over 400 species of salamanders.  They are found throughout the world with the exception of the arctic and Antarctic.

Frogs and Toads - The Order Anura
Frogs and toads are the most common amphibians and they account for almost 80% (approximately 4,800 species) of all amphibian species.  All frogs and toads have four limbs with the hind limbs typically being longer than the forelimbs.  The caudal vertebrae are fused into a structure called the urostyle.  Fertilization is external with the exception of three species, which have internal fertilization.  The tailed frog, Ascaphus truei, is an example of a species of frog that has internal fertilization and it may be found in British Columbia and the northwestern U.S.  As with other amphibians, frogs typically hatch from eggs and go through a larval stage before transforming into the adult form.  Frogs also may have direct development and or they may be viviparous.  Anurans are found worldwide with exception of the Polar Regions.  

All photos: J. V. Feltham

Amphibians

Reptiles
 

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