For most of earth’s history, there was no life on land.  One reason was that, until photosynthesis had contributed enough oxygen to form an ozone layer, too much ultraviolet light from the sun bombarded the land, making life impossible.  By the Siluran Period, plants and arthropods had made the transition to land 100 million years before the first amphibians would adapt to terrestrial environments.  Although moving from aquatic to terrestrial life seems like an enormous change for a fish, a variety of modern ray-finned (actinopterygian) fish, such as mudskippers and walking catfish, have adapted to terrestrial life by crawling on their fins and adapting various parts of their bodies for breathing oxygen from the air. The following images are of modern catfish which can breathe oxygen from the air and maneuver themselves onto land.

Primitive ray-finned fish possess a swim bladder which they can use to breathe atmospheric air.  This swim bladder branches from the esophagus, just like the lungs of amphibians.  





Modern lungfish have lungs (hence their name). Presumably, their prehistoric relatives had lungs as well before they left aquatic habitats.  The following picture of a lungfish shows both its gills and lungs.

In lungfish, the lungs are long structures which join with the esophagus.

The lungs of a salamander are also long organs which join with the esophagus, as seen in the following photo.

     Why would fish evolve lungs before they began to inhabit the land?  The lack of oxygen in aquatic environments (such as swamps) can cause mortality in fish since slow-moving waters can have very low levels of dissolved oxygen (especially at warmer temperatures).  Any fish with lungs has an advantage in these environments since it can supplement its oxygen supply from the gills with oxygen from the air. 

    The first amphibians did not need to evolve new bones such as the scapula and clavicle of the shoulder, bones for the hip, the humerus, radius and ulna for the arm, or the femur, tibia, and fibula for the leg—these bones were already present in the fins of advanced sarcopterygian fish. Perhaps these fossil fish used their fins/limbs to leave water briefly to move from one body of water to another.  Perhaps these ancient fish used these bones in their fins to move themselves along the bottom.  The early amphibians simply modified the fins/limbs of their fish ancestors to permit better movement on land. The pectoral girdle was separated from the bones of the skull, fingers and toes developed from cartilaginous fin rays, and the pelvis fused to the spinal column for support.      

     Eusthenopteron was a sarcopterygian fish which had bones in its fins homologous to the limb bones of land vertebrates (humerus, radius, ulna, femur, tibia, fibula).  The shoulder (pectoral) girdle was still attached to the skull so that there were a few bones which were lost in the first amphibians when the shoulder separted from the skull. 
ACANTHOSTEGA       Acanthostega is the oldest tetrapod known from Greenland, 360 million years ago.   While it is recognized as the most primitive amphibian, there are a number of aspects of its skeleton which link it to sarcopterygian fish and demonstrate that it was not fully adapted to locomotion on land.
     Although Acanthostega had limbs and digits, it couldn’t have supported its weight and walked on these limbs since there were almost no modifications of the sacral vertebrae to interact with pelvic bones.   Interestingly, it seems that the number of digits in a tetrapod limb wasn’t fixed at five in the first amphibians: Acanthostega had 8 fingers, Ichthyostega had 6 toes, and  Tulerpeton had six fingers.  Ichthyostega is a more advanced amphibian from the Upper Devonian. 

     Modern amphibians are classified into 3 orders, 34 families, about 400 genera and more than 4000 species.  Although they were once the only group of land vertebrates, they are now the smallest group. 

     Of the three groups of amphibians, there is one group, the legless caecilians, which do not occur in the United States.  Two caecilians are pictured in the following images.

Chtonerpeton indistinctum


Siphnops paulensis



    Among vertebrates, amphibians have the greatest diversity in their modes of reproduction.  Fertilization can be internal (occurring inside the female’s body) or external (occurring outside the female’s body) after both the male and female have released their gametes.  Most salamanders use internal fertilization and females pick up packets of sperm (called spermatophores) with their cloacal lips.  Females can often store sperm inside their bodies for a certain period.  In two North American species of salamander, the sperm simply activate the process of reproduction but do not contribute genetic information.  Most frogs reproduce through external fertilization.  Caecilians utilize internal fertilization with the inverted cloaca of males functioning as an intromittent organ (functionally equivalent to the penis).  Many caecilians lay eggs which are guarded by the female.  About half the caecilians give live birth and may use uterine secretions (“milk”) to feed their offspring.

     Many amphibians do not undergo an aquatic larval stage.  The majority of salamanders and some frogs (typically in tropical regions) undergo direct development in which the larval stage matures inside the egg and the hatchlings resemble small adults.  Metamorphosis is not as dramatic in salamanders as it is in frogs since the larval salamanders are very similar to the adults in their overall form.   Salamander larvae lose their external gills and undergo a few internal changes to adapt to life on land.  Many salamanders paedomorphic, retaining larval characteristics (such as external gills) as adults. 

     In frogs, the tadpole larvae are very different from the adults.  As tadpoles mature into adults, the tail is reabsorbed, the larval teeth is shed, the mouth becomes much wider, and the long intestine (which is needed to digest the plant material that makes up a significant amount of a tadpole’s diet) is reduced and may stretch only 15% of its original length. 

The hormones prolactin and thyroxine are important mediators of amphibian metamorphosis.  Some desert toads exist as tadpoles for as few as 8 days, other species of frog (such as bullfrogs) may spend 2 years in their tadpole stage.