Evolution of Amphibians

• The first land dwelling creatures with four legs, known as tetrapods
• They evolved from lobe-finned fish in the late Devonian to early Carboniferous
• They then ventured forth into terrestrial environments, where they would evolve into reptiles, birds and mammals

• Lobe-finned fish are lungfish 
  
• These fish had the ability to breath in and out of water
• They had robust fleshy fins with an arrangement of central bones inside, similar to the structure of bones in a hand (which would have allowed for mobility and support - especially on land) - However a disadvantage of this design was that there was no strengthening girdle connecting the bones at the extremities to the rest of the skeleton which created an area of weakness
• They had functional lungs (modified swim bladders) - this allowed them to breath out of water
• They could move from one water source to another and exploit life on land, with the absence of predators
• Fossil remains of these were found in Devonian Strata and can be found on earth today

• The four fins of the lobe-finned fish are the four limbs of the early amphibian's skeletal structures were very similar
• Their limbs were in the same position on their bodies
• They both lacked claws or nails
• The skull morphology, the jaw bone and teeth of the lobe-finned fish and amphibians were very much alike
• The amphibian's skull became more slender, with the temporal and opercular bones becoming smaller in size, and the jaw bones becoming more fused together
• The teeth of of both the lobe-finned fish and amphibians were complex
• Early amphibians still had a tail fin, suggesting it still spent a great deal of time in the water
• Early amphibians still had traces of small bony scales on the skin, a trait of fishes

Early amphibians had many similar characteristics to lobe-finned fish but also had some unique charactersistics which allowed them to adapt to life on land. These included:

• The development of a girdle connecting the limb bones to the skeleton fro better movement on land
• A more robust skeleton strengthening the vertebral column and rib bones, for support on land
• Eyelids formed to help keep eyes moist as it was no longer always submerged in water
• The development of a double-loop circulatory system and a three chambered heart to pump mixed blood before and after it had been to the lungs (for more efficient gas exchange)
• A tounge formed within its mouth, which could be used to catch pray as well as having a sensory role
• Ears adapted so it could detect soundw aves through the thin medium of air, allowing it to listen for prey or predators

However amphibians still had to remain close to a water source as gas exchange still took place through skin, so needed to be kept moist. They also had to lay their eggs in water so they wouln't dry out and the young would hatch with gills and metamorphose to develop terrestrial features.