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Origin and Evolution of Amphibia

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					         Origin and Evolution of Amphibia
The first major groups of amphibians developed in the Devonian period from lobe-finned fish
similar to the modern coelacanth and lungfish, which had evolved multi-jointed leg-like fins
with digits that enabled them to crawl along the sea bottom. Some fish had developed
primitive lungs to help them breathe air when the stagnant pools of the Devonian swamps
were lacking in oxygen. They could also use their strong fins to hoist themselves out of the
water and onto dry land if circumstances required it. Eventually, their bony fins would evolve
into limbs and they would become the ancestors to all tetrapods, including amphibians,
reptiles, birds, and mammals. Despite being able to crawl on land, many of these prehistoric
tetrapodomorph fish still spent most of their time in the water. Ichthyostega was one of these
tetrapods and had four sturdy limbs, a neck, a tail with fins and a skull very similar to the
lobe-finned fish, Eusthenopteron. Amphibians evolved adaptations which allowed them to
stay out of the water for longer periods. However, they never developed the amniotic egg
which prevented the developing embryo from drying out and which allowed early reptiles to
move on to the land to reproduce. They still need to return to water or find a damp place to
lay their shell-less eggs and most have a fully aquatic larval stage.




                          Possible paths of Lissamphibia evolution.

There are large gaps in the fossil record but the discovery of a batrachian from the Early
Permian in Texas in 2008 provided a missing link with a lot of the characteristics of modern
frogs. Molecular analysis suggests that the frog–salamander divergence took place
considerably earlier than the palaeontological evidence indicates. However the date of the
divergence of the caecilians deduced by molecular phylogenetics agrees with the fossil
record.

The first true amphibians appeared in the Carboniferous Period, by which time they were
already moving up the food chain and occupying the ecological position currently claimed by
such animals as crocodiles. Amphibians were once the top land predators, sometimes
reaching several meters in length, preying on the large insects on land and many types of fish
in the water. During the Triassic Period, the better-adapted reptiles began to compete with
amphibians, leading to the reduction of their size and importance in the biosphere.
Lissamphibia, which includes all modern amphibians and is the only surviving lineage of
amphibians left, could have branched off from the extinct groups Temnospondyli and/or
Lepospondyli at some time between the Late Carboniferous and the Early Triassic according
to the fossil record. The relative scarcity of fossil evidence does not permit an exact date, and
the most recent molecular clock study based on multi-locus data suggest a Late
Carboniferous–Early Permian origin of extant amphibians.

				
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