Ichthyostega
Ichthyostega is an early tetrapod and lived during the Late Devonian period. Being one of the first animals with legs, arms, and finger bones, Ichthyostega was a hybrid between a fish and an amphibian. Ichthyostega had legs but its limbs probably weren't used for walking as once believed, but were used instead to negotiate its way through the swamps of the time.
Primitive amphibians like Ichthyostega differed from animals like Crossopterygians in that although Crossopterygians had lungs, they used their gills as the primary means of acquiring oxygen. Ichthyostega used lungs as its primary means of breathing. Primitive amphibians like Ichthyostega had a special type of skin that helped them retain bodily fluids and deter desiccation whereas Crossopterygians did not, and a stronger skeletal structure allowed Ichthyostega to live more comfortably with the increased burden of weight on land. Moreover, Crossopterygians used their bodies and tails to move about while using their fins for balance while Ichthyostega instead used its limbs for locomotion and its tail for balance.
Amphibians gained an advantage by moving to live on land in that there was less competition for food and they could avoid large predatory fish that ruled the seas. The move also came with disadvantages such as the new requirement of a moist gas-permeable skin in order to aid inefficient lungs. Water was still a requirement too because the jelly-like eggs of an amphibian cannot survive out of water, so reproduction could not occur without it. Water was also needed for the external fertilization that is characteristic of amphibians. Most land-dwelling animals have since developed various methods of internal fertilization.
Ichthyostega was succeeded by Eryops, an amphibian that developed the ability to walk on land.
Referenced By
Amphibia | Amphibian | Amphibians | Eryops | Lissamphibia
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