The first fishes to develop lungs were the lobe-finned fishes called Crossopterygians and the lungfishes called Dipnoans.
The Crossopterygians are considered tobe the ancestors to land animals.
The Crossopterygians had evolved "land animal-like features":
• A more impervious skeleton
• Enhanced nostril development
• Leg-like bone features in their fins.One type of Crossopterygian was Eusthenopteron.
Eusthenopteron was one of the first fishes to venture out onto land. This occurred toward the end of the Devonian Period.
Animals likeEusthenopteron may have come out unto land for the following reasons:
• Their ponds dried out during a draught and they had to cross over land to find a new pond.
• They needed to find new food sources.Primitive Amphibians Develop True Legs and Spend Extended Time on Land
Ichthyostega lived during the Late Devonian Times. Ichthyostega was a cross between a fish and an amphibian. Ichthyostega had legs andwalked and was a true tetrapod. With true legs, it could live on land for extended periods.
Primitive amphibians like Ichthyostega differed from animals like Crossopterygians (picture above) in thefollowing ways:
• Although the Crossopterygians had lungs, they used their gills as the primary means of acquiring oxygen. Conversely, Ichthyostega used lungs as the primary means of getting oxygen.• The primitive amphibians like Ichthyostega had a special kind of skin that helped them retain bodily fluids and deter desiccation. Crossopterygians did not have this feature.
• The skeletons ofanimals like Ichthyostega had grown stronger than those of Crossopterygians. Stronger skeletons allowed the primitive amphibians to live more comfortably with the increased gravity on land.
• TheCrossopterygians used their bodies and tails to move about while using their fins for balance. Animals like Ichthyostega used their limbs for locomotion and their tails for balance.