Limbs:
The move to land had to be facilitated by a different limb structure than fish posess. The limbs had to be slowly altered to carry more body weight for the different locomotion and gravity conditions of terrestrial organisms. These limbs were derived from fish fins. When they were developing, the aim was not to become a terrestrial organism, but probably to aid in propulsion and short excursions out of the water.
Cladistical analyses again point to the Sarcopterygians
as the closest relatives in tems of limb structure, (again, Sarcopterygians
encompass both Dipnoi and Rhipidistians). This conclusion was reached
from a 1989 cladistic study.
Lungs:
The other preadaptation that goes in stride with the development of land friendly limbs, is the development of lungs. This preadaptation was developed to aid fishes survival AS FISH, not to immediately propel them into a terrestrial environment.
In the short survival induced trips out of the water, lobe fins would have had to support their body weight, which is where the tie in with limb development becomes apparent. For the succesful development of lungs as a survival aid, that adaptation had to be linked to the limb adaptation to be effective.
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