Ichthyostega and Acanthostega both show a combination of fish and land vertebrate characters. Bones of the head and neck region have grooves suggesting the presence of internal gills, that were probably functional in the young, if not in the adults, and a very fish-like tail. On the other hand, their pectoral and pelvic girdles are similar to those of terrestrial vertebrates, indicating that they had extensive musculature attaching to the limbs that could raise their bodies off the ground. They were certainly able to swim, but probably also able to walk on land. That is, they were amphibious. It is extremely difficult to imagine how such girdles and limbs could have evolved in an aquatic environment. Both Ichthyostega and Acanthostega and their closest fish relatives were relatively large animals, approximately a meter from snout to tail. Their large body size would have enabled them to absorb heat from the sun if they were on land, in the same way as crocodiles bask in the sun. Having a high body temperature would have enabled them to be active both on land and for short periods of time in the water. I have just submitted a paper to a scientific journal, written with two other biologists, arguing the fish initially came on land to take advantage of the energy of the sun. This is only practical in fairly large animals. Small amphibians are only known several million years after Ichthyostega and Acanthostega. Animals smaller than about 1 meter in length loose their body heat too rapidly to make basking practical.
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