Complex sentence: Eusthenopteron was a member of the group known as the Crossopterygii, or lobe-finned fishes; it was probably near the main line of evolution leading to the first terrestrial vertebrates, the primitive amphibians.
Split into simpler sentences: Eusthenopteron was a member of the group known as the Crossopterygii (lobe-finned fishes). It was most likely near the main line of evolution leading to the first terrestrial vertebrates, primitive amphibians.