Original: The fin-limbs of sarcopterygiians show such a strong similarity to the expected ancestral form of tetrapod limbs that they have been universally considered the direct ancestors of tetrapods in the scientific literature.
Simplified: The fin-limbs of sarcopterygiians are very similar to what people believe was the ancestral form of tetrapod limbs. This has caused them to be believed to be the direct ancestors of tetrapods in the scientific literature.