Mark, having become the interpreter of Peter, wrote down accurately everything that he remembered of the words and deeds of our Lord without, however, recording in order what was said and done; for neither did he hear the Lord nor follow him; but as I said, he attended Peter who adapted his instructions to the needs of his hearers. Mark committed no error in writing certain matters just as he remembered them. He took thought for one thing, not to omit or falsify any of the things that he had heard.—Eusebius
Mark, the disciple and interpreter of Peter, handed down in writing the things which his Master proclaimed.—Irenmus (180 A.D.)
Note the prominence of Peter in Mark's Gospel.
(1 Pet. 5. 13)