Our Gospel of Mark is a Shortened Version of a Longer Text

For Matthew is understood to have taken it in hand to construct the record of the incarnation of the Lord according to the royal lineage, and to give an account of most part of His deeds and words as they stood in relation to this present life of men. Mark follows him closely, and looks like his attendant and epitomizer.   For in his narrative he gives nothing in concert with John apart from the others: by himself separately, he has little to record; in conjunction with Luke, as distinguished from the rest, he has still less; but in concord with Matthew, he has a very large number of passages. Much, too, he narrates in words almost numerically and identically the same as those used by Matthew, where the agreement is either with that evangelist alone, or with him in connection with the rest. [Augustine De Consensu Evangelistarum I.2.4]