The Ending of St. Mark's Gospel

In The Gospel according to St. Luke the late Professor Creed accepted the view of Welhausen and Meyer that the original version of Mark was intended to close with the words ἐϕοβοῦντο γάρ. The view avoids the difficulty of explaining the fact that neither Matthew nor Luke knows any other ending; it is certainly hard to see how a document could have gained so wide a circulation that the authors of these two Gospels knew it, and yet had at some previous date been so little known that the only surviving copy had lost its original ending.