A friend of mine asked me about the longer ending of Mark today. Mark 16:9-20 poses an interesting question because they are missing from some important early witnesses. There are also a additional verses found in some manuscripts (See Metzger, Textual Commentary on the New Testament, pp. 102-6).
As I was looking back through Mark 16:9-16 today, I was reminded about how this passage is such a collage of resurrection appearance episodes found in Matthew, Luke, and John. Mary Magdalene appears to Jesus as in John. There is an echo of the Lukan Emmaus story in Mark 16:12-13. Jesus gives the disciples a Matthean-like Great Commission, along with some pieces about the handling of serpents (which may reflect knowledge of Acts 28:3-6). The ascension of Jesus in Mark 16:19 also appears to indicate knowledge of Luke 24 and Acts 1, especially the final verse about the disciples preaching.
Recently, I have also noticed a couple features that may add to further evidence of Johannine influence. First, the deeds in 16:17-18 are called signs. Jesus' actions referred to as signs in John's Gospel. And second, Mark 16:9-16 is the only place in the Synoptic Gospels where the narrator uses the verb pisteuo. The verb is actually used three times vv. 14, 16, 17. In John, pisteuo is often used by the narrator.
These pieces of evidence along with the questionable manuscript history of Mark 16:9-20 suggests that this passage was added to Mark after its original writing and that the author of it most likely made use of the gospel traditions in the other three gospels.
For further reading, see Perspectives on the Ending of Mark: Four Views published by Broadman & Holman.