But what could be defined as the "original form of Mark"? If Mark was not the first such Gospel, i.e. if Marcion's Gospel preceded Mark, then would that be considered "original Mark"?
For this exercise I would propose that there was some form of the Gospel of Mark that very closely resembles the canonical version and that Canonical Mark contains primarily additions to a prior version of Mark. I believe that the most significant additions are at the end of the story. But I'm definitely not certain about how the "original" story may have ended. So, what cases can be presented for various endings?
Proposed "original endings":
1) Mark 15:39 And when the centurion, who was standing right in front of Him, saw that He died in this way, he said, “Truly this man was the Son of God!”
2) Mark 16:7-8 But go, tell His disciples and Peter, ‘He is going ahead of you to Galilee; there you will see Him, just as He told you.’” And they went out and fled from the tomb, for trembling and astonishment had gripped them; and they said nothing to anyone, for they were afraid.
3) Something else.
Endings past 16:8 are regarded as certainly later additions.
However, there is a possibility (option 3) that that the "original ending" of Mark is not preserved at all. It may have gone past 15:39, but been different from what we find in the current Gospel.
Argument in favor of #1:
Throughout the Gospel of Mark the scenes are almost entirely composed of scriptural references and/or references to the Pauline letters. Indeed, the crucifixion scene makes extensive use of scriptural references. Yet, the scriptural references, as far as I can see, stop with 15:39 (or perhaps technically with 15:36).
15:40 names Mary Magdalene with no introduction, as if we should know who she is, and then provides a retroactive account of women that had apparently been a part of the story all along but were never mentioned before. Quite strange.
The nature of 15:40-47 seems gratuitously explanatory, unlike the rest of Mark. It all looks quite contrived, which is out of character for Mark.
16:7 seems almost impossible to reconcile with the rest of the story. The entire narrative of Mark works diligently to discredit Peter specifically and all of the disciples in general. Peter and the rest of the disciples had just abandoned Jesus. They were not witnesses to the crucifixion. Why would they be summoned now? Are they really going to be made witnesses to the resurrection when they didn't even witness the crucifixion? I hardly think so.
Argument in favor of #2:
All the extant texts contain material up through 16:8.
Almost all variants of the Gospel story are witnesses to longer endings that contain Mary Magdalene and Joseph of Arimathea.
One could argue that the "original" ending ran though 16:8, but did not include 16:7. In other words one could argue for the ending:
This would address one of the objections of position #1, while still essentially preserving the ending up through 16:8.
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The arguments I've presented are not extensive or necessarily very sophisticated. What more sophisticated arguments can be put forward in favor of these propositions?