The Anchor Bible Dictionary describes the "longer ending" of
Mark, usually versified as 16:9-20, as an amalgam of the other three
Gospels that does not appear in the earliest manuscripts.
Is there a fairly solid consensus about that? Is there a consensus date
for the composition 16:9-20, and if so, what is it?
What theories are there of the composition of the longer ending? There
are parallels, but it seems more like paraphrasing than literary
dependence. For example, one compilation suggests:
What grounds are there for supposing that the longer ending was compiled
from the other three Gospels, rather than the other way around? I am
aware that the longer ending of Mark is supposed to have non-Markan
stylistic features, so let's grant for the sake of argument that the
longer ending was not composed by the author of the rest of Mark. But
might it not represent an early non-Markan Resurrection account that
circulated independently at first, then became attached to Mark, and
served as a source for Matthew, Luke and John?