Identify the Bone, Win a Prize

I found this small bone yesterday, and I'd like to identify it.  Any ideas?

It was found right on the coast, embedded in sticky clay, out of context.  I noticed the patch of discoloration in the clay, a rich brown, familiar from the North Sea fossils I'd seen in Holland.

We are leaning toward a marine mammal, but would like your input.  What is it?  What's your evidence for it?

Please take particular note of the fourth image from the top, showing a series of punches/impressions in the surface of the bone, which I'd like to interpret as predation -- bite marks.  Check it out, see what you think.

Incentive?  I did mention a prize?  Yes sir, I did.  The person who correctly identifies this bone (understanding, of course, that I have no clue what it is) wins a roll of duct tape.  No kidding, a brand new roll of duct tape, still wrapped in its original packaging.