The species isthe ultra-rare Phymatoceras rude (hurray, another species from my “wants list” down !), and it usually comes from the striatulum subzone of the upper lias, the Peak Mudstone member at Ravenscar, where the beds were protected by the downthrow of the Peak Fault from the erosion that everywhere else at the end of the upper toarcian eroded a lot of the uppermost toarcian.

Now wait, you will say, if these beds were eroded everywhere else, then how come this ammonite can be found at Port Mulgrave ?

There are two possible theories (and I think I can trust the data given by the seller, who wasn´t even selling it as a Phymatoceras, so that excludes the third option that the seller tried to make a bit of extra money by selling a rare ammonite that comes from somewhere else…) :

1. When you look closely at the ammonite, you see a rusty brown sandy substance in the umbilicus.
   I think this is middle jurassic sandstone, and that the ammonite was naturally eroded from the striatulum subzone and then re-bedded when the aalenian sandstone was sedimented. I have seen small badly eroded fragments of ammonites in these sandstones before, but never a recognizable ammonite.
   I think somebody even told me about a nautilus from the sandstones as well…
2. Somebody found this ammonite at Ravenscar and lost it at Port Mulgrave. Much less interesting…

AndyS

P.S.: If you are the seller of this ammonite : I know you might think that the sales price was a little low now that you´ve read this.
Console yourself with the tought that you´ve earned yourself a copy of the book once it´s ready !