Hammatoceras ammonite in situ,
picture courtesy of R. Taylor

It came from the Grey Sandstone beds of the Grey Sandstone member of the upper lias,
?dispansum Zone.

We had briefly discussed it over facebook and consensus was that it could be a
Hammatoceras. I offered to prep it as well, price as usual – a photo for the book.
Unfortunately we had at that time already left Yorkshire again, so Robert posted
the ammonite to me, and after arrival  I could confirm that it is actually a
Hammatoceras.

I took a good look and did some exploratory prep of the inner whorls on one side.
Unfortunately it did not look so good, no inner whorl became visible, there were
only slight brown discolorations so I put it on side for a while.

All literature available to me did not mention any Hammatoceras from Yorkshire
so I contacted the Zoé Hughes, curator for ammonites at the NHM,
and Crispin Little, senior lecturer at Leeds University who had a project
cataloguing some finds of a student of his from the Ravenscar area,
but both confirmed they had not seen a Yorkshire Hammatoceras.

So when the time came to do the final prep on the ammonite
(I had promised to give it back to Robert this summer 🙂 ),
I decided to check the other side, where a part of the inner whorl was visible.

Hammatoceras ammonite as found,
picture courtesy of R. Taylor

Since most of the outer whorl on this side was eroded away, it was an easy decision to
remove the remnants of the outer whorl, bowling it out so it could still be seen from
the other side.

After about 10 hours total this is how it looks now :

Hammatoceras cf. semilunatum, 10 cm inner whorl

It was relatively difficult to prep, what is preserved of the shell is mostly sideritic, at the
surface probably converted to limonite,  but overall very soft and brittle. The innermost
whorls are mostly not there.

Hammatoceras cf. semilunatum, 15 cm, keel view

Diameter of the inner whorl is 10 cm, including the crushed outer whorl it is about 15 cm.
I would tentatively put this towards Hammatoceras cf. semilunatum (QUENSTEDT, 1885)
– it has about 46 ribs on the whorl, and an umbilical width of about 30 %, which fits nicely.

Congratulations to Robert on this rare find – just goes to show what still can be found by
persistent collecting and a bit of luck !

And of course thanks very much for the opportunity to prep & photograph this ammonite !

AndyS