“show me the keel, please”, because especially with liassic ammonites, a safe
identification of an ammonite cannot be done without having a view of the keel.
The following example illustrates this best…
This nodule with a partially prepped ammonite was shown by David in the Yorkshire
Fossil Hunters group on facebook a while ago.
The fossil was found on the Holderness Coast as a glacial erratic, so age wise this can
basically be anything – from lower jurassic to cretaceous.
The keel of the ammonite was not prepped at this state, so identification was pure
speculation – Schlotheimia and Caenisites were proposed candidates.
Intrigued by the nodule and the mystery, I offered David to try and prep this ammonite.
We met this summer in Robin Hoods Bay, exchanged the ammonite, and David also
allowed me to photograph another beautiful and rare ammonite,
a Xipheroceras dudressieri, which you will see in another blog post soon…
The matrix the ammonite is embedded in – a greyish-brown, relatively hard clay ironstone
like concretion – did not aid a lot in the identification – it could still be
hettangian (Schlotheimia), sinemurian (Caenisites), or even pliensbachian (Pleuroceras)
or some non-liassic formation unknown to me.
When I returned home after my vacation, I started prep work on the ammonite.
The matrix was still just soft enough to be air abraded using iron powder,
so my first check was wether this ammonite actually had a keel
(Caenisites, Pleuroceras,…) or just a furrow (Schlotheimia..).
After about an hour of carefully working towards the keel, first with an air pen, and then
with an air abrader, it was clear that the ammonite does actually have a keel, so it had to
be Caenisites – but wait, the keel is relatively strongly crenelated – could it be a Pleuroceras ?
Counting the ribs it would have to be a Pleuroceras apyrenum, and there is a bed that actually
produces nodules like these, so I went on based on this assumption…
Seing the beautiful preservation of the ammonite
(a milk cholcolate brown shell on a solid calcite core –
doesn´t that make your mouth water 🙂 ), I decided to completely prep the side
of the ammonite still completely hidden in the concretion as well.
It is never without risk to prepare an ammonite from both sides, the ammonite might
break, or the innermost whorl might just blow through.
Since the concretion had some calcite-filled shrinkage cracks as well which might be
breaking points, I had to prep it most carefully and as vibration free as possible, so I
did most of the work with the air abrader, after securing the innermost whorl with a
drop of super glue, from the other side, just to be safe 😉
A few hours on, the furrows on the side of the keel I exposed got deeper and deeper…
Back to the books – no, there is no Pleuroceras where the furrows are that deep, even
taking into account that this specimen has shell preserved.
That means, it has to be a Caenisites after all, more specifically a Canisites turneri .
I had previously only seen smaller specimen without shell, so the crenelated keel
on larger whorl sizes is new to me.
After about 10 prep hours, here in it´s full beauty :
In summary, I think this shows how difficult it is to identify an ammonite without
having full view of all the diagnostic characteristics.
It´s been an absolute pleasure to prepare this ammonite
(but remember : Don´t ask me, I will ask you…), and of course it yielded a few more
good pictures of this species for the book and this blog.
The ammonite will be returned in the next couple of weeks to David, with special thanks .