Specimen as found

It looked to me like a quarter section of a large Oxynoticeras, where some (I could see two)
Gagaticeras had been washed into the body chamber.
There was a little bit left of the inner whorl of that ammonite, and I decided that this might
potentially look nice when done.One additional specimen had crumbled off the relatively
brittle stone, it had lain a while in the water already.

At home, this was put on the back list of fossils that I would occasionally take another look
at, certainly not top priority. When I was air abrading another specimen, I tested the broken
off specimen, it abraded relatively easily, allthough the matrix was very sandy
(that should have told me something in the first place…).

Broken off Bifericeras, diameter of ammonite = 3 cm

Bifericeras bifer is a very rare ammonite in the beds around Robin Hoods Bay, or,
to be more precise it is rarely found.
In all the years since 1989 I had only found Bifericeras bifer in 2016, after a cliff fall cut
through the specific beds of the bifer biohorizon.

The ammonites in that fall were contained in highly pyritic lenses, unfortunately
neither very well preserved nor easy to prep.

Pyritic lens with multiple, badly preserved Bifericeras, diameter of biggest ammonite = 3 cm

So seeing this specimen from 2017, I was deligthed to have another Bifericeras bifer,
which had not been in my collection from Robin Hoods Bay yet, and more easily to
prep as well !

The specimen instantly went to the top of my prep list.
Preparation was not as easy as I had thought at first, the sandy matrix was made up
of harder and softer layers on the milimeter scale so I had to  alternate between air pen
to remove the harder layers and the air abrader to remove the softer layers which
were luckily mostly around the fossils.

I had finished the ammonite that is visible on the first photo and another one to the right
and was already smoothing the matrix between them, when I noticed another ammonite,
and while air abrading that one, even another one, better and bigger than all the previous
ones…

The finished specimen with 4 Bifericeras bifer within the body chamber of a large Oxynoticeras sp.

Detail of the better preserved ammonites – biggest Bifericeras bifer diameter = 3.5 cm

The ammonites were washed into the body chamber of the larger ammonites and were
sheltered there from compaction or getting  otherwise damaged before or during
fossilization, the shells all seem to be complete to the mouth border, though they are
most likely not adult.

The most diagnostic characteristic for Bifericeras bifer is the double row of spines,
the spines being connected through a rib and the inner ones
often pointing inwards towards the center of the ammonite.  The innermost whorls up to
between 7—10 mm are smooth.

Detail of Bifericeras bifer showing double spines connected through a rib

Bifericeras vitreum (SIMPSON, 1855) and

I do actually have a single Bifericeras vitreum, a small sharp shell preserved specimen
contained in a very similar sandy matrix as the Bifericeras bifer shown above.

Bifericeras vitreum, diameter = 2 cm

View of the venter of Bifericeras vitreum with sharp ribbing, convex across the venter

Bifericeras vitreum has rather sharp ribs that run across the venter in a slightly convex manner.
This specimen has also been carefully prepped with air abrasion.

Bifericeras donovani from the base of the taylori subzone has so far eluded me,
allthough from the Wine Haven paper (see literature below) it appears they are
relatively abundant.

The only question now remains is : Is the ammonite that „provided shelter“
to the Bifericeras in it‘s body chamber really an Oxynoticeras or more likely
to be a Gleviceras ?

Section of Oxynoticeras sp with sutures showing, width = 9 cm

There really is not much to go from, but the venter seems to be very sharp and the
whorl lacks the fine ribbing characteristic for Gleviceras, so I‘d go for a late
Oxynoticeras grp. oxynotum, which with the sandy matrix lithology nicely fits
with (Hesselbo & Jenkyns numbering) beds 44-49 of the bifer biohorizon and their description
in PAGE 2004.

Detail of biggest Bifericeras bifer

So in the end, that „ugly duckling“ of an incomplete ammonite has (been) transformed into
a stunning „swan“ of a multi ammonite, multi species specimen with a story to tell.

On the same day I found this specimen, two other, no less interesting ammonite
specimen were found, more about these soon…

AndyS

• Schlegelmilch, R.,Die Ammoniten des süddeutschen Lias, 2nd revised and extended edition,
  Stuttgart & New York 1992

• Howarth, M.K., The Lower Lias of Robin Hood´s Bay, Yorkshire, and the work of
  Leslie Bairstow, Bulletin  58/2 of The Natural History Museum, London 2002

• Meister/Eberhan/Blau/Dommergues/Feist-Burkhardt/Hailwood/Hart/Hesselo/Hounslow/
  Hylton/Morton/Page/Price,
  The Global Boundary Stratotype Section and Point (GSSP) for the base of the Pliensbachian
  Stage (Lower Jurassic), Wine Haven, Yorkshire, UK, 2006

•  Page,K.N., Normanby Stye Batts – Miller´s Nab (Robin Hood´s Bay),
  North Yorkshire (NZ 972 025- NZ 952 075), in : British Lower Jurassic Stratigraphy,
  Geological Conservation Review Series, JNCC Peterborough 2004, pp. 250-262