I must admit in my more than 20 years of collecting on the Yorkshire  coast I have not really had much luck with finding Arnioceras or with prepping it. When I see beautiful large slabs of Arnioceras, skillfully  prepared, I’m often somewhat frustrated with my own feeble attempts of prepping this sort of matrix from Robin Hoods Bay, where I most often collect. Another german collector once told me that Arnioceras material from the glacial drift (e.g. Holderness coast) does seem to prep easier, with the matrix being less “sticky”, may be due to longer weathering. So while I do still have some slabs of Arnioceras blocks found in Robin Hoods Bay waiting to be prepped in my cellar, most of the better pieces shown here are actually purchased from other collectors and orginating from the Holderness coast. Maybe I should give my slabs a few years of additional weathering in the garden…

To really see the differences between the Arnioceras species you need to look closely :

Arnioceras acuticarinatum is the easiest of the lot to identify. It has rursiradiate ribs, i.e. when you look at the whorl of an ammonite with the aperture to the right, you see that the ribs while going over the flank from umbilicus to venter point away from the aperture, they´re leaning backward towards the venter. It has quite an “acute”  (sharp, well-developed) keel as one might translate the latin species name. The partial one shown is actually found by myself, but I do have pictures of better specimen from other collections for the book…

Arnioceras kridioides in contrast has no pronounced keel and a very short stadium without ribs on the inner whorls. Keel wise it’s almost like a Gagaticeras.

Arnioceras falcaries has convex ribs, it might be mistaken for A. acuticarinatum at first sight, but it differs in having less dense ribs especially on the inner whorls and
a keel with small side keels. Both A. falcaries and A. acuticarinatum are smooth until about 1 cm diameter.

This specimen of Arnioceras semicostatum, the Index fossil of the semicostatum zone, was actually found in situ on the most seaward fringe of the reef in the middle of Robin Hoods Bay in 1991 by myself – I had gone there especially on a very low spring tide and could not believe my luck when I found the small limestone nodule in situ containing the ammonite (according to HOWARTH 2002, even Bairstow did not find any !) As HOWARTH writes in his 2002 paper about the chances of repeating Leslie Bairstow’s collection in Robin Hoods Bay today “Such a collection would be difficult to repeat today, because so many of the accessible ammonites have been removed from the Bay.” – 10 years on, this feels even more true, especially due to the reef being more and more overgrown with algae – in the 1930s and 1950s pictures in the same paper, the reefs do look a lot less overgrown.

The smooth stadium of A. semicostatum is usually about 2 cm in diameter, it’s keel is a bit more pronounced than A. kridioides, but less developed than A. acuticarinatum, a bit like A. falcaries, but just a touch of side keels at larger sizes.

As a reference for all the different names of ribs, sections, shell elements etc of ammonites there is a very nice web page on Tonmo by Kevin Bylund :

I found it when looking for the english name for backwards bending ribs – rursiradiate – which are actually called retroradiate in german literature…

AndyS