Arnioceras or A frustratingly sticky matrix

Arnioceras semicostatum, 4 & 4.5 cm, Holderness Coast

Arnioceras semicostatum, 4 & 4.5 cm, Holderness Coast

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, 4 cm, Robin Hoods Bay

Arnioceras acuticarinatum, 4 cm, Robin Hoods Bay

Arnioceras acuticarinatum, keel view

Arnioceras acuticarinatum, keel view

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, 3.5 & 4 cm, Holderness Coast

Arnioceras kridioides, 3.5 & 4 cm, Holderness Coast

Arnioceras kridioides, keel view

Arnioceras kridioides, keel view

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, 4.5 cm, Robin Hoods Bay

Arnioceras falcaries, 4.5 cm, Robin Hoods Bay

Arnioceras falcaries, keel view

Arnioceras falcaries, keel view

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.
Arnioceras semicostatum, 3.5 cm, Robin Hoods Bay

Arnioceras semicostatum, 3.5 cm, Robin Hoods Bay

Arnioceras semicostatum, keel view

Arnioceras semicostatum, keel view

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.
Arnioceras miserabile, 2cm, Robin Hoods Bay

Arnioceras miserabile, 2cm, Robin Hoods Bay

Arnioceras miserabile, keel view

Arnioceras miserabile, keel view

Arnioceras miserabile is the “odd one out”, being a small, almost totally smooth ammonite with no pronounced keel, but just an edge where the flanks meet. It is easy to mistake inner whorls of Arnioceras semicostatum as A. miserabile, a safe identification is only possible, when A. miserabile shows typical signs of adulthood, i.e. complete body chamber with preserved aperture and/or crowding of the sutures, i.e. decreased distance between sutures before the body chamber.  There are theories that this “species” may actually be a microconch of another species.
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
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2 Comments

  1. Adrian

     /  September 3, 2012

    Andy, i have to agree with you.When collected from the holderness coast fossils are a lot easier to prep.Keeley and i have been and collected a small arnie block and a paltecioceras?The matrix is a lot softer and comes away very easily compared to RHB material.As you know we have found both of the ammo’s mentioned above with you and like you say the arnie blocks are a real pain to attempt to prep.The rolling by the sea on their journey down the coast and the time it takes to get there definately make them easier to prep,especially if i can do them!

    Reply
  1. The lost Arnioceras or The hunt for a name | Yorkshire Ammonites (and other fossils ) revisited

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