A as in Amauroceras or A hopeless case ?

You have to start somewhere so why not begin with A ?

This middle lias (spinatum zone) nodule was found in the shingle on July 17, 2011 around Castle Chamber. The nodule was split in the middle and revealed the typical mix for such a nodule: Plenty of bivalves, a fragmentary, larger Pleuroceras, a smaller Pleuroceras (possibly apyrenum ?) and – looking at it again – a larger Amauroceras, broken straight down the middle, a relatively clean break as well. Larger Amauroceras aren´t all that common, so the nodule was bagged.

I did not look at it again until after May 2012, since it was neither amongst the perceived “highlights” of that holidays´collecting nor did it look easy to prep with the large complex surfaces of the nodule halves that would have to be glued together to make it safe to prep. I did not want to risk damaging the whorls while prepping around it with a pen and I do not have a large stonesaw available. A hopeless case, doomed to end up on the eternal prep backlog ?

So when I looked at it again in May 2012, I put the two halves of the nodule back again to see how they fit – and found they fit so well that I could not break them apart again ! Making a virtue out of necessity, I closed the remaining gap a bit further by gently hammering the two halves together with a hammer and letting a lot of low viscosity super glue run into the crack…

About a week later, I started to prep the ammonite and could soon expose the outer whorl of that 6 cm / 2.5″ Amauroceras lenticulare. I was amazed that you almost have to know where the crack was to see it…

Amauroceras belong to the Amaltheidae family of middle liassic ammonites. There are 2 species to be found on the Yorkshire coast :

  • Amauroceras ferrugineum, a smaller, relatively common species and
  • Amauroceras lenticulare, a larger species.

Apart from the obvious difference in size they differ in …. well I don’t really know that !
Anyone having broken apart a large, well-preserved A. lenticulare to see what the inner whorl looks like ? I haven´t !!!
Looking at the sutures of A. ferrugineum and A lenticulare at approximately same size on page 83 of the Lias SCHLEGELMILCH, they do look different in that A. ferrugineum has a more complex one, while A. lenticulare´s suture is more simple, less “fractal”. But still, you would need to break up your A. lenticulare specimen to find out, those sutures were taken at 7.5 to 8 mm ! In a way, the suture of A. ferrugineum does look more “adult” at this small stage, could this be a sign for a “dwarf” variation ?

The large Amauroceras lenticulare is rarely found in a condition appealing to the collector; the larger shells apparently were quite fragile
and often are only preserved as partials or badly crushed. Furthermore, their thin discus shaped shells are hard to recognize, this may lead to a certain
collection bias so that the smaller Amauroceras ferrugineum appears to be much less rare – not sure if they really are.

AndyS

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