A rare Amaltheid ammonite: Amaltheus reticularis

For Yorkshire, it is rare that Amaltheus occurs together with Pleuroceras – if you check the first table in my Pleuroceras blog post https://andysfossils.com/2016/01/31/pleuroceras-the-other-amaltheid-ammonite/, you’ll notice it is really just a few beds where this could theoretically happen.

So when you find an Amaltheus together with a Pleuroceras, you already know it’s really special…

Recently in one of the facebook groups, Phil Rann posted a picture of what looked like an Amaltheus together with Pleuroceras in the same rock, found on the Holderness coast , in a rock that seemed very much like the Yorkshire ironstone.


Phil very kindly offered to send it to me.

After about 3 weeks (and one week where it unnervingly seemed to be stuck in the carrier’s unknown depot, and I already feared it lost) it arrived.

I completed the preparation on this specimen and here it is now :

Amaltheus reticularis, 31 mm

I have interpreted this ammonit as an Amaltheus reticularis (SIMPSON, 1843)

It is apart from Pseudoamaltheus engelhardti the only Yorkshire Amaltheid ammonite I’ve not yet had in my collection.

The specimen is 31 mm in diameter, and shows relatively strong sigmoidal ribbing with 18 ribs at maximum diameter, with the remaining part of the body chamber constituting approx. 40 % of the last whorl. Umbilical width is about 22 % at 31 mm.

The ammonite fragment in the upper right hand corner is of a Pleuroceras paucicostatum. On the same small block there is also a small P. apyrenum, a small Amauroceras ferrugineum and fragments of a Pleuroceras elaboratum beside the Amaltheus reticularis, making it a perfect match of the equivalent of Hawsker bed 33 where also the holotype came from.

Like Amauroceras it does not have the typical braided Amaltheus keel :

Amaltheus reticularis – keel view

The venter is first relatively rounded and then sharpens up on the body chamber, only a few small crenations can be seen from ribs crossing over.

The next picture shows the specimen in UV light, a calcite vein runs across the whole little block and has also replaced a small part of the ammonite, unfortunately also the last suture before the body chamber.

View of specimen in UV light

The suture of Amaltheus reticularis exhibits unusual frilling of the first lateral saddle
(Howarth 1957, p. xix), which with this specimen due to above mentioned calcite vein can only be seen in its “infancy”, taken at around 15 mm diameter:

Suture at 15 mm – 2.5 x magnification stacked from 9 shots

I’ve always been fascinated by the holotype specimen that is exhibited in Whitby museum (WM 217) with it’s spiral ornamentation criss-crossing with the ribbing on the outer whorl and thus creating a net pattern (hence the name, reticulum = latin for net).

It is also the very first ammonite figured in Buckman’s classic 1909
“Yorkshire Type Ammonites”, and I’ve scanned it in from my Buckman original, since all other available published pictures simply don’t do it justice:

Amaltheus reticularis figure from Buckman´s “Yorkshire Type Fossils” from 1909

The figure of the holotype shows also a Pleuroceras preserved with the Amaltheus, it also looks like a Pleuroceras paucicostatum. The figured ammonite is 61 mm in diameter, so considerably larger than the specimen at hand.

All of the other specimen figured in literature I know of are from Howarth’s Amaltheidae (if you do have any other specimen or know of any other figured specimen, please let me know !)
On a visit to the Sedgwick Museum Cambridge in 2012 I did actually take some snapshots of a display that included the two specimen Howarth figured :

Amaltheus reticularis – Sedgwick Museum Cambridge
Amaltheus reticularis – Sedgwick Museum Cambridge

Unfortunately, these have not been 3d scanned in the GB3D project (http://www.3d-fossils.ac.uk), probably
because they’re not type specimen, so we‘ll have to make do with these bad photographs of mine…

GeoEd cast of Amaltheus reticularis, 40 mm

The cast of the more complete specimen above (from GeoEd Ltd https://www.geoed.co.uk) is more finely ribbed with about 22 ribs at the same diameter, with a sharper keel, but generally a good match in morphology.

It is hard to see the beginning spiral ornamentation on the specimen – I had to tease it out using some grazing light over the body chamber :

Amaltheus reticularis – spiral ornament on body chamber

It does seem to be a bit more robust both in terms of ribbing and whorl thickness and the suture is unfortunately not completely “readable”, but I think that this is likely within the variation of the species, as the stratigraphical age is such a good match. The more robust ribbing lends some more weight to the theory that A. reticularis may be derived from A. margaritatus.

There are really no other specimen I know of to compare to other than the ones figured by Buckman and Howarth, so I hope I have added a little to the knowledge about this very elusive ammonite.

Many thanks again to Phil Rann for sending me this specimen !

Literature


M.K. HOWARTH 1957, The Ammonites of the Family Amaltheidae in Britain, Palaeontographical Society

K.N. PAGE : Castlechamber to Maw Wyke, North Yorkshire in : British Lower Jurassic Stratigraphy, JNCC Peterborough, 2004

S.S. BUCKMAN : Yorkshire Type Ammonites, Part 1, William Wesley and Son, London, 1909