Amaltheidae – One of my favourite ammonite families

Amaltheus stokesi, completely septate, 13 cm

Amaltheus stokesi, completely septate, 13 cm

It must be the beautiful braided keel that most of the members of the family Amaltheidae show that makes it a favourite amongst collectors.
The Amaltheidae family of ammonites includes the genera
  • Amaltheus, with it´s subgenus Pseudoamaltheus
  • Amauroceras, which we´ve already looked at in an earlier post
  • Pleuroceras
Amaltheus (Pseudoamaltheus) is supposed to occur in Yorkshire. I think I´ve seen a badly preserved, flattened one on a crumbling block at Hawsker once, but I´m not sure since large Amaltheus margaritatus can also sometimes show sections with spiral ornamentation. But I´ve never seen one in a collection – If you have one from Yorkshire – let me know !
We´ll deal with the Pleuroceras genus later, so this article will be about the Amaltheus genus.
In the middle lias of the Yorkshire coast, Amaltheus is not particularly rare, although nice specimen can be hard to come by. As with many Yorkshire ammonites from the various beds of the lias, the best preserved Amaltheus ammonites come from nodules.
In the order of (perceived, ascending) rarity of occurrence the species are :
  • Amaltheus stokesi
  • Amaltheus wertheri
  • Amaltheus subnodosus
  • Amaltheus striatus
  • Amaltheus bifurcus
  • Amaltheus margaritatus
  • Amaltheus laevigatus
  • Amaltheus gibbosus
  • Amaltheus gloriosus
  • Amaltheus reticularis
This order of course is just perception, not based on any well founded statistics. Of the 3 first species I have more than 10 specimen in my collection, of the next group of 3 species up to 5 specimen in my collection, the last group of 4 species I have only one or none. This might give you some idea about chances of finding them…
The first one, Amaltheus stokesi, is also the largest, I have found specimen up to 22 cm / 9″.
The shown specimen is almost complete including the body chamber, there seems to be a predetermined breaking point (when eroding) between body chamber and phragmocone, because most specimen are found as just the phragmocone. It does often occur together with A. wertheri and A. bifurcus.
Amaltheus stokesi, almost complete specimen, 20 cm

Amaltheus stokesi, almost complete specimen, 20 cm

Amaltheus  wertheri in constrast is one of the smaller Amaltheus species, I´ve never seen specimen exceed 4 cm. It’s an almost smooth species with just a slight crenelation of the keel and a very pronounced constriction at the aperture.
Comparison of small Amaltheus : A. bifurcus (left), A. wertheri (top), A. stokesi (right), all approx. 2.5-3 cm

Comparison of small Amaltheus : A. bifurcus (left), A. wertheri (top), A. stokesi (right), all approx. 2.5-3 cm

In the subnodosus subzone the species A. subnodosus is the naming index species. it’s got an inner whorl with strong ribs, each crowned with a little tubercle.  Ribs stay strong on larger whorls, while tubercles fade away at around 2 to 3 cm. Strong keel.
Amaltheus subnodosus : Side view, keel view, aperture view (from left)

Amaltheus subnodosus : Side view, keel view, aperture view (from left)

Amaltheus striatus starts similar to A. subnodosus, ribs & tubercles on the inner whorls are not as strong and fade away until at around 2-3 cm the shell becomes almost smooth apart from fine shallow ribs. Strong keel.
Amaltheus striatus, 4 cm

Amaltheus striatus, 4 cm

Amaltheus bifurcus is a rarer Amaltheid from the stokesi subzone, occurring together with A. stokesi and A. wertheri. Similar to A. stokesi, but stronger ribbing at similar sizes, thicker whorls. Usually specimen do not get much larger than 6-8 cm, I do have a very eroded (doubtful) specimen at about 10 cm.
Amaltheus bifurcus, 6 cm, completely septate

Amaltheus bifurcus, 6 cm, completely septate

Amaltheus margaritatus is the index fossil of the margaritatus subzone. It does not overlap with A. stokesi, is seen as the direct descendant from A. stokesi and can grow to similar sizes. The more differentiated keel is the main difference between A. margaritatus and A. stokesi, but since the species do not overlap and the rocks they´re found in in Yorkshire are rather distinctive (ironstone nodules = A. margaritatus, grey limestone nodules = A. stokesi), mixing up of the two species ist almost impossible.
It is often found together with A. subnodosus and A. striatus.
Large A. margaritatus 7cm, A. striatus (below) 3.5 cm, A. subnodosus (left) 4 cm

Large A. margaritatus 7cm, A. striatus (below) 3.5 cm, A. subnodosus (left) 4 cm

Amaltheus laevigatus is almost smooth on the inner whorl and develops fine ribbing after about 1 cm.
Rather rare, I’ve found only 1 specimen in more than 20 years.
Amaltheus laevigatus, 2.5 cm, not a replica !

Amaltheus laevigatus, 2.5 cm, not a replica !

Amaltheus gibbosus  is very rare in Yorkshire – I only have a specimen that I purchased from M. Marshall / M. Forster that came from Staithes.

Amaltheus gibbosus, 3 + 3.5 cm

Amaltheus gibbosus, 3 + 3.5 cm

Both Amaltheus gloriosus and Amaltheus reticularis are apparently extremely rare (or I have not been looking in the right places) – I have not found a specimen of these.
I do have a replica Amaltheus reticularis that I´ve shown you in a previous post.
As a bonus (this is my 20th post !) here´s another picture of a combination of the 3 stokesi subzone Amaltheus :
Large A. stokesi 14 cm, 2 small A. wertheri 2 cm, A. bifurcus in aperture 5 cm

Large A. stokesi 14 cm, 2 small A. wertheri 2 cm, A. bifurcus in aperture 5 cm

AndyS
Leave a comment

2 Comments

  1. Adrian

     /  July 7, 2012

    One of my favorite ammonites too Andy,but you already know that.We must go and look for some more next time your over.
    An excellent insight into the photography side of your work too.
    Adrian and Keeley

    Reply
  1. Twice bought and three times found or Why finding the fossil yourself is best… | Yorkshire Ammonites (and other fossils ) revisited

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