To be or not to be – a Harpoceras…

To be or not to be ... a Harpoceras ?

To be or not to be … a Harpoceras ?

Wait a second, you may say, I’ve seen this one before, but last time you called it an
Eleganticeras…!  Yeah, I know – sometimes you have to look somewhere else to find an
error in an ammonite identification.  In this case, it was my error
(you can see prominently featured it in the Eleganticeras post here),  and I only fully
realized it when I saw (and bought) an ammonite offered on eBay that looked very similar.

Harpoceras aff. serpentinum, 10 cm from Hawsker Bottoms (left) and Harpoceras serpentinum, 8 cm, from Altdorf/Germany (right)

Harpoceras aff. serpentinum, 10 cm from Hawsker Bottoms (left) and Harpoceras serpentinum, 8 cm, from Altdorf/Germany (right) – Thanks to Arno Garbe !

In my defense, I have to say that I’ve never really been 100 % comfortable with calling
it an Eleganticeras… I’m not really fully sure about the species of this one even now,
but I’m much more comfortable calling it a
Harpoceras aff. serpentinum (SCHLOTHEIM, 1813).

Because of the circumstances I vividly remember finding this specimen at Hawsker
Bottoms some years ago…

I was at Hawsker Bottoms and on my way back to Robin Hoods Bay and had decided to
inspect the channels and gullies that furrow the low tide reef, sometimes a good place to
find the odd nodule that rolled down the cliff  (and roll, or better shoot they do, I once
witnessed one flying at head height across the shore…) and found this round pyrite
nodule of- if memory serves me right- about 12 cm diameter. I gave it a whack, and the
top of the slightly weathered, crumbly nodule split off showing the ammonite, but part
of the shell on the body chamber came off as well, because there was an oil filled hollow
there – just as on the Cleviceras I have shown you before.
I bagged all parts and re-affixed the shell bits after removing the oil, and filling the cavity
with glue, which I only succeeded half-decently in because I did not quite get the level of
glue correct – but try to reconstruct a broken egg-shell of a blown out egg, and you’ll
understand what this was like…

The rest of the nodules’ pyrite shell developed severe pyrite rot in the years following, so
at some point I decided to fully remove the remaining matrix and so it represents itself
today.

For me, this was a typical Cannonball nodule from bed 33, and therefore allthough it
looked slightly different, had to be an Eleganticeras…

But was it a Cannonball nodule ? Would I be able to distinguish it from a Curling Stone
(bed 37) nodule in the slightly worn and obscured with algal growth state it was in ?

In hindsight, I don’t think so. But even so, if it really was a Cannonball nodule, there is
no reason whatsoever why this cannot be an early Harpoceras. These animals knew
nothing of the arbitrary boundaries of ammonite zones and subzones we draw today,
these boundaries have to be “fuzzy”and only hold true for a statistically very significant
majority of ammonites, but not necessarily for all, given there is a continuity of the
record, i.e. no hiatus or non-sequence in sedimentation.

Harpoceras serpentinum (it has a large list of synonymies including Harpoceras
alternatum and Harpoceratoides alternatus) is relatively rare in Yorkshire, it can be
distinguished from its later, more common descendant Harpoceras falciferum by the
less (if at all) expressed spiral groove and the slightly angled (instead of vertical
to undercut) umbilical walls.

The below pictured group of Harpoceras falciferum (SOWERBY, 1820) from Sandsend
was purchased from Byron Blessed.
Harpoceras falciferum, 12 cm and 8 cm, Sandsend

Harpoceras falciferum, 12 cm and 8 cm, Sandsend

Fragmentary Harpoceras falciferum on back of two other specimen, would have been about 30 cm in diameter if complete

Fragmentary Harpoceras falciferum on back of two other specimen, would have been about 30 cm in diameter if complete

I like specimen like this one, since it represents all stages of growth of the ammonite
species from a moderate size of 8 cm to what (if it were complete) must have been close
to the size of the magnificent “Harpoceras mulgravium” at Whitby museum. What is
especially evident is the umbilical width ratio growth at large sizes -i.e. the ammonite
shells became more evolute at large sizes – the 8 and 12 cm specimen have an U/D of
about 0.27, the large fragment on the back of the two smaller ammonites has an
estimated U/D of 0.5 !

What is also evident of course from this story is that it  most of the time really matters
for ammonite identification to better know exactly which bed the ammonite came
from – which of course can be difficult when collecting on the Yorkshire coast…

There´s only a few members of the Hildoceratidae left we still need to take a look at
including Harpoceras subplanatum and Hildaites sp. and they are also some of the
rarest Hildoceratidae in Yorkshire – we´ll cover them in one of the next posts…

 

AndyS

 

Leave a comment

1 Comment

  1. mike scher

     /  December 9, 2014

    This is an incredible and beautiful work, the photos are just amazing, how about pics of large multiple plates.

    Reply

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