All posts for the month September, 2012

Disappointments, delights and a small surprise

You might have guessed, I’m currently spending some time prepping the finds from my last Yorkshire visit…

Here are a few updates on the fossils I’ve found and their prep status :
Lytoceras, no inner whorl :(

Lytoceras, no inner whorl 😦

The lower lias Lytoceras has become a disappointment,  as I was working around the whorl towards the inside of the nodule, several gaps in the shell appeared, where the whorl had broken apart, probably during the process of the fossilization.  The outermost part of the whorl is squashed, as I was “digging” for the inner whorls, nothing was there…
This is not unusual for luridum subzone Lytoceras, but they can be found with inner whorls preserved, as shown in the next picture of a specimen found ex-situ during another visit:
Lytoceras fimbriatum, 12 cm, with inner whorl :)

Lytoceras fimbriatum, 12 cm, with inner whorl 🙂

The Pleuroceras I´ve shown you earlier also  has become a disappointment after a promising start – there´s a big hole (more like half the whorl missing) in the outer whorl, the inner whorl is there, but it´s brittle Calcite. Oh well, another one for the graveyard of failed prep attempts aka the gravel patch beside our house…
Pleuroceras, whorl stops :(

Pleuroceras, whorl stops 😦

But there have also been delights and a small surprise…
An upper toarcian Ammonite that had recently fallen from the cliff and was just covered by a thin layer of soft mudstone went  directly to the air abrader, details that are usually lost when they roll around in the waves (or when they are embedded in hard mudstone nodules)  could be recovered, like the wonderful complete spines or this especially delightful small spiny Peronoceras (fragment ?) inside the aperture of the larger Peronoceras subarmatum.
Catacoeloceras subarmatum, 7 cm

Peronoceras  subarmatum, 7 cm

Aperture of the Catacoeloceras, with another small Catacoeloceras fragment inside

Aperture of the Catacoeloceras, with another small Catacoeloceras fragment inside

With some fossils, they may not look like anything when found but can yield surprises :
Peronoceras turriculatum, as found

Peronoceras turriculatum, as found

While roughing this Peronoceras turriculatum from the nodule, I noticed a small shelly fossil appearing after a matrix piece of about 1 x 1 cm size was dislodged by the airpen.  I stopped the airpen to take a closer look :
Peronoceras turriculatum, during preparation

Peronoceras turriculatum, during preparation

It’s a small aptychus, part of the ammonites jaw apparatus !
Apytchus inner mould, 10 x 7 mm

Apytchus inner mould, 10 x 4 mm

Since it is not inside the shell of the ammonite, it can not be safely concluded that it belongs to the ammonite, but form and size (may be a bit on the small side) seem about right, and it’s at least close to the aperture of the Peronoceras…
Peronoceras turriculatum, 7.5 cm, with aptychus

Peronoceras turriculatum, 7.5 cm, with aptychus

There is very little literature about Dactylioceratid aptychi, the only article I found is by Ulrich Lehmann describing remains of the jaw apparatus inside a Dactylioceras tenuicostatum
(Palaeonology Vol. 22, part 1, pages 265-271). He does notice that the jaw apparatus that was found inside that Dactylioceras was also smaller than expected.
Inside my prep box I then went looking for the bit of matrix that flew away (I did not see where it landed) because it took with it most of the actual shell of the aptychus. This was about half an hour after I started work on the Peronoceras and my prep box was filled with hundreds if not thousands of similar matrix shreds… While contemplating the time it would take going through the splinters one by one I picked up a few likely looking ones and after about 5 pieces I found it –  you have to lucky sometimes  !
Apytchus shell, 9 x 7 mm

Aptychus shell, 9 x 7 mm

This has been tweaked somewhat in Photoshop to better show the structure of the aptychus – the colors are not 100% exact.
In my experience, every time I come to Yorkshire to collect I usually get to take with me one special piece – this time, it´s got to be the ammonite with the aptychus !
And, as a little bonus, since this is my 30th post, here is one of the highlights of  the previous visit in April : A double Androgynoceras lataecosta, 6 & 7 cm –
what makes this one special is a Goniomya bivalve which with its v-shaped ornament sits decoratively on the whorl of the ammonite…
Androgynoceras lataecosta, 6 & 7 cm, with Goniomya bivalve

Androgynoceras lataecosta, 6 & 7 cm, with Goniomya bivalve

AndyS
by andysfossils on September 12, 2012  •  Permalink
Posted in Ammonites, Preparation

Posted by andysfossils on September 12, 2012

https://andysfossils.com/2012/09/12/disappointments-delights-and-a-small-surprise/

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
by andysfossils on September 2, 2012  •  Permalink
Posted in Ammonites, Preparation

Posted by andysfossils on September 2, 2012

https://andysfossils.com/2012/09/02/arnioceras-or-a-frustratingly-sticky-matrix/