Recent prep results and What´s in the queue ?

Recent prep results, numbers see text

Recent prep results, numbers see text

You might have noticed, I´ve deviated from my usual schedule of posting an article about every 2-3 weeks…
There are a couple of reasons, none bad, which have kept me from posting.
Reason number 1 is that commitments from my daytime job have kept me unusually busy for January and February and this will stay that way at least until mid march,
so you´ll have to wait for  a new full article until about 3-4 weeks time.

Reason number 2 is I´ve been working on several full articles, but due to my perfectionism I was not satisfied with what I could have posted…

In the “unfinished posts” queue is the first part of the Dactylioceras article, dealing with the lower toarcian Dactylioceras species.
When looking at some of the ammonites I was photographing (every little prep fault  somehow gets exaggerated when you look through a lens…),
I found that most of them needed some form of re-prep to comply to the same standard I´ve been trying to adhere to for the book.
This is for example the reason why #7 in the photograph, a Dactylioceras (Orthodactylites) clevelandicum,  went back to the top of my prep queue :
The inner whorls needed some more attention with the fine air pen and the air abrader – it had been found in 2002 and basically went straight to the drawer at that time.

Reason number 3 is I need to clean up my prep slate before I go for my traditional spring collection tour to make space for potential new finds,
so the proportion of time prepping was higher that the one on writing…
All of the ammonites (and other fossils) have been prepped last weekend, in case you´re wondering what they are here´s the list :

  1. Dactylioceras (Orthodactylites) tenuicostatum,  7 cm
  2. Double of Dacytlioceras commune, 5 & 4 cm, thanks to Dr. Mike Howarth for helping to correct my inital thoughts on this one…
  3. A Plagiostoma sp. bivalve, 6 cm,  from the apyrenum subzone of the middle lias, a “first” for me, I´ve never seen one before from the Yorkshire lias…
  4. A combo of Amaltheus stokesi (5.5 cm) , Amaltheus bifurcus (2.5 cm) , Amaltheus wertheri (2 & 1 cm)
  5. Pleuroceras hawskerense, 6 cm
  6. Dactylioceras (Orthodactylites) semicelatum, 5 cm
  7. Dactylioceras (Orthodactylites) clevelandicum, 9 cm

Another article that´s in the “unfinished posts” queue for a long time already is about pathologies on Yorkshire coast liassic ammonites, for the simple reason that
literature about pathologies was somewhat thinly spread across a wide range of publications, most of the time with few pictures (so important for the amateur collector !).
But thanks to Prof. Dr. Keupp from FU Berlin this has now changed (http://www.geo.fu-berlin.de/geol/fachrichtungen/pal/eigenproduktion/Band_12/index.html) :
A brand new copy of his almost 400 page thick, large format, just released new atlas on cephalopod palaeopathologies has landed on my desk, I had only very little time to study it yet, but what I´ve seen so far is
spectacular (pictures galore !) and will surely set the scientific standard on this topic for years to come (unfortunately it is currently only available in german).
So through this new publication my “sick ammonites from Yorkshire” post will take a giant leap forward and will be published after the first part of the Dac post…

AndyS

 

 

 

 

 

 

 

 

 

A moment frozen in time, part II

In the first part of this blog post I described the Yorkshire liassic crinoids, now´s the time to show you the starfish.
It feels like starfish are even a bit more rare than crinoids – at least with crinoids you get the obvious isolated segments (ossicles) in most sediments.
Disarticulated starfish remains are less obvious, though I guess they must be there as well, most Yorkshire lias sediments are just difficult to prepare for microfossil analysis.
Most articulated starfish remains in my collection are from the figulinum – stokesi subzones of the Yorkshire coast, as are the first three.

Palaeocoma milleri (PHILLIPS,1829)
This is the most “common” brittle star from the Yorkshire coast and can reach considerable size – my largest specimen has an arm length of 6″ / 15 cm –
 “Yorkshire” seas also fed the brittle stars better 😉 – Dorset specimen of the same species are usually considerably smaller.
(In his 1964 revision of the brittle stars of the british jurassic (“Die Ophiuren des Englischen Jura”) HESS sums some davoei/margaritazus zone
Palaeocoma species  (P. egertoni, P. gaveyi) under one species P. milleri.)
Palaecoma milleri, 9 cm diameter

Palaecoma milleri, 9 cm diameter

This one was found in a fresh cliff fall on a block of softer sediment, sensitive to both drying out/cracking and falling apart when subjected to water.
Mike Marshall kindly dry-cut the block to size and stabilized it with some epoxy putty. Having thus safely arrived home, a couple of minutes of
air abrading brought the winding arms of the 9 cm diameter brittle star to light.
Palaecoma milleri, width of view = 24 cm

Palaecoma milleri, width of view = 24 cm

Palaecoma milleri, detail of center

Palaecoma milleri, detail of center

This is the said large specimen of Palaeocoma with the arm length of 15 cm (stretched out)  and an “arm span” (as embedded) of more than 22 cm.
The long arms sink deep into the thinly bedded sandstone with harder and softer layers, with the very tips less than 0.5 mm thick.
This fossil was prepped with alternating runs of air pen (for breaking through the harder layers) and air abrader (for the detailed work around the fossil).
There is a little curiosity at the missing arm (5 o´clock position) : There is a small very thin little arm seemingly coming out of the stump, it is tempting to assume
it could be a replacement arm (allthough it looks too small for it).
I have not seen any pictures of what replacement arms with brittle stars look like, I you have, please let me know…
Palaecoma milleri, block as found

Palaecoma milleri, block as found

Palaecoma milleri, width of block 22 cm

Palaecoma milleri, width of block 22 cm

This starfish was found in March this year. It sits in a little block about 23 cm wide and was again prepped with air pen / air abrader.
The central disc is about 3.5 cm wide – the arms must have really been long on this one, if they had been preserved…
While prepping it I mistakenly thought the arm in the 4 o´clock position was connected to the end coming up in the 5 o´clock position – they are not,
as I had to notice when finding that the arm in the 5 o´clock position grew thinner while it was going down, instead of thicker to connect with the other arm…
The embedding rock seems to originate from beds around the oyster bed, showing some oysters as well.
Palaecoma milleri, width of view approx. 20 cm

Palaecoma milleri, width of view approx. 20 cm

Recent brittle stars are known to gather where there is a large supply of nutrients, i.e. in the deep sea on decaying whale carcasses.
This picture shows a section of a larger slab that contains such a fossilized gathering – the reason for it is not known (I can safely say it wasn´t a whale, though…)
The slab was found in 1999 on the way back from a long day out at Hawsker (I told you, it always happens to me like this…);
as the tide came in already, it had to be left on the beach and was hidden in a secretively marked spot.
After a very restless night and anxiously awaiting the next low tide, the 40 kg slab was carried back to Robin Hoods Bay in a large Rucksack and carried up the hill in a fishermen´s box with the help of my friend Klaus…
The reddish covering sediment was found to be preppable using potassium hydroxide (KOH) pellets, and over the course of some months
it was treated in a large cement tub. It is now awaiting finishing touches in a big enough air abrading chamber (planned for 2013…)
The slab had an eroded Amaltheus stokesi on the back, so it must originate from the stokesi subzone.

Tropidaster pectinatus FORBES, 1850
Tropidaster pectinatus, 4 cm wide

Tropidaster pectinatus, 4 cm wide

This small starfish sits on the same slab as the Palaeocoma specimen of the last picture. It is only relatively small (4 cm diameter) and very fragile.
While looking at some photographs of the slab some time ago, I notied a second Tropidaster specimen on the slab as well which I had not seen when looking at it with the “naked” eye…

Luidia murchisoni (WILLIAMSON, 1836)
This is the elusive sun starfish. I have only ever found one specimen myself, but could not salvage it without risk (huge block, last day of the holiday…)
I notified a local collector who had it cut out of the block with a diamond still saw and had it prepped professionally ,it is now on public display in a shop in Robin Hoods Bay.
Luidia murchisoni in situ (center), to the right the snout of dear beach dog Lucy

Luidia murchisoni in situ (center), to the right the snout of dear beach dog Lucy

As a surrogate, here´s an eroded specimen I got off Mike Marshall :
Luidia murchisoni, 10 cm diameter

Luidia murchisoni, 10 cm diameter


?Sinosura sp.
 
Bilder kl. brittle stars
The small brittle stars associated with the Hispidocrinus scalaris crinoid I showed you in the previous post are just too small to be identified – a little over 1 cm diameter.
I have tentatively put them towards Sinosura, since larger specimen of Sinosura have been found with these crinoids.
?Sinosura sp., 1 cm diameter

?Sinosura sp., 1 cm diameter

?Sinosura sp., 1 cm diameter

?Sinosura sp., 1 cm diameter

These tiny little brittle stars are preserved so life-like, with even some of the finest hairs still attached, truly a moment frozen in time.

Addendum November 25, 2012 :

I´ve recently had the opportunity to prep a beautifully preserved P. milleri which is, unlike all the other P. milleri shown here before (did you notice ?) not prepped in oral aspect (from the underside),
but in aboral aspect (like you would see it crawling around in and over the sediment). It is perfectly preserved and could be teased out of the relatively soft shale with low air abrader pressure in about 45 minutes.
Congratulations to the finder (D. Clark) and thanks again for letting me picture it here !

Brittle star Palaeocoma milleri, width approx. 20 cm, collection D. Clark

Brittle star Palaeocoma milleri, width approx. 20 cm, collection D. Clark

AndyS

A moment frozen in time, part I

Hispidocrinus scalaris, showing crown from underside, width = 6 cm

Hispidocrinus scalaris, showing crown from underside, width = 6 cm

The title of this blog says it already : “…and other fossils”…, so now’s the time to introduce you to (for a change) my other fossil passion :
echinoderms, more specifically starfish and crinoids. It really came more as a by-product of hunting for ammonites, mostly when my rucksack was already full on the way back, tiredly stumbling across the large blocks while taking a different route across the boulder strewn beach on the way home. It also happens that I pick up rocks with crinoid or starfish content when I´m frustrated because there is nothing else (meaning decent ammonites) to find – almost as if you have to be in a certain state of mind to notice these fossils, when your attention is no longer focused on other fossils, similar to when you start hearing strange sounds in the dark night when your visual sense becomes useless…
This of course has to do something with the difference in the “search mode” when your looking for ammonites vs. starfish / crinoids : With ammonites, on the Yorkshire coast you´re mostly looking for concretions, while with starfish / crinoids you´re carefully surveying the surfaces of (potentially large) fallen blocks – this also explains the of course totally unobjective observation that I tend to find crinoids / starfish mostly on the return from a location : I tend to use the easier way across the large blocks, and it´s usually later in the day, which makes for nice low angled light and better chances to see the delicate fossils…

For intact, articulated preservation, fragile animals like starfish and crinoids need fast embedding, because decay sets in quickly, e.g. for brittle stars, already after half a day. Brittle stars cannot free themselves when they´re suddenly covered by more than 5 cm of sediment – most of the brittle star fossils on the Yorkshire coast have probably been created by rapid burial in sediment, like a sediment avalanche – truly a moment frozen in time.
Some of the crinoids that can be found articulated are thought to have been deposited in scour troughs.
In Yorkshire, subzones where the liklihood of finding articulated crinoids or starfish are greatest are in my experience :
  • taylori
  • obtusum
  • oxynotum
  • maculatum
  • figulinum
  • stokesi
  • tenuicostatum
  • falciferum
Still – “greatest liklihood” does not mean they’re common fossils, finding an articulated crinoid or starfish remains a great rarity. The falciferum zone is the odd one out here, since benthic conditions were mostly anoxic during this time (very little oxygen at the bottom) – so there are no starfish, and crinoids that occur were usually attached to drifting logs at the surface of the sea.
So the crinoids and starfish I’m showing you here now really represent more than 20 years collecting – please do not get the impression that fossils like these can be found on a day’s trip to the coast – unless of course you are extremely lucky !
Some of these have been purchased, some reside in collections other than my own and are pictured here by kind permission of their respective owners – you will see that in the notes for the fossils.
Generally preparation of starfish and crinoids most of the time is a difficult and time consuming task.
Their delicate structures are usually firmly embedded in the sediments and can only be retrieved if there is a difference between the surrounding matrix and the fossils which are usually preserved as calcite or pyrite. This difference can either be a difference in hardness, when the fossils are harder than the matrix – these fossils can be prepared mechanically, i.e. needles, airpens or – you might have guessed – air abrader. If there is no difference in hardness, there can also be the option of chemical preparation – when the fossils are more resistant to certain chemicals like acids or alkaline solutions than the matrix. In rare cases, the environment (or rather what we put into it – sulphur dioxide / nitrogen dioxide from burning fossil fuels reacting with water to form sulfurous/sulfuric resp. nitric/nitrous acid) or certain naturally occuring humic acids can do the job for us – but that is only really the exception.
This first part will be about the Yorkshire liassic crinoids, so without much further ado, here they are :

Eocomatula interbrachiatus (BLAKE, 1876)
Eocomatula interbrachiatus, width = 13 cm

Eocomatula interbrachiatus, width = 13 cm

This is one of the first more or less articulated crinoid crowns I´ve found on the Yorkshire coast.
Eocomatula interbrachiatus with Oistoceras ammonite

Eocomatula interbrachiatus with Oistoceras ammonite

Another specimen found by Keeley and Adrian which they kindly let me prep and photograph shows the crown in association
with an Oistoceras ammonite, which places the crinoid in the figulinum subzone of the lower Pliensbachian.
Eocomatula interbrachiatus as found, width = 24 cm

Eocomatula interbrachiatus as found, width = 24 cm

Eocomatula interbrachiatus prepared, height of detail = 12 cm

Eocomatula interbrachiatus prepared, height of detail = 12 cm

These crinoids only have a very short stem (only a few segments), which I´ve never found preserved so far.

Hispidocrinus scalaris (GOLDFUSS, 1831)

Hispidocrinus scalaris, slab as found, width = 30 cm

Hispidocrinus scalaris, slab as found, width = 30 cm

Hispidocrinus scalaris, prepped, width = 22 cm

Hispidocrinus scalaris, prepped, width = 22 cm

Hispidocrinus scalaris, detail with crown and small brittle stars

Hispidocrinus scalaris, detail with crown and small brittle stars

This is the crinoid specimen that really got me seriously started to try and collect all the known Yorkshire liassic crinoids. It was found on a very warm summers day in July 2007 when I was out collecting with friends and had found nothing at all up to this point – I picked up this 30 x 20 cm slab with some faint crinoid traces and put it in my bag so it at least felt like I had found something… At home I started prepping it with an air abrader (iron powder) and followed the visible arms of the crinoid into the rock. To my amazement more and more stems and arms appeared until after about 60 hours of prep work almost the whole slab showed a deathbed of multiple Hispidocrinus crinoids in an early state of decay. When looking carefully at the slab, you can even notice tiny little brittlestars that have either fed on the decaying crinoid or had used the crinoid to achieve a higher up filter feeding position or even benefit from the crinoids´excretions and got embedded with it.
Hispidocrinus scalaris, slab as found, width = 15 cm

Hispidocrinus scalaris, slab as found, width = 15 cm

Hispidocrinus scalaris, prepped

Hispidocrinus scalaris, prepped

Another specimen was found in a small 12 x 15 cm which was initially split after finding it since the outside did only show very faint crinoid traces.
As the split showed a crinoid inside, it was glued back together again and air abraded in about 20 hours.
It shows two crowns, with the larger one having all it´s arms folded unto itself. And of course there´s another small brittlestar (5 o´clock position at the bottom) …

Seirocrinus subangularis (MILLER, 1821)

Seirocrinus subangularis lens with ossicles, width = 18 cm

Seirocrinus subangularis lens with ossicles, width = 18 cm

Seirocrinus subangularis, detail of lens cross section

Seirocrinus subangularis, detail of lens cross section

This lens comes from the tenuicostatum zone of the lower Toarcian and shows remains of Seirocrinus subangularis and was prepared using an air abrader.
On the inside, this lens consists of almost 100% of crinoid ossicles amalgamated into a solid calcitic core.
Seirocrinus subangularis, crown on lens, width = 15 cm

Seirocrinus subangularis, crown on lens, width = 15 cm

Other side of lens with ammonite Dactylioceras tenuicostatum, width = 15 cm

Other side of lens with ammonite Dactylioceras tenuicostatum, width = 15 cm

Another specimen showing a beautiful crown of Seirocrinus was found by my friend Klaus, who paintakingly prepared this
specimen using potassium hydroxide pellets over the course of a couple of weeks, the final touchers were again made with an air abrader.
This specimen also handily shows a Dactylioceras tenuicostatum ammonite on the back of the nodule which tremendously helps dating it !

Isocrinus robustus (WRIGHT, 1858)

Isocrinus robustus, oxynotum subzone, 9 cm

Isocrinus robustus, oxynotum subzone, 9 cm

Isocrinus robustus, stem detail

Isocrinus robustus, stem detail

This crinoid was a surprise find on my birthday this year ! It sat neatly on the edge of a fallen rock most likely of oxynotum zone age and when found I had thought it was a Hispidocrinus in an unusual, almost 3D preservation. The surprise came at home when I prepped it using the air abrader and noticed that it did not have the characteristic spines of a Hispidocrinus. Dr Mike Simms kindly identified it as an early Isocrinus robustus.

Pentacrinites dichotomus (MCCOY, 1848)

Pentacrinites dichotomus, width of colony 20 cm

Pentacrinites dichotomus, width of colony 20 cm

This beautiful colony of Pentacrinites dichotomus from the Jet Rock in the Whitby area was found by Mike Marshall who prepared it by stabilizing the exposed weathered side of the thin fragile slab with a layer of epoxy putty and prepared it from the other side using an air abrader. The colony is nicely set into a slab of top jet dogger to stabilize it.

Jet with juvenile ?Pentacrinites

Small juvenile ?Pentacrinites crinoid on jet, width = 6 cm

Small juvenile ?Pentacrinites crinoid on jet, width = 6 cm

This beautiful section of a piece of solid black jet with a juvenile crinoid anchored to it was kindly given to me by a friend. It gives more evidence to the observation that some crinoids have used floating logs for anchoring themselves to during their lifetime and filter fed in the surface waters.

Balanocrinus gracilis (CHARLESWORTH, 1847)

Balanocrinus gracilis, ossicles with juvenile Amaltheus (1 cm diameter) ammonites

Balanocrinus gracilis, ossicles with juvenile Amaltheus (1 cm diameter) ammonites

Have you spotted the crinoid remains on this picture ? – don´t get distracted by the associated juvenile (~1 cm ) Amaltheus ammonites !
Of this one I yet have to find a crown. Disarticulated ossicles are no rarity in the stokesi and maculatum subzones, but sometimes extraction can be impossible.
The second part describing the starfish will be published next.
AndyS

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

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

Old beliefs proven wrong or Young and Old Oxynoticeras

Oxynoticeras simpsoni, 9 / 5 / 3.5 / 2.5 cm

Oxynoticeras simpsoni, 9 / 5 / 3.5 / 2.5 cm

You might have wondered why I called the Oxynoticeras in one of the last posts Oxynoticeras simpsoni and not Oxynoticeras oxynotum. I did actually have that one labeled as O. oxynotum originally, more or less assuming that this was the more common species and not looking for the differences. I had only the large Oxynoticeras where you could not overlook it’s typical O. simpsoni characteristics labeled as such. So when I looked through my collection, I actually found that I had only one potential O. oxynotum, all the other ones were O. simpsoni, just different growth stages !

This picture shows the O. cf. oxynotum , found in 1990, a lucky split, inner whorls as they could be prepped then (with a prep needle).

Oxynoticeras cf. oxynotum, 3.5 cm, 1990 style prep

Oxynoticeras cf. oxynotum, 3.5 cm, 1990 style prep

Today I gave it a little re-prep with the air abrader, just a 5 min job.
Oxynoticeras cf. oxynotum, 3.5 cm, re-prepped today

Oxynoticeras cf. oxynotum, 3.5 cm, re-prepped today

Here is a similar sized Oxynoticeras simpsoni :
Oxynoticeras simpsoni, 3.5 cm

Oxynoticeras simpsoni, 3.5 cm

So here are the main differences between the species : O. oxynotum has a smaller umbilicus at larger sizes, the whorls do not get as thick, the whorl section is more oval with a sharp keel,
there is a bit of a soft wave-like ribbing that almost vanishes at the point where the ribs bend forward towards the keel,  while O. simpsoni has a larger umbilicus, thicker whorls, and the whorl section thins out towards the venter, making it more lanceolate. O. simpsoni has very fine ribbing that continues towards the keel and can produce a crenelated keel. In comparable beds around the world, Oxynoticeras seems to be more common without shell, e.g. pyritized. I wonder if the authors describing the different species ever saw them with shell ?
At small sizes, with the shell on, they´re almost identical.
This is a “classical” larger Oxynoticeras simpsoni, 9 cm, an inner whorl of a larger, eroded specimen, the largest I have with almost complete shell on :
Oxynoticeras simpsoni, 9 cm

Oxynoticeras simpsoni, 9 cm

Both O. oxynotum and O. simpsoni apparently get very large : HOWARTH 2002 describes specimen between 25 and 40 cm !
As usual: If you do have a large specimen like that, let me know !
I have a small Oxynoticeras specimen with a crenelated keel – SCHLEGELMILCH lists this as a characteristic of another 2-3 cm small species O. bucki, which HOWARTH 2002 later lists as a synonym of O. simpsoni – I tend to agree with that, allthough since this one has its complete shell preserved, the other characteristic of O. bucki, as SCHLEGELMILCH describes it – a very simple suture – cannot be seen.
Oxynoticeras "bucki", 2 cm

Oxynoticeras “bucki”, 2 cm

AndyS

Keeping the shell on or A million tiny needles…

Microderoceras birchi, width of view 4 cm

Microderoceras birchi, width of view 4 cm

A Microderoceras birchi, shell fully preserved, with a tiny little ammonite sitting on one of the spines.

The most difficult way to prep an ammonite is to try to keep its shell on – making it look almost like it used to when the animal was still alive.  The ability to do so of course depends on a few things. First and foremost : The shell has to be preserved (obvious). Then there has to be a way to separate shell and matrix without the internal mould inside the shell separating from the shell first. For this to be possible there are a few conditions :

  • The shell has to be relatively solid
  • There has to be a difference between matrix and shell, be it in terms of hardness or chemistry
  • There has to be a way to use this difference to remove the matrix without damaging the shell too much.
For shell preserved ammonites in soft clays, washing and brushing is the easiest version where these conditions are true. The only problem is :  None of the beds in the Yorkshire liassic are soft clays…
Most shell preserved ammonites in Yorkshire come from more or less hard nodules that formed around the decaying animal. With some of the nodules, especially from some beds in the upper sinemurian, it is the hardness difference between shell and nodule that can be used to extract an ammonite with shell more or less intact.
Generally ammonite shells are most commonly preserved in calcite that was transformed from the original aragonite. Calcite has a Mohs hardness (a relative scale of hardness from 1 to 10 with diamond being 10) of 3 – not really all that much, considering that the steel of a chisel or a prep needle has a Mohs hardness of between 5.5 and 6.5.
So using a prep needle, you can scrape softer matrix off the shell, but you will always scratch it more or less, depending how careful you work.
In recent years, air abrasive techniques have become affordable for collectors, in essence shooting small abrasive medium particles at the fossil using a compressed air stream, like a million tiny needle-pricks. The elegant thing about air abrasive technique is that you can (within boundaries) tailor your air abrasive medium to the hardness difference between matrix and fossil. A more commonly used abrasive medium is iron powder with a hardness of about 4, but almost anything is possible e.g. walnut  shell powder , glass, aluminium oxide. Iron powder is quite popular in Germany because it is just slightly harder than calcite (it will “smear” ammonite shell just a bit) but harder than most soft shales, or oolithic sediments with a high silt content, is relatively cheap, and quite recyclable, i.e. you can re-use it many times. Another quite commonly used medium is aluminium oxide which you can use on harder matrices like limestone nodules as well, but with a Mohs hardness of 9, it will easily shoot holes through your fossil if you’re not really careful.
Here are some more examples of what can be done using iron powder air abrading:
When there are chemical differences between fossil and matrix they can sometimes be used to prep fossils as well – which we’ll explore sometime later…
AndyS