All posts for the month November, 2012

Protogrammoceras or A rare immigrant from the Tethys

Protogrammoceras (M.) geometricum and Amaltheus bifurcus, both 6 cm

In the stokesi subzone of the Yorkshire coast, there are very few cephalopods other than Amaltheus. There is the very occasional Lytoceras, maybe even a very rare nautilus and a few immigrant ammonites from the Tethys, a new ocean that had begun to form during the triassic (for a visualization of the movement of continents during the ages see e.g. http://www.scotese.com/earth.htm)

One of these immigrant genera from the Tethys is Protogrammoceras, an early member of the Hildoceratidae family of ammonites, that is going to be one of the dominant ammonite families in the Toarcian, with genera like Hildoceras, Harpoceras, Eleganticeras, Hildaites, Pseudolioceras etc.

Much of the evolution leading to Protogrammoceras is assumed to have occurred in the Tethys, so appearance of Protogrammoceras in Yorkshire seems rather “sudden”.

Protogrammoceras is also rather a rare ammonite in Yorkshire, finding good specimen requires intense searching of the hard flat limestone nodules of the stokesi subzone.

There are 2 species that I´ve found so far :

Protogrammoceras (Matteiceras) geometricum (PHILLIPS, 1829)

Protogrammoceras (Matteiceras) nitescens (YOUNG & BIRD, 1828)

There are 2 further species known from Yorkshire (HOWARTH 1992) :

Protogrammoceras (Protogrammoceras) paltum (BUCKMAN, 1922) from the paltum subzone

Protogrammoceras (Protogrammoceras) turgidulum (FUCINI, 1904) from the hawskerense subzone

As usual, if you do have a specimen of these species in your collection, and would allow me to photograph it for the book, please let me know…

Again, M.K. HOWARTH has provided the perfect reference for these ammonites in the form of a monograph of the Palaeontographical Society :”The Ammonite Family Hildoceratidae in the Lower Jurassic of Britain, London, 1992″

The two species mentioned above can be separated by their rib densities (measurements were taken from the specimen shown here):

Diagram of rib density differences between P. (M.) nitescens and P. (M.) geometricum

Protogrammoceras (Matteiceras) nitescens (YOUNG & BIRD, 1828)

P. (M.) nitescens has lower rib densities on the outer whorls, ribs are more angled backwards (rursiradiate) on the outer half of the whorl than
P. (M.) geometricum.

Protogrammoceras (M.) nitescens, 7 cm, “bowl” preparation

Protogrammoceras (M.) nitescens, 8 cm, only aperture was visible before prep

Protogrammoceras (Matteiceras) geometricum (PHILLIPS, 1829)

P. (M.) geometricum has higher rib densities throughout, the ribs do not angle backwards as much as P. (M.) nitescens

Protogrammoceras (M.) nitescens (left, 7 cm) and Protogrammoceras (M.) geometricum (right, 6 cm)

Possible Intermediates

This interesting cluster of 8 Protogrammoceras, 1 Amaltheus stokesi and various bivalves was found in July 2009.
By their rib densities , of the 8 Protogrammoceras 2-3 are P. (M.) geometricum or intermediates, while the other 5 are P.(M.) nitescens.

Cluster of 8 Protogrammoceras, between 6 and 8 cm

The following 2 large specimen don´t really fall into the rib density range given by HOWARTH with ribs/ whorl over 40, but they are also
larger than the specimen HOWARTH used for his rib density observations.

Large 10.5 cm Protogrammoceras, possible intermediate from P. occidentale ?

Large 9 cm Protogrammoceras, possible intermediate from P. occidentale ?

Identification of species by rib densities is certainly no exact science, but they could also be intermediates from P. (P.) occidentale ?

AndyS

by andysfossils on November 11, 2012 • Permalink

Posted in Ammonites

Posted by andysfossils on November 11, 2012

https://andysfossils.com/2012/11/11/protogrammoceras-or-a-rare-immigrant-from-the-tethys/

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

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, 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, 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

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

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

As a surrogate, here´s an eroded specimen I got off Mike Marshall :

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

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

AndyS

by andysfossils on November 4, 2012 • Permalink

Posted in Other fossils, Preparation

Posted by andysfossils on November 4, 2012

https://andysfossils.com/2012/11/04/a-moment-frozen-in-time-part-ii/

Yorkshire Ammonites (and other fossils ) revisited

All posts for the month November, 2012

Protogrammoceras or A rare immigrant from the Tethys

A moment frozen in time, part II

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