No, you did not miss anything, I just decided to do part 2 before part 1,
because it fits so nicely with my previous post – a few of the ammonites for this
post was actually found on th e same day as the Bifericeras from the last post !
The whole set will be in 3 parts, part 1 will be about the Schlotheimia species
that can be found in Yorkshire only as erratics, but we will of course include
the Redcar & Cleveland Schlotheimiids (with some gaps).
Part 3 will take longest, because many of the ammonites from this part still
require careful (re)preparation and more research -part 3 will cover the genus
Phricodoceras, for which there is mounting convincement that it does not
belong, as previously thought, to the Eoderoceratidae, but to the S chlotheimiidae.
But back to Angulaticeras…
There are 2 species of Angulaticeras that I can show you, one is an early bucklandi
subzone species, that I plucked out of Byron‘s unprepped Redcar box a few years ago
for a good price, and the other a relatively rare denotatus subzone species from
Robin Hoods Bay.
Angulaticeras often has a thin ventral groove created by rib ends projecting over
the venter line. This ventral groove is most often lost on larger specimen.
In contrast to Schlotheimia, Angulaticeras shows secondary ribbing,
i.e. bifurcating ribs, from very early on in the development.
But let‘s start with the bucklandi subzone one from Redcar :
Angulaticeras cf. charmassei (D´ORBIGNY, 1844)
A big, robustly ribbed species. Angulaticeras can get quite big, I‘ve seen Somerset
specimen well above half a meter.
The next one is the denotatus subzone species from Robin Hoods Bay :
Angulaticeras sulcatum (SIMPSON, 1843)
I found the first fragmentary specimen of this species in 1996 as a small pyritic inner
mould, only a mere sliver of an ammonite:
I discovered my second specimen when formatting a little bit of matrix with an Eparietites
and found that there was a very small 7 mm Angulaticeras at the back :
the day as mentioned above, found more signs of ammonites :
The nodule showed signs of 3 ammonites, unfortunately one had been split in half.
Pleurotomaria gastropod sitting on it as a bonus !
The 2 remaining pieces of the nodule with the split ammonite were glued together again
after marking the position of the ammonites carefully.
abrader once the smallest signs of ammonites showed themselves, revealing 2 small
further gastropods in the process :
The crowning specimen came along when in discussions on the Yorkshire fossil
hunters group on facebook, Mark Hawkes posted a much larger fragment of a
Robin Hoods Bay ammonite that was also identified as Angulaticeras and which
Mark thankfully sold to me (also for a good price, thanks again Mark !).
This specimen is about 10 cm in width and includes inner whorls and part of a
still fully chambered next whorl, I would estimate it would have been more than
20 cm in diameter including the missing body chamber. It is very finely ribbed,
the ventral groove, being present in the juvenile, is now fully closed.
The matrix looks very much the same as the one for the smaller specimen, so it is
assumed it comes from a similar range of beds.
The Schlotheimiinae are known for large size differences of microconch [m] and
macroconch [M], and this is what we may see here as well, allthough the small
specimen show no signs of being mature, so could be juveniles as well.
What makes this specimen even more remarkable, besides its large size and
rarity, is the sharply localized, round holes in the shell, which are also
complemented by similar holes on the other side of the ammonite…
Since there´s a lot more to this specimen, it´s well worth another blog post
so we´ll explore this a bit(e) more in the near future, taking this to a
whole new dimension 😃
AndyS
Schlegelmilch, R.,Die Ammoniten des süddeutschen Lias,
2nd revised and extended edition, Stuttgart & New York 1992
Howarth, M.K., The Lower Lias of Robin Hood´s Bay, Yorkshire,
and the work of Leslie Bairstow, Bulletin 58/2
of The Natural History Museum, London 2002
Edmunds,M., Varah, M., Bentley, A., The Ammonite Biostratigraphy
of the Lower Lias ‘Armatum Bed’ (Upper Sinemurian – Lower Plieansbachian)
at St. Peter´s Field, Radstock, Somerset, Proceedings of the
Geologists´ Assocation 114, 2003