Asteroceras obtusum, 9 cm, Charmouth / Dorset
(Starting this post with a Dorset Asteroceras – bit cheeky, I know, but it´s going to be the only Dorset specimen in this post, I promise !)
Asteroceratinae is a subfamily of ammonites more usually associated with the Dorset coast, the beautiful, sometimes large, ammonites in all shades of calcite from deep brown to light yellow come to mind. In Yorkshire species like Asteroceras or Eparietites are less common in such nice a preservation, and I have not seen the beautiful brown & honey colors – greys & blacks are more prevailing in Yorkshire coast specimen; body chambers without inner whorls can be seen quite often on the scars, complete specimen can usually only be found in concretions, which are usually soft enough to prep the ammonites completely with shell – carefully done, it reveals shell details that you rarely see from anywhere else.
Eparietites impedens, shell detail
Asteroceras obtusum, fine crenelation on keel on air abraded specimen, Robin Hoods Bay
The green shelled Asteroceratinae from the old, long closed Conersby quarry in Scunthorpe, Lincolnshire are of course a visual highlight; when the shell is removed, predominantly white chambers fillings with additional greens/yellows/pinks & browns and dark chamber walls produce a striking contrast, that is highly sought after (and paid for) these days…
From what I´ve seen, theses so called “Scunnies” belong to species that are basically the same as on the Yorkshire coast.
Aegasteroceras sagittarium, 5.5 cm, Scunthorpe
Eparietites impedens, with shell, 7.5 cm, Scunthorpe, purchased from M. Marshall
Eparietites impedens, shell removed & polished, 12 cm, Scunthorpe
Opportunities to find Asteroceratinae ammonites in Robin Hoods Bay have become relatively rare; more often than not, the beds containing these ammonites are covered by sand or rocks in Robin Hoods Bay. Larger specimen are particularly rare, but they are there – the picture below was shot on day in March 2012 where a window down to the beds below a boulder field had opened – I was counting 6 large Asteroceras in an area of about 20 x 20 m – all of them were eroded like the one pictured. Two days later, that window had closed. Also remember – this is an SSSI exposure, please collect responsibly !
Asteroceras sp. in situ, approx. 20 cm
HOWARTH lists the following species of the Asteroceratinae subfamily in the Bairstow collection from Robin Hoods Bay:
Asteroceras obtusum 23 ribs / whorl
Asteroceras confusum 30-34 ribs / whorl
Asteroceras stellare 32-40 ribs / whorl
Asteroceras blakei 26-28 ribs / whorl
Aegastoceras crassum 22-26 ribs / whorl
Aegasteroceras sagittarium 22-26 ribs / whorl
Caenisites turneri 42 ribs / whorl
Caenisites brooki 40 ribs / whorl
Eparietites impedens shallow ribs, keel very prominent, no real keel furrows
Eparietites bairstowi large specimen with almost no ribbing
Epophioceras landrioti almost like an Echioceras
As a rule of thumb, the genera can be distinguished in the following manner :Asteroceras – more or less thick whorls with keel
Aegasteroceras – more or less thick whorls without keel
Caenisites – thinner whorls with keel, fine deep ribbing, deep keel furrows, more evolute than Eparietites, umbilical width between 40 and 50 %
Eparietites – thinner whorls with very prominent keel, fine shallow ribbing, more involute than Caenisites, umbilical width between 22 and 35 %
Epophioceras – looks more like a Gagaticeras or an Echioceras, only in this subfamily due to similar suture.
(Numbers given for ribs/whorl and umbilical width are from SCHLEGELMILCH and GUÉRIN-FRANIATTE, for references see below)
Of course, some of these species can also be found on the Cleveland coast and in the glacial drift at the Holderness coast.
Asteroceras obtusum (SOWERBY, 1817)
Asteroceras obtusum seems to be relatively rare, I have only a few specimen in my collection. It´s whorls are thicker than the ones
from A. blakei and it has the least ribs/whorl of the species.
Asteroceras obtusum, 5.5 cm, Robin Hoods Bay
Asteroceras obtusum, 5.5 cm, Robin Hoods Bay, keel view
Asteroceras blakei SPATH, 1925
Asteroceras blakei seems to be the most common species at Robin Hoods Bay, at least in my collection…
I´ve shown you Keeley & Adrian specimen of this species before (link)
, here is another one :
Asteroceras blakei, 6.5 cm, Robin Hoods Bay
Aegasteroceras cf. crassum SPATH, 1925
This is a thicker species of Aegasteroceras which has even less of a keel compared to Aegastoceras saggittarium.
Keel shot here only, the specimen needs a bit of re-prep. It was on my “wants list” before, I “found” it in my drawers while
working on this post…
Aegasteroceras cf. crassum, 7.5 cm, keel view
Aegasteroceras sagittarium (BLAKE, 1876)
I have many Aegasteroceras saggittarium in my collection, but none of the are very well preserved.
Here is a keel shot only to show the almost non-existent keel. There are some more that need re-prepping…
Aegasteroceras sagittarium, 5.5 cm, Robin Hoods Bay, keel view
Caenisites tuneri (SOWERBY, 1824)
Caenisites turneri seems to be very rare from Robin Hoods Bay, this is the only specimen I have found so far, and it is badly crushed in the inner whorls.
Caenisites turneri, 5 cm, mostly crushed, Robin Hoods Bay
Caenisites brooki (SOWERBY, 1818)
Caenisites brooki is also quite rare and sits in the middle between Eparietites impedens and Caenisites turneri : It´s middle keel rises above the
side keels and it has deeper ribbing than Eparietites. It has deep keel furrows, it´s umbilical width is somewhat variable, I guess there are many intermediates
between the genera.
Direkt comparison between Eparietites impedens (left) and Caenisites brooki (right), both 3.5 cm, Robin Hoods Bay
Caenisites brooki, 10 cm, Scunthorpe
Caenisites cf. brooki, 3 specimen 4-5 cm, Robin Hoods Bay, purchased from B. Blessed
The 3 ammonites in this specimen seem to be intermediates beween C. brooki and C. turneri –
thicker whorls than C. brooki, a bit more evolute, but the keel rises above the side keels.
Eparietites impedens (YOUNG & BIRD, 1828)
The Eparietites shown here is a find from July 2013. It is a beautiful specimen, that is comprised of most of the shelled inner whorls of a larger specimen
shown in the previous post – sadly the outer whorl was mostly eroded. It´s most diagnostic feature is the strong keel that has no real side furrows.
I´m particularly fond of this specimen since it has most of it´s shell preserved, and it has such a nice fine structure…
Eparietites impedens, 8 cm, Robin Hoods Bay
Species I think I do not currently have in my collection from Yorkshire include :
Eparietites bairstowi HOWARTH 2002 (some doubtful large specimen, mud filled chambers with no inner whorls)
Epophioceras landriotti (D´ORBIGNY, 1849)
Asteroceras confusum SPATH, 1925
Asteroceras stellare (SOWERBY, 1815)
S. GUÉRIN-FRANIATTE : Ammonites du Lias Inférieur de France, Centre National de la Recherche Scientifique, Paris 1966
R. SCHLEGELMILCH : Die Ammoniten des süddeutschen Lias, Gustav Fischer Verlag, 2. Edition 1992
M.K. HOWARTH : The Lower Lias of Robin Hood´s Bay, Yorkshire, and the work of Leslie Bairstow, Bulletin of The Natural History Museum Geology Series Vol. 58/2, London 2002