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Some upper Sinemurian ammonites from Gloucestershire

Oxynoticeras sp. (ammonite A)

Ammonite A

Cheltonia sp.(ammonite B)

Ammonite B

Bifericeras sp.(ammonite C)

Ammonite C

Bifericeras sp? (ammonite D)

Ammonite D

Angulaticeras sp. (ammonite E)

Ammonite E

The ammonites on this page were collected from a location in Gloucestershire. The locality consisted of excavations within an area of around one square kilometre, with the general geology being that of the Charmouth Mudstone Formation. The specimens collected were not in situ, but weathered out from excavation arisings. The excavations were around 10m deep, and the bedding seemed to run either level, or to shallow gradient (that was just a casual observation, not direct measurement).

I've only attempted to identify to genus level, although I now have a copies of various books that have enough information to be reasonably accurate to species level, with some. At the end of this piece, I've had a go at identifying the range of ammonite zones represented by the specimens.

I cast my eye over what must have been a thousand plus specimens at the site, picking up just under a hundred that appeared fairly representative. Unfortunately, my reasons for being there weren't primarily fossil related, so the sampling was far from scientific! I'll consider each ammonite separately:

Ammonite A

This was the most common; in fact in places there were hundreds of them, though in varying states of decay as they were all pyritised. Sizes were up to around 60mm. The oxycone profile is very distinctive, and that narrowed the possibilities down, somewhat. When considering it within the context of Ammonite E being present I decided it was most probably Oxynoticeras sp. The ribs fade towards the outer edge of the ammonite, and it is well compressed, with a sharp keel.

Ammonite B

This was another very common find. Superficially it is similar to ammonite A at a quick glance, but on closer inspection it has no keel and a blunt venter. Cheltonia sp. seems to fit the bill. Sizes were up to around 20mm.

Ammonite C

This ammonite along with the two above constituted over 99% of what was visible at the site (that's just an estimate, by the way). Ammonite C was almost as common as the other two. There are no ribs visible at the centre, but when they appear they become quite prominent. On the venter they turn forward to form a slight chevron that is a little flattened on top. Close inspection shows some to have two rows of spines on each side of each whorl, although in most cases they've broken off. This one got me scrathing my head for a bit. It seemed to match the description in Arkell (1957) for Hemimicroceras, but elsewhere I'd read that was synonymous with Bifericeras sp. The descriptions I had for that genus, however, didn't really fit. On getting a copy of Schlegelmilch (1976) it became clear that the various species of the Bifericeras genus have quite a range of morphologies, so I'll settle with Bifericeras sp. Sizes were up to about 30mm.

Ammonite D

This is one of only two of its type I picked up. It has more ribs than ammonite C, but still has two rows of spines each side, and these are more obvious. The whorl section is depressed, and the ribs are barely visible on the venter. I considered Crucilobiceras sp. but the whorl sections in that genus are compressed. So, I reckon its another species of Bifericeras.

Ammonite E

This one's very distinctive, and the only one of its type I could find. It has the appearance of a Schlotheimiid ammonite, and considering it being found with ammonite A it had to be Angulaticeras sp.

Ammonite F

One of the biggest ammonites I picked up, and again the only one of its type. The venter has a central keel with sulci either side, and the whorl section matches that for Paltechioceras sp. in Schlegelmilch (1976).

Conclusion

The depth of excavation (Worssam et al. 1989, 27), and genera present (Simms et al. 2004, 9), suggest the Oxynotum subzone of the Oxynotum zone through to the Raricostatum subzone of the Raricostatum zone. When the OSGB36 National Grid coordinates of the locality are plotted on the map of ammonite zones in Worssam et al. (1989) fig 7, it seems to corroborate that in general terms.

A final interesting point is in what is not represented in the sample, and the presence of Paltechioceras. There are no genera representative of the top of the Densinodulum subzone, or the base of the Raricostatum subzone as far as described in Simms et al. (2004). The only specimen relating to the Raricostatum zone is Paltechioceras. However Dean et al. (1961) do mention that there are accounts of Paltechioceras occurring at the base of the zone. That could be the case here.

I may update this piece at some time as I'm trying to get my hands on some more related literature. Furthermore, there are a few specimens I may add that vary subtly to what's shown above.

2007

Paltechioceras sp. (ammonite F)

Ammonite F

Section through venter of ammonites F, A and B respectively

Section through venter of ammonites F, A and B respectively

References

Arkell, et al. 1957. Mesozoic Ammonoidea. In Treatise On Invertebrate Paleontology. Part L. Mollusca 4. Cephalopoda, Ammonoidea. L80-L347. Kansas & New York.

Dean, W.T., Donovan, D.T. and Howarth., M.K. 1961. The Jurassic ammonite zones and subzones of the North Western European Province: Bulletin of the British Museum of Natural History, v.4, pp. 435-505.

Donovan, D.T. et al. 2005. The lower part of the Lias Group in south Gloucestershire: zonal stratigraphy and structure. Proceedings of the Geologists' Association, 116, pp45-59.

House, M.R. 1993. Geology of the Dorset Coast. Geologists' Association guide No.22.

Schlegelmilch, R. 1976. Die Ammoniten des süddeutschen Lias. Gustav Fischer Verlag. Stuttgart, New York.

Simms, M.J., Chidlaw, N., Morton, N. and Page, K.N. 2004. British Lower Jurassic Stratigraphy, Geological Conservation Review Series, No.30, Joint Nature Conservation Committee, Peterborough.

Worssam, B.C, et al. 1989. Geology of the country around Tewkesbury. Mem. Geol. Surv. GB.