Click on article of interest below. For full list see the homepage.
Contents are as of 06 November 2009. For later additions see homepage.
Copyright © 2006- All rights reserved
Unknown (see text)
Unknown (see text)
The ammonites on this page were collected from Charmouth, Dorset, in an approximately 0.5 mile stretch of coast starting from Evan's Cliff and continuing east. They were picked-up from the shingle, or from mudslides at the cliff base, and for inclusion here I've used the criteria that they must be pyritised and appear uncrushed. That, and the location, means they are most likely from the Stonebarrow Pyritic Member of the Charmouth Mudstone Formation. Biostratigraphically that's the Raricostatum Zone of the Upper Sinemurian. Note that other ammonites preserved in different ways are found here, as well, but they are most likely to be from above or below this. However, some pyritised specimens do also occur at these other levels, but less frequently, and some are included here. An interesting point is that the ammonites on this page continue on biostratigraphically from the piece accessed from the menu on the left "Some upper Sinemurian ammonites from Gloucestershire", although geographically we are now considering an area further to the South that experienced breaks in deposition below, and at the top of, the Raricostatum zone.
These ammonites were collected over several visits during the last few years. Some of them aren't in exactly the best condition, but I think they're reasonably representative of what the casual collector is likely to find. I know identification is a problem if you're an amateur as the basic books available only figure a limited number of what they consider to be the most likely finds. They usually give no full description of morphology, or any additional contextual data like the method of preservation for a typical locality. Fortunately, there are good sources of information available if you're prepared to search and the books and papers listed in the references at the base of this page do provide good descriptions, and additional information, even if it may be a little heavy going and not all in English!
As usual, I'll add the caveat that you shouldn't take my identifications as definitive. Finding these things and learning more about them is just a hobby, and I'm still honing my identification skills; so, if you want to be sure with your specimens I'd suggest getting the books and having a go yourself.
I've picked up a number of these up to around 80mm diameter. They have prominent ribs on the sides which fade as they pass on to the venter. The venter has a single thin keel as can be seen in the second photo down. I've taken it as Echioceras raricostatoides rather than Echioceras raricostatum due to the number of ribs (the latter seems to have noticeably fewer from what I've read) and also that the literature seems to indicate it's the most common species of Echioceras at the locality in question. Another, larger, specimen of Echioceras is shown at the bottom of the page.
I've only found one of these. It looks like Oxynoticeras but is less compressed (in other words it looks fatter). The sutures appear similar to the diagrams in Buckman (1917).
Again, I've only got this one, and it's not exactly a brilliant specimen. However, it does show the distinctive features of this species: compressed, lots of ribs, and tubercles towards the venter.
The small ammonite as shown in the sixth picture down on the right is quite frequently found. The ribs flattening on the venter in a chevron are common to a number of genera including Bifericeras and Promicroceras. There are no tubercles evident in the specimens I've found indicating it is most likely the latter. However, it is not a genus of the Stone Barrow Pyritic Member, but of a slightly lower level. I'll say some more on that at the end.
Unknown specimen (bottom right)
I'm not sure what to make of this one. It looks like Gagaticeras, but that's an ammonite of the Oxynotum zone -- a zone not present here.
A rather tatty looking specimen shown below! It must have been knocking around in the shingle for a while. However, you can just make out tubercles and only feeble ribbing. Having read what's likely to be found, and using a process of elimination arrives at this being Eoderoceras sp. I'll have to try and find a better one.
Several visits collecting seem to have produced a selection of genera representative of the Stonebarrow Pyritic Member including Echioceras, Crucilobiceras, Gleviceras and Eoderoceras. There are, however, a few anomalous specimens. I'm not entirely convinced of the Promicroceras identification as it belongs to the Obtusum zone. However, that zone is only just below the Raricostastum zone here due to the absence of the Oxynotum zone. I've also read several sources that indicate that Promicroceras are found pyritised, so I'll assume it is Promicroceras, in light of the evidence I have.
The specimen titled "unknown" is a little frustrating. It looks like several ammonites I've read about, including Gagaticeras, but I don't think it's that for the reason given above. It looks too big for Promicroceras. Someone did tell me it was Androgynoceras, but they come from much higher in the succession, and I'm not sure those beds are even exposed at the western end of Evan's cliff where I picked it up from on top a mudslide. Also, I've not seen any evidence of Androgynoceras being preserved as this specimen; they are usually in nodules if they're found uncrushed.
As usual, I may update this if dig up more information, or find some better specimens.
Arkell, et al. 1957. Mesozoic Ammonoidea. In Treatise On Invertebrate Paleontology. Part L. Mollusca 4. Cephalopoda, Ammonoidea. L80-L347. Kansas & New York.
Buckman, S.S. 1917. Jurassic Chronology: I Lias. Quarterly Journal of the Geological Society, 73, 257-327.
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, 4, 435-505.
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.