Thursday, 11 May 2017

Identifying Ormsaigbeg's Fossils

Ormsaigbeg's 'Jurassic Coast' boasts some magnificent Cephalopod fossils, the Cephalopods being the class that includes the modern octopus and squid.  What makes the Ormsaigbeg specimens so unusual is their size - there are several which are around 300mm in diameter, which means their full size when they were alive some 130 million years ago might have been twice that.

This is one of the ammonites. There are several good specimens, and they all look much the same, so they're probably all the same species - but identifying it is the devil's own job.

Researching the internet didn't produce much, but the Geological Survey's 'Regional Guide' to the area - my copy is dated 1961 - states that the Ormsaigbeg rocks belong to the Inferior Oolite, and the zone fossil is....

.... Ludwigia murchisoni, an ammonite that might well be the one we see. This is a better specimen from the Museo Caffi di Bergamo. Unfortunately, the 'Guide' states that another common fossil in this area is Graphoceras concavum which also looks very like the Ormsaigbeg ammonites.

The ammonites became extinct at the same time as the dinosaurs, but other the Cephalopod along this coastline is a nautilus - in this case, neatly cut in half so that we can compare it with....

....a modern example of Nautilus pompilius.

Here, the only clue to our nautilus' identity is that a similar beast is found in rocks of the same age in Skye, and this is Cenoceras cf striatum.

All of which leaves us a little, but not much further forward. If anyone has a friend who is a professor of palaeontology with a particular interest in Jurassic cephalopods, perhaps he/she might be able to help.


  1. One of the major keys to identification is the suture line. On your first fossil pic, just to the right of the middle, is a very wavy line, looking a bit like a frond. That's where the join between two segments was made.
    Graphoceras concavum (see )has a similar suture line, but look at the middle of the fossil, and you'll see it is quite sunken. However there are lots of ammonites, and it isn't certain that the male and female weren't different.

  2. Quite interesting, though I know nothing about any of it! I have never seen these, except for, of all places, the stone-paved floor of an elevator in a building I once worked in. There were a few small fossils, about 4 inches across, in the stone. It always felt like we were stepping on ancient history (which I guess we were, in a way).

  3. This entry is just one of the many reasons why you will be so sorely missed. Great photos, I wished I had found these examples. Would have made my holiday !

  4. Many thanks for the comments. It always seemed to me that using the suture line as a way of distinguishing species was very arbitrary, particularly as it isn't always visible. Short of the Jurassic coast of Dorset, these are the largest ammonite specimens I 've seen in Britain, yet they are little known.