An unusual ammonite from the Yorkshire Coast.

The ammonite pictured on this page was found in a glacial erratic pebble at Mappleton on the Holderness coast of Yorkshire by Mr Chris Cone. As can be seen the final whorl section that is preserved does not show bilateral symmetry. One side shows simple ribbing similar to the inner whorls while the opposite side to this shows a pattern of alternate strong and weak ribs with tuburcules on the outer ends of the stronger ribs.

(Apologies to any palaeontologists for my rather vague terminology- more accurate suggestions welcome.)

There is no sign that this represents preservation of an internal mould on one side and an internal mould on the other except that the ribbed side seems slightly narrower as measured from the keel than the tuburculed side. There seems to be feint traces of the shell preserved on some of the inner sections of the whorls which show the plain ribbing.

The specimen has been examined by the local museum and members of the local geological society. The Palaeontology department at the Natural History Museum, (London) have identified the specimen as Agassiceras decipiens (Spath) It has been suggested that the asymmetry is caused by shell damage due to attack by a predator or due to disease.
Jim Craig writes:

I've observed similar regrowth with the gault ammonites I collect from Folkestone. These range from minor deformities often caused, I suspect, by recovery of the shell from some kind of attack (fish, crab, squid etc.) to other pathologies or genetic abnormalities interrupting the ammonites growth pattern creating bizarre forms such as in the example you have posted.

Any further suggestions as to what is going on here, identification or requests for further information on this specimen are welcome.

The following photographs show the same specimen from several angles.

Drawing by Angela Griffin

Images and text copyright Nigel Whittington 1999/2000. Permission to reproduce single copys for the purpose of study or research only is granted. Commercial reproduction prohibited.