Photos are of a Moroccan ammonite from the Cretaceous period (Albian stage approx. 100 mya), cut longitudinally and polished.
Photo1 covers an area 16mm X 12mm.
This is my own simple sketch of what an ammonite might have looked like, based largely upon the present day nautilus (although latest thinking has the ammonite more closely related to cuttlefish, squid and octopus than the nautilus):
http://brucewilliams.ifastnet.com/ammon ... 0title.jpg
The first photo shows the chambered structure of the ammonite. Like the present day nautilus, the ammonite “lived” in the last chamber (or camera) only, secreting new chambers as it grew. However it maintained a living link with each chamber via a long thin tube (the siphuncle) passing back through each chamber wall (or septum).
The siphuncle was used to control the level of water and gas in each chamber as a form of buoyancy control. In the case of the ammonite the siphuncle was positioned ventrally (whereas it's located centrally in the nautilus).
In order to see the siphuncle, the ammonite must be longitudinally cut exactly halfway through to expose the “tube”. In this specimen this objective has only been partly achieved, exposing the siphuncle in 5 chambers only (about a third of the way up). It can be seen that the chamber walls (septa) are especially modified to allow the siphuncle to pass from chamber to chamber - this is more obvious in photos 2 and 3.
During the process of fossilisation the chambers become filled with calcite and other fine grained sediment. How this might have occurred is discussed in this article:
http://gsa.confex.com/gsa/2006CD/finalp ... 104207.htm
All the very best for 2007!
Bruce
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