Amazing suturing on this lovely ammonite and great detail, allowing us to see how he grew, adding to his size, chamber by chamber, building out his spiral shape.
Ammonite shells had many chambers divided by walls called septa. Nautiloids had simple septa with a single arc whereas ammonites developed septa with intricate folds, lobes and saddles. They also developed delicate feather-like or fern-like lacey patterns, called sutures, on the outer shell. You sometimes see them on polished or water worn specimens and in the photos of this fellow below.
The chambers were connected by a tube called a siphuncle which allowed for the control of buoyancy with the hollow inner chambers of the shell acting as air tanks to help them float. A bit like internal water wings you might use to learn how to swim as a kid.
We can see the edges of this specimen's shell where it would have continued out to the last chamber, the body-chamber, where the ammonite lived. Picture a squid or octopus, now add a shell. That's him!
