There is tremendously robust suturing on this lovely ammonite, Holcophylloceras mediterraneum, (Neumayr 1871) from Late Jurassic (Oxfordian) deposits near Sokoja, Madagasgar. Madagascar is a treasure trove of outstanding fossil species and this ammonite is no exception.
The shells had many chambers divided by walls called septa. 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.
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 and a ton of water.