A 110 million-year-old fossil of Cleoniceras ammonite, found in Madagascar. Ammonites are extinct cephalopods that lived in shells. Their closest modern relatives are nautiluses, octopi, squid, and cuttlefish. Like the nautilus, ammonites gradually added onto their shell to accommodate their increasing body mass. As they extended the shell they built a wall behind them, closing up the now too-narrow portion of the shell as they moved into the larger portion of the spiral.
Unlike the nautilus, the morphology of the tissue wall ammonites built between the chambers is not just a smooth curved wall. Instead it has a bizarrely complex 3-dimensional fractal shape. These are called “suture patterns” and mark the intersection of the septum walls with the shell. Scientists can’t agree why these walls are so complexly furrowed or even how they formed.