10.6" Ammonite In Septarian Nodule - Madagascar
Ammonites were predatory mollusks that resembled a squid with a shell. These cephalopods had eyes, tentacles, and spiral shells. They are more closely related to a living octopus, though the shells resemble that of a nautilus. Ammonites appeared in the fossil record about 240 million years ago and they barely survived several major extinction events. The last lineages disappeared 65 million years ago at the end of the Cretaceous.
The exact mechanism for how the cracks form in the concretions is a mystery. One possible mechanism is the dehydration of the clay-rich core of a concretion causing it to shrink and crack. Another is the cracks being due to the expansion of gases produced by the decay of organic matter within a concretion. Earthquakes have also been a suggested as yet another mechanism.
The cracks in the concretions are then filled in with minerals such as calcite (yellow) and aragonite (brown) and sometime pyrite causing the very interesting patterns, which have often been described as dragon's skin. They are frequently found as geodes with hollow, calcite crystal filled cavities. More rarely the fossils that originally started the formation of the concretion are still preserved in the septarian.