7.6" Ammonite In Septarian Nodule - Madagascar

This is a 7.6" ammonite (Perisphictes) on Septarian rock, from Betsiboka Region, Madagascar. The outer shell of the ammonite is gone, revealing the intricate suture pattern of the inner shell. The ammonite is unpolished while the Septarian is polished. There are calcite crystals growing in the gap between the ammonite and the rock.

Ammonites were predatory cephalopod mollusks that resembled squids with spiral shells. They are more closely related to living octopuses, though their shells resemble that of nautilus species. True ammonites appeared in the fossil record about 240 million years ago during the Triassic Period. The last lineages disappeared 65 million years ago at the end of the Cretaceous.

What an ammonite would have looked like while alive.
What an ammonite would have looked like while alive.

Septarian or septarian nodules are concretions containing angular cavities or cracks, called "septaria", filled with calcite and aragonite. A concretion is a hard, compact mass of rock that often forms around decaying organic matter. In the case of septarian nodules, the concretions formed around decaying sea-life in a marine environment.

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. The cracks could also be caused by gas expansion produced by the decaying organic matter within a concretion. Earthquakes have also been suggested as yet another mechanism.

The cracks in the concretions are then filled in with minerals such as calcite (yellow), aragonite (brown), and sometimes pyrite, causing the very interesting patterns. They have often been described as looking like 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.
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DETAILS
SPECIES
Septarian, Unidentified Ammonite
LOCATION
Ambondromamy, Betsiboka Region, Madagascar
SIZE
7.6" ammonite, 8.2" tall Septarian
CATEGORY
ITEM
#113739