This is a very unusual specimen, it's a large ammonite fossil preserved inside of a septarian nodule. These nodules are mined near the village of Ambondromamy in the Betsiboka Region of Madagascar, but finding complete fossils in them is unusual. This is a large, polished, septarian nodule, with an ammonite which would be about 9" wide preserved inside of it. You can see the distinctive crack patterns of the septarian particularly on the backside. The base has been cut flat so that it displays nicely on a flat surface.
Septarian nodules are concretions containing angular cavities or cracks, called "septaria" These cracks have been filled in by various minerals causing distinctive patterns. The process which creates the spetaria remains a mystery. A number of mechanisms, e.g. the dehydration of clay-rich, gel-rich, or organic-rich cores; shrinkage of the concretion's center; expansion of gases produced by the decay of organic matter; brittle fracturing or shrinkage of the concretion interior by either earthquakes or compaction; and others, have been proposed. Item #31830.
This is a very unusual specimen, it's a large ammonite fossil preserved inside of a septarian nodule. These nodules are mined near the village of Ambondromamy in the Betsiboka Region of Madagascar, but finding complete fossils in them is unusual. This is a large, polished, septarian nodule, with an ammonite which would be about 9" wide preserved inside of it. You can see the distinctive crack patterns of the septarian particularly on the backside. The base has been cut flat so that it displays nicely on a flat surface.
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