Marine Fossils and Mississippian Geodes

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This geode is from the Mississippian-age Harrodsburg Limestone
in Central Indiana.

The geode is about 20cm x 15cm (max. dimensions) in size.
One side is the normal lobed-form geode commonly found. The other
side presents a pseudomorph of a planispiral marine fossil.

The fossil is about the size of a soft-ball or possibly considerably larger,
since the living chamber is not discernible. The fossil is about 23mm from
side to side near the start of the coil. At the putative end of the coil, it is
about 120mm from side to side.

In image #6, you can see the fossil appears to end at about 10 o'clock on
the coil. There appears to be a damaged exterior section of the coil
(image 6, from 12 o'clock to 4 o'clock).

The fossil seems to be bi-laterally symmetrical. It may be a pseudomorph
after an internal cast (steinkern). It may represent a gastropod (perhaps a
Bellerophont) or even an Ammonoid. Perhaps someone will recognize it.

The fossil is geodized and lined with quartz crystals. The "normal"
lobe of the geode also seems hollow, based on the weight.

I have several other geodes from the Harrodsburg Limestone in which a
marine fossil (brachiopod, crinoid stems) appears to have served as a
focus for geode formation.

Harry Pristis

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