the fossils
stromatoporoid
a sponge, not a coral · it built the reef
The tell
Concentric layered domes with NO cells and no eyes. In a broken face it looks like a stack of thin sheets, one over another.
Look for low bumps on the surface with faint star-shaped grooves running out of them. Those are astrorhizae, the sponge's outflow canals, and nothing else on this chart has them. What a stromatoporoid does not have is cups that held polyps, which is what separates it from any coral.
Often walked past as a boring gray lump. They were reef-builders, and a big one is a genuinely uncommon find.
What it gets mistaken for
- petoskey stone
- kona dolomite
- concretion
- plain rock
The facts
- also calleda sponge, not a coral · it built the reef
- age~385 million yrs
- where it turns upCharlevoix & Alpena Co. · statewide float
- the field marklayered like a cabbage · low bumps with star-shaped grooves
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