sedimentary
Rugose Coral Fossil (Horn Coral)
Order Rugosa (common genera include Zaphrentis or Heliophyllum)
Form: Conical or horn-shaped; Hardness: 3 (calcite) to 7 (if silicified); Color: Grey, tan, or brown; Luster: Dull/Earthy; Structure: Visible septa (radial lines) and growth rings (rugae) on the exterior shell.
- Hardness
- 3 (calcite) to 7 (if silicified)
- Color
- Grey, tan, or brown
- Luster
- Dull/Earthy
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Physical properties
Form: Conical or horn-shaped; Hardness: 3 (calcite) to 7 (if silicified); Color: Grey, tan, or brown; Luster: Dull/Earthy; Structure: Visible septa (radial lines) and growth rings (rugae) on the exterior shell.
Formation & geological history
Formed in shallow, warm marine environments during the Paleozoic era (approx. 488 to 251 million years ago). These were solitary coral polyps that built calcium carbonate structures around themselves.
Uses & applications
Primarily used as educational specimens, for paleontological study, and as decorative collectibles for fossil enthusiasts.
Geological facts
Rugose corals are called 'horn corals' due to their shape. They are extinct, having died out during the Great Permian Extinction. Their growth rings can sometimes be used to estimate the length of a year in the ancient past.
Field identification & locations
Identified by their distinctive cone shape and the presence of radial septa inside the cup. Commonly found in limestone outcrops, stream beds, and gravel pits across North America (especially the Midwest and Ohio River Valley).
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