sedimentary

Fossilized Blastoid

Pentremites (Class Blastoidea)

AI-generated identification · may be incorrect

Five-fold radial symmetry, nut-like shape (theca), beige to grayish-tan color, hardness approximately 3-4 on the Mohs scale, calcium carbonate composition. These fossils often display clear ambulacral grooves emanating from a central point.

Identified June 23, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed in marine environments during the Mississippian and Pennsylvanian periods (approximately 300 to 360 million years ago) through the mineralization of a marine echinoderm.

Uses & applications

Primarily for education, scientific research, and hobbyist fossil collecting. They are not used in industry but are valued by the geological community for mapping stratigraphic layers.

Geological facts

Blastoids were sedentary marine animals related to modern starfish and sea urchins. They used hair-like appendages called brachioles to filter food from seawater. They went extinct during the Great Permian Extinction.

Field identification & locations

Identify by the distinct five-sided star pattern on a rounded, flower-bud-like body. Commonly found in limestone outcrops in the Midwestern United States, particularly Illinois, Kentucky, and Indiana. Look for weathered limestone piles at roadcuts.