sedimentary
Fossilized Blastoid
Pentremites (Class Blastoidea)
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.
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Physical properties
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.
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.
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