fossil

Fossilized Blastoid (Sea Bud)

Pentremites sp. (Echinodermata, Blastoidea)

Hardness: 3-4 (calcite composition) or 6.5-7 (if replaced by silica/chert). Color: Dark brown to blackish. Luster: Dull to sub-vitreous. Structure: Five-fold radial symmetry with distinctive ambulance fields (grooves).

Hardness: 3-4 (calcite composition) or 6
Color: Dark brown to blackish
Luster: Dull to sub-vitreous

Identified July 23, 2026More fossil →

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Physical properties

Formation & geological history

Formed during the Paleozoic Era, especially abundant in the Mississippian period (approx. 325-350 million years ago) in shallow marine environments. Over time, the hard calcite structure of the echinoderm was fossilized and mineralized in sedimentary strata.

Uses & applications

Collected as geological and paleontological specimens, educational display, and decorative fossil items.

Geological facts

Blastoids were echinoderms related to modern sea stars and sea urchins. They attached to the seafloor via a stalk and fed using fine filter-feeding structures called brachioles attached to their five radiating grooves.

Field identification & locations

Identified by its small, rounded, nut-like or bud-like shape featuring five distinctive petal-like or ribbon-like groove patterns (ambulacra) converging at the top. Commonly found in limestone deposits across North America, particularly in the Midwest US.

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