Blastoids (Class Blastoidea) are an extinct type of stemmed echinoderm. Often called sea buds, blastoid fossils look like small hickory nuts. They originated, along with many other echinoderm classes, in the Ordovician period and reached their greatest diversity in the Mississippian subperiod of the Carboniferous period. Blastoids persisted until their extinction at the end of Permian, about 250 million years ago. Although never as diverse as their contemporary relatives, the crinoids, blastoids are common fossils, especially in many Mississippian-age rocks.