Onychocrinus Onychocrinus is an extinct genus of feathered crinoids that lived during the Carboniferous period, approximately 353.8 to 318.1 million years ago. It belonged to the family Synerocrinidae.DescriptionOnychocrinus exhibited the typical anatomy of crinoids, characterized by a cup-shaped calyx and multiple feathery arms used for suspension feeding. The dorsal cup was relatively low, and the arms were branching and often contracted in fossilized specimens.Fossil recordsThis genus is known in the fossil records of the Carboniferous period of United States and Canada.