What is "petalocrinidae"

Petalocrinidae is an extinct family of cladid crinoid from the Early Ordovician to Lower Devonian. Fossils of petalocrinoids have been found in China, Europe and the United States.

All petalocrinoids have five, flattened arms formed through the fusion of the branches of the ancestral crinoid arm, an Ordovician crinoid possibly similar to the Silurian Gissocrinus or Arachnocrinus. The shape of these fused arms varies from genus to genus, and from species to species. The upper surface of the arms had distinctive patterns of grooves which are thought to have born captured food particles towards the mouth. The purpose of the arms, themselves, remain unknown, though, some researchers suspect that they may deterred predators by presenting a would-be attacker with a largely inedible chunk of calcite.