How to recognise them

Small circular plates shaped like polo mints (10 mm or less); rarely small hexagons.

Fossil Info

Crinoids are marine animals, commonly called sea lillies (but they are not plants!).  Fossil crinoids from Ireland were attached to the seafloor by a stalk up to 1.5 m long. At the top of the stalk were several long feathery arms attached to a swollen area called the cup.  The crinoid’s arms could open like an umbrella to collect food: tiny floating particles drifting by on ocean currents. Most of the crinoid’s hard skeleton is made up of circular plates called ossicles (shaped like polo mints) joined together by ligaments. When crinoids die the ligaments rot away and the ossicles can become scattered by waves and currents. These little circular ossicles (approx. 4 – 10 mm across) are the most common signs of crinoids in rocks. Sometimes the plates aren’t fully separated and instead, we can see a stack of the plates (like a stack of polo mints).

Fun Fact

Crinoids still exist today! Deep-water crinoids are attached to the seafloor using a long thin stalk up to 1 metre long. Shallow-water crinoids are free swimmers and have no stalks.