Pentacrinus fossils are usually just the columnals – the stem-like sections that connected the lower part, which is also called the hold-fast keeping the animal attached to the sea floor, the columnals connected that to the main body of the crinoid animal, the tentacles and feeding structures. Pentacrinus columnals are – wait for it – pentagonal in shape, in contract to the circular ones so common in Mississippian rocks. They might be only four or five millimeters across – pretty small – but the species that lived in the Rierdon Sea had columnals that were perfect 5-point stars in cross-section. Really distinctive if you can find one – but it’s harder than you might think. Many are the times that I’d start looking for one in some unknown limestone, only to give up and fall back on the more reliable sequence approach to determining if it was the Rierdon or not.
—Richard I. Gibson