Pentacrinites fossilis like most echinoderms, is composed of numerous calcite plates which are arranged into different organs.
Pentacrinites fossilis is divided into 3 sections:
The stem raises the calyx and arms away from the drift wood and is composed of a stack of numerous 5-sided star-shaped plates. The stem also bears flexible, fingerlike appendages called cirri that are used to attach the individual to the driftwood and other crinoids.
The calyx is formed of two bands of five plates that form a small cup-shaped body. From these the bases of the arms arise. The top of the calyx is covered by numerous small polygonal plates and the mouth and anus are found on this surface. Five arms originate in the calyx but these frequently divide, similar to tree branches, so that there may be over fifty arms in total.
The arms are formed of stacks of U-shaped calcite plates and bear numerous projections called pinnae (like the pinnae/”leaves” of a fern frond). It is on these pinnae that tube feet, covered in mucus, extend into the water and catch plankton. The arms are only weakly moveable. The arms are ‘U’ shaped and have a grove running down their centre. These groves run the length of the arm and onto the calyx and convey food to the mouth.
Pentacrinites fossilis is thought to have evolved from the isocrinids a group of free living crinoids which lived on the sea floor.
Their evolution into a pseudo-planktonic lifestyle enabled them to take advantage of feeding areas unavailable to other crinoids. However, they appear to have become extinct at the end of the middle Jurassic and have left no direct living descendents. Other isocrinids however still populate the world’s oceans usually at a water depth of exceeding 150 metres.