Pirasocrinidae

Spine of the anal sac (tegmen).

The spines of the anal sac formed the ribs of a large "umbrella" that sat on the distal end of the anal sac of the crinoid. It is possible that the spines were an effective defense mechanism against predators or that they served as a deflector to stop food-carrying water currents.

Articulated pyrasocrinids are generally very rare in the fossil record. These crinoids are thought to have had weak sutures between the plates, making them very vulnerable to disarticulation. The vast majority of remains are found in the form of loose anal and brachial spines and occasional calyxes.