Crinoids: Beauties of Echinoderms | Science 2.0
Crinoids are exceptionally beautiful and graceful members of the phylum Echinodermata. They look like an underwater flower swaying in an ocean current. But make no mistake, they are marine animals. Imagine a flower with a mouth on the top surface which is surrounded by foster arms. Awkwardly, add an anus right next to this mouth.
Crinoids with root-like anchors are called sea lilies. They have graceful stems that grip the ocean floor. Those that live in deeper water have fairly long stems that can reach 3.3 feet or a meter in length. Then there are other varieties that swim freely with only vestigial stems. They make up the majority of this group and are commonly referred to as feathered or comatulid stars.
Unlike water lilies, feathered stars can move on tiny hook-shaped structures called cirri. It is these same cirri that allow crinoids to cling to the surfaces of the seabed. Like other echinoderms, crinoids have pentaradial symmetry. The aboral surface of the body is dotted with plaques of calcium carbonate, forming an endoskeleton similar to that of starfish and sea urchins.
These give the calyx a cup shape, and there are few or no ossicles in the buccal (upper) surface, an area we call the tegmen. It is divided into five ambulacral zones, including a deep groove from which the tubular feet protrude, and five interambulacral zones between them.
Crinoids are alive and well today. They are also among the oldest fossils on the planet. We have beautiful fossil specimens dating from the Ordovician – if we ignore the enigmatic Echmatocrinus Burgess Shale. And there can be quite a few of them. Crinoid fossils, and particularly disarticulated crinoid columns, can be so abundant that they sometimes serve as primary support clasts in sedimentary rocks.
The specimen you see here is an amazing example from the Upper Cretaceous, Santonian Age, Uintacrinus – named after the Uinta Mountains of Utah. It shows a plate of known mortality from the Mancos shale. These adorable ones are best known to the Smoky Hills Niobrara Formation of central Kansas.