Classes of the Phylum Echinodermata (currently in Atlas)
What are echinoderms?
Echinoderms are marine invertebrates such as sea stars, sand dollars, and crinoids. The Echinodermata (+Hemichorata) are the most closely related invertebrates to humans. The word echinoderm means “spiny skin” and it is an apt description for these organisms, as many of them contain exoskeletons covered in small spines or ridges. Additionally, echinoderms have a unique hydraulic vascular system which allows them to regulate the water pressure within canals in their limbs for feeding and locomotion.
Echinoderms have pentameral (5-part) symmetry. The most distinctive part of their anatomy is the water vascular system. The interconnected tubes of the water vascular system run the length of the limbs of the organism and ends in tube feet. The areas with openings for the tube feet are the ambulacral areas. These are used to acquire food when filter feeding and for locomotion. Crinoids are very different from starfish or sand dollars. Crinoid’s bodies sit atop long stems which hold them to the seafloor, and their arms are directed upward for filter feeding.
How are echinoderms preserved in Cincinnatian strata?
Echinoderms have an excellent fossil record. Because their exoskeletons are built of calcite, echinoderm plates preserve well. However, it is rare to find complete articulated specimens. Crinoids tend to disarticulate only a few days post-mortem, and sea stars and delicate crinoid arms are commonly broken off and destroyed by geologic processes. The most common echinoderm fossils found in the Cincinnatian are the small crinoid ossicles, which are small, circular pieces resembling Cheerios, that make up the stalk of the crinoid. These ossicles preserve well and are highly abundant in Cincinnatian rocks.