Echinoderm Fossils

From Ohio History Central
Revision as of 17:11, 27 April 2013 by (Talk)

The Upper Ordovician edrioasteroid Cystaster stellatus on a cobble from the Kope Formation in northern Kentucky. In the background is the cyclostome bryozoan Corynotrypa.

Echinoderms are a diverse group of marine organisms with radial symmetry that first appeared in the Cambrian Period. Most people are familiar with starfish but several other groups are important in the fossil record and are prized by collectors. The most abundant group of echinoderms found in Ordovician through Pennsylvanian marine rocks in Ohio is the crinoids. These animals resemble plants in that they have a long stalk that was anchored to the sea floor and a flower-like cup that housed the filter-feeding body parts of the animal. The skeletal material is calcium carbonate. Upon death of the animal, the stalk-like portion broke up into a series of circular plates and the body (calyx) broke up into individual plates. These disarticulated remains are very common in some marine rocks and sometimes are a major constituent of the rock. Rarely, a calyx is found intact with all, or a portion of the stalk attached.

Starfish (asteroids) and brittle stars (ophiuroids) are known from Ordovician, Silurian, Devonian, and Mississippian rocks of Ohio. They are rare fossils but an outcrop in the Cuyahoga Formation (Mississippian) of Cuyahoga County contained approximately 4,500 individuals of Strataster ohioensis per square meter.

Blastoids were echinoderms with a hickory-nut sized and shaped body attached to a long stalk, similar to that of a crinoid. They are known from Silurian, Devonian, and Mississippian rocks. They are common in some units.

Among several groups of rare echinoderms known from Ohio are edrioasteroids, which are small, circular animals with starfish-like arms on the dorsal surface. They are usually found attached to the shell of a brachiopod or other shelled invertebrate.