Isorophus cincinnatiensis   CLASS - Edrioasteroidea ORDER - Isorophida FAMILY - GENUS/SPECIES - Isorophus cincinnatiensis ROCK UNIT - Maysville Group AGE - Late Ordovician (445 million years old) LOCALITY - Cincinnati, Ohio, USA Edrioasteroids are one of the oldest groups of echinoderms, with representatives dating back at least to the Early Cambrian (535 million years ago), if not earlier in the late Proterozoic. The disc-shaped form of Isorophus is typical for many (but not all) edrioasteroids. These echinoderms encrusted hard surfaces, such as the brachiopod shell (Rafinesquina alternata) seen above. Normally rare in the fossil record, they may be found by the thousands on individual bed surfaces (which were presumably favorable to encrustation). PRI 511K