by Kyle Hartshorn and Jack Kallmeyer, Dry Dredgers
In the spring of 2002, attendees of Cincinnati’s annual gem, mineral, and fossil show, GeoFair, cast their ballots to elect an official city fossil. As the votes were tallied, an unlikely winner emerged: the edrioasteroid Isorophus cincinnatiensis. Looking for all the world like an upside-down starfish on a scaly pincushion, this plucky upstart had unseated a local favorite, the trilobite Flexicalymene, as well as three other contenders of varying phylogentic heritage. The mayor proclaimed Isorophus the victor, but more than a few wondered: what, exactly, is an edrioasteroid and why did it deserve to be the official city fossil of Cincinnati?
The answers lie in the Late Ordovician. Roughly 450 million years ago, much of the American Midwest was submerged in a shallow sea. The region we call Ohio, Kentucky, and Indiana was south of the equator, in the tropics or subtropics. The warm waters teemed with abundant marine life: thickets of bryozoans, submarine mudflats dappled with brachiopods, obtuse stromatoporoid mounds, coral not-quite-reefs, and swaying groves of crinoids. And scattered here and there, edrioasteroids: round little echinoderms encrusting whatever surface their larvae had glommed onto.
Like their distant relatives the starfish, brittlestars, sea cucumbers, and sea urchins, edrioasteroids possessed a calcite skeleton composed of numerous plates, a water vascular system, and pentaradial symmetry, with (usually) five ambulacra spiraling outward from a central mouth. These ambulacra were food-gathering organs, using mucous, cilia, and perhaps tube feet to sift small particles out of the passing currents. The oral surface also featured a gonopore (from which gametes were released), hydropore (an inlet to the water vascular system), and anal pyramid (from which waste was expelled). On the opposite side of the edrioasteroid, a ring of marginal plates and fleshy lower surface more-or-less permanently attached the animal to some solid substrate, suggesting an ecology similar to that of modern barnacles. Some were flattened or dome-shaped; others, globose or even stalked, with the oral surface perched atop an extended, scaly pedunculate zone.
Edrioasteroids range throughout the Paleozoic, from the Cambrian to the Permian, but the Late Ordovician was the acme of their diversity. Even judged against these contemporary occurrences, the Cincinnati region has one of the most diverse and prolific edrioasteroid faunas in the world, with six genera (Carneyella, Cryptogoleus, Curvitriordo, Cystaster, Isorophus, and Streptaster) and eleven species known. The class is well represented stratigraphically, with specimens found in almost every Cincinnatian formation, Kope through Whitewater. All are isorophids, a clade united by uniserial ambulacral floor plates, an uncalcified aboral region, and a well-developed series of maginal plates. A second major edrioasteroid lineage, the edrioasterids, is lacking in the Cincinnatian proper, but represented in the subjacent Lexington Limestone by Edrioaster priscus (Miller and Gurley, 1894).
Cincinnatian edrioasteroids are often found affixed to the shells of brachiopods, particularly the large, concavo-convex Rafinesquina, and bivalves, such as Ambonychia. Other, albeit less frequent, hosts include bryozoans, sponges, corals, and crinoid columns (the latter uniquely represented by Cincinnati Museum Center specimen CMC IP 26324). During periods of low sedimentation, edrioasteroids even colonized the solidified seafloor itself.
Paradoxically, edrioasteroids can be difficult to find, but where found, they may be present in the hundreds, if not thousands. This peculiar pattern is likely a taphonomic artifact. Cincinnatian strata abound with brachiopod shell beds, hardgrounds, and other environments suitable for edrioasteroid colonization. However, the echinoderms’ delicate thecae would have easily disarticulated after death, so only a rapid burial could guarantee preservation. Thus, where events such as storms or turbidity currents smothered entire seafloor communities alive, the resulting bedding planes can preserve many square meters of edrioasteroid-encrusted brachiopods. Close inspection of these beds can reveal interesting biotic interactions. For example, clusters of edrioasteroids vying for space on the same brachiopod.
These edrioasteroid assemblages have sparked the interest of many researchers over the decades, including Bruce Bell, Colin Sumrall, David Meyer, René Shroat-Lewis, Carlton Brett, and others. Their efforts have long been aided by Cincinnati’s local paleontological society, the Dry Dredgers. Members of this group are responsible for many an edrioasteroid in the Cincinnati Museum Center’s collections. In the early 2000s, the Dredgers even hosted several “edrio digs”, field trips dedicated to the systematic excavation of large, edrioasteroid-rich slabs for research and eventual donation.
The Dry Dredgers were also instrumental in organizing the campaign to choose Cincinnati’s city fossil, again in partnership with the Cincinnati Museum Center. Precisely why so many voters chose Isorophus may never be known, but its victory was appropriate. Plenty of cities have trilobites, corals, crinoids, and brachiopods. But only Cincinnati is known for its bonanza of exotic edrioasteroids.