SOFT-BODIED COELENTERATES IN THE PENNSYLVANIAN OF ILLINOIS

Soft-bodied coelenterates are the dominant animal in the Pennsylvanian Essex fauna. Their abundance exceeds that in any other known fossil fauna and their diversity at least equals the diversity of the famed Jurassic Solnhofen coelenterate fauna. Eight named taxa are reported from the Essex coelenterate fauna. Twelve medusae that may be new taxa are mentioned. Only one of these is described. Three possible hydrozoans are present: Mazohydra megabertha, Drevotella proteana, and Etacystis communis. The last species was previously unassigned. I regard it as a young stage or a portion of an adult colonial hydrozoan. I provisionally assign it to the order Siphonophora. The most abundant type of object in the concretions at Pit 11 is one that has informally been termed the “blob”. These objects are regarded as fossils of scyphozoan coelenterates. This conclusion is supported by their general form, the presence of impressions of coelenterate organs on some specimens, and the presence of predatory snails with Janthina-like habits on the presumed lower side of some specimens. The majority of the “blobs” are assigned to the new genus and species Essexella asherae. The remainder are placed in the new genus and species Reticulomedusa greenei or are unnamed. All of the modern planktic scyphozoan orders are believed to be represented in the Essex fauna. The order Cubomedusae is represented by Anthracomedusa turnbulli which differs little in morphology from the tropical “sea wasps” of modern oceans. The order Coronatae is apparently represented by Octomedusa pieckorum. The order Semaeostomae is represented by Lascoa mesostaurata new genus and species. The order Rhizostomeae is apparently represented by Essexella asherae new genus and species and Reticulomedusa greenei new genus and species. These fossils extend the known ranges of their respective orders from the Jurassic back to the Pennsylvanian. Essexella asherae differs from somewhat similar modern members of the orders Semaeostomeae and Rhizostomeae in having a rather resistant skirt extending down from the circumference of the main part of the bell to almost totally enclose the oral lobes and tentacle-like structures. The mouth bears a manubrium like tube that splits into eight complex long tapering oral lobes that are either unfused or only slightly fused. Outside the oral lobes are slim tentacles that come off the oral lobes or subumbrella or both. The skirt and enclosed organs may have formed an efficient trap for prey. The striking variety in form seen in this species is believed to primarily reflect differing post-mortem histories. The Essex medusae probably inhabited warm nearshore marine waters. A few may have also penetrated into river mouths and similar indentations of the shoreline. Their burial probably took place in quieter more shoreward situations where they were intermittently swept by strong onshore winds. The predators of the Essex medusae probably included one or more species of the shelled gastropod Strobeus and the shell-less Tullimonstrum gregarium. The morphology of the Essex coelenterates suggests that many of them were little different from those living today. Their abundance and variety suggests that at least part of the Pennsylvanian zooplankton was as abundant and varied as today's zooplankton.