THE MAZON CREEK CNIDARIAN ESSEXELLA: THE WORLD TURNED UPSIDE DOWN

The forms called “blobs” by local collectors are the most common animals in the brackish (Essex) component of the Pennsylvanian Mazon Creek Biota of Illinois, comprising more than 40% of the fauna in some localities. Foster (1979) reviewed the Mazon Creek cnidarians and named the blobs Essexella asherae, assigning them to Scyphozoa (Rhizostomeae). The key evidence given in support of a medusan affinity was the occasional presence of small gastropods on the “skirt” region, believed to be analogous to a modern snail species that feeds on planktic cnidaria. The skirt itself was described as “a membranous curtain of rather resistant material that hangs below the bell”; although Foster admitted that a similar feature is not found in any modern scyphozoans. Various pustules and ridges occurring in that region were suggested to be “oral lobes, tentacles, clusters of tentacles, tentacle-like structures from the oral lobes, folds in the oral lobes, and gas bubbles from decay of structures.” We propose an alternative interpretation: the animal represents an anemone, possibly analogous to burrowing forms of the family Actiniidae. Examination of hundreds of specimens of Essexella suggest that the high variability in preserved morphology is probably due to a combination of differences in the original orientation in the sediment, degree of post-burial decay, and the superimposition of upper and lower features due to compaction. There are nevertheless basic similarities in most of the preserved forms. Many of the specimens have substantial relief compared to most other Mazon Creek animals, indicating a relatively tough and firm structure. “Tentacles” are re-interpreted as contracted muscles, especially as strong longitudinal retractor and parieto-basilar muscles. The “bell”, which is highly variable in shape and size, is instead suggested to be an inflated pedal disk. Linear surface structures may represent verrucae. Similar sets of features are found in the modern burrowing anemone Phyllactis. Its size and morphology also suggest that Essexella is a candidate for the tracemaker of Conostichus, a common Pennsylvanian trace attributed to burrowing anemones.