A COMPARISON OF ENVIRONMENTAL CONSTRAINTS ON LATE MISSISSIPPIAN (CHESTERIAN) ECHINODERM FAUNULES FROM EAST-CENTRAL KENTUCKY, U.S.A

Mississippian echinoderms are well-known and widespread, but very few studies have examined these echinoderms in a community context. In an attempt to do this, two echinoderm “gardens,” separated by 80 km, were collected form the shallow, open-marine Ramey Creek Member (Chester, Hombergian) of the Slade Formation in east-central Kentucky. One garden from Powell Co., KY, was developed on a storm bed (muddy calcarenites); two very similar faunules developed sequentially at this locality, each buried rapidly in a mud fall-out event with little transportation. Both faunules are dominated by thicket-like development of mid-level, suspension-feeding Archimedes bryozoans, out of which higher-level crinoid and blastoid suspension feeders apparently projected. Lower-level suspension feeders included smaller ramose and fenestrate bryozoans and six species of brachiopods. Rare echinoids seemed to have prowled the margins of the gardens. Although thirteen crinoids species are present, only three were very common, and their presence seems to largely reflect environmental conditions established by the bryozoans in this moderate-energy setting.

The other garden in Carter Co., KY, actually reflects three closely related environmental settings: shoal-top (coarse calcarenites), shoal-margin (fine-grained, muddy calcarenites), and basinal (argillaceous calcilutites). Overall, echinoderms in these settings are not as abundant as in Powell Co., and bryozoans are rare. The high-energy shoal-top faunule is dominated by four crinoid species and a low-level hyalosponge. In contrast, the moderate-energy shoal-margin faunule is dominated by brachiopods, Archaeocidaris echinoids and an unidentified ophiuroid. The low-energy basinal faunule largely contains Archaeocidaris and the semi-infaunal crinoid Agassizocrinus. Although energy is a commonly cited control on echinoderm community development, at only the Carter Co. locality was it likely a major control. In the other community, however, it seems that the possibility of synecological, mutualistic relations with bryozoans may have been more important.