TAPHONOMY AND PALEOECOLOGY OF PARACONULARIA PLANICOSTATA (CNIDARIA, SCYPHOZOA) FROM THE LATE MISSISSIPPIAN BIG COVE FORMATION (NEWFOUNDLAND) AND LOWER WINDSOR GROUP (NOVA SCOTIA), EASTERN CANADA

Exceptionally abundant, three-dimensional Paraconularia planicostata (Dawson) form part of a low-diversity, putative hydrothermal vent community in lithologically heterogeneous limestones in the Late Mississippian (mid Visean) Big Cove Formation near Aguathuna, southwestern Newfoundland. A similar biota occurs in the laterally equivalent lower Windsor Group at Irish Cove, Nova Scotia. Specimens of P. planicostata exhibit minute nodes and a single low, internal carina at the facial midlines, but none of the specimens exhibits a schott (apical wall). At both localities, the truncated ends of many P. planicostata are overlapped by other fossils, including (at Aguathuna) microbial rinds, and some specimens exhibit one or more holes, the edges of which, again, are overlapped by other fossils. The inner surface of largely hollow conulariids may be lined by microbial rinds and/or euhedral sparry calcite. Additionally, the inner and/or outer surface of many conulariids exhibits small spirorbid annelids. At Aguathuna, the attitude (trend and plunge) of P. planicostata and elongate tube worms appears to be random, though many of the conulariids are oriented with the oral end facing directly or obliquely downward, and they may be telescoped within a worm tube or occur in localized, log-jam-like concentrations. At Irish Cove, the apertural end of the conulariids is preferentially directed toward the northeast. At Aguathuna, approximately 60% of geopetal structures in conulariids, terebratulids, and worm tubes are inverted or tilted. These observations indicate that P. planicostata and associated fossils underwent short distance transport prior to final burial. Moreover, some fossils were transported after they had been filled with geopetal sediment and cement. Probably in association with transport, conulariids were subjected to mechanical breakage, and they served as attachment substrates for spirorbids, possibly before and after death. This scenario seems inconsistent with the hypothesis that the biota of the Big Cove Formation was a low temperature hydrothermal vent community that inhabited deep, generally quiescent marine waters, but it is consistent with the alternative hypothesis that this biota inhabited shallow, brackish inlets occasionally subjected to storms or seismic events.