Paleoenvironmental Analysis Of The Upper Silurian Callocystitidae (Cystoidea, Rhombifera) In The Central Appalachians

Abstract

The dominantly transgressive Upper Silurian interval of the Keyser Limestone (U. Sil. - L. Dev.) in the central Appalachians has yielded the most diverse fauna of rhombiferan cystoids found in any single formation in North America. All forms are members of the Callocystitidae, a diverse family of the Glyptocystitida. Nine lithofacies are recogniz­ed within this stratigraphic interval, representing all major epeiric sea environments from supratidal through shallow shelf below wave base. Identification of cystoid genera and species by disarticulated part studies (comparison of isolated plates to the corresponding element on whole thecae of known taxa) demonstrates their restriction to three discrete assemblages on the open shelf. Cystoid bearing lithofacies represent a contemporaneous and laterally coexisting sequence of skeletal sands and silt which include: 1. well sorted calcarenite (both biosparudite and biosparite), 2. poorly sorted biosparite, 3. open shelf (fossiliferous) calcisiltite. Maximum cystoid diversity is recorded in the calcisiltite, diminish­ing progressively in coarser shoreward calcarenites. Callo­cystitid distribution is suggested to have been primarily depth controlled. These cystoids were intolerant of turbid water, high sedimentation rate and of high energy nearshore sands and infirm mud bottoms. A new genus Laosacystis monterey is described and assigned to the subfamily Staurocystinae on the basis of its four protuberant ambulacra.