Crinoids from the Nada Member of the Borden Formation (Lower Mississippian) in eastern Kentucky

Thirty-four crinoid species, including four new species, are reported from the Nada Member of the Borden Formation in eastern Kentucky. The dominant crinoid group is monobathrids (18 species), but diplobathrids, disparids, cladids, and flexibles are also present. The four new species are the camerates Blairocrinus protuberatus, Uperocrinus acuminatus, and Aorocrinus nodulus, and the cladid Atelestocrinus kentuckyensis. The majority of the species in this fauna were previously known from what has traditionally been considered the "upper part" of the Burlington Limestone in the Mississippi River Valley and what is now recognized as the Burlington Pelmatozoan Assemblage III. The current study confirms the conclusion of Lane and DuBar (1983) that the Nada is middle Osagean in age, being deposited earlier than the well-documented Borden delta crinoid assemblages of north-central Kentucky and Indiana. The middle Osagean age of the fauna indicates a previously unrecognized unconformity between the Nada and the overlying Meramecian-age Renfro Member of the Slade Formation. In addition, this is the first well-documented middle Osagean fauna from a siliciclastic facies in North America. The dominance by camerate crinoids is enigmatic, but may be related to either a low rate of sedimentation or greater larval dispersal abilities.