INVERTEBRATE ICHNOLOGY OF THE MISSISSIPPIAN HORTON BLUFF FORMATION AT BLUE BEACH, NOVA SCOTIA, CANADA

The Lower Mississippian (Tournaisian) Blue Beach Member of the Horton Bluff Formation at Blue Beach, Nova Scotia has been interpreted to represent a lake-margin environment with periodic marine influence. It yields extensive fossil assemblages that encompass some invertebrate body fossils and numerous plants and vertebrate fossils, including early tetrapod bones and footprints. Particulary well preserved at Blue Beach is a diverse suite of invertebrate ichnofossils, which can be assigned to two ichnofossil assemblages. Assemblage 1 consists of profuse, monotaxic beds of Palaeophycus, which we interpret to represent shallow burrows in permanently subaqueous settings. Assemblage 2 is dominated by arthropod walking and resting traces with subordinate grazing traces. It occurs in the same strata as fish swimming traces (Undichna) and abundant tetrapod footprints. Assemblage 2 ichnotaxa include Arborichnus, Cruziana, Diplichnites, Diplopodichnus, cf. Gordia, Kouphichnium, Limulicubichnus, Paleohelcura, Planolites and Rusophycus. Rusophycus is numerically dominant, followed by the walking traces. These ichnofossils are characteristic of the Scoyenia ichnofacies, and thus are indicative of unstable substrates that were periodically subaerially exposed. They indicate deposition along the Blue Beach Member lake margin or in very shallow, fluctuating parts of the lake system. Although the invertebrate traces are in the same strata with the tetrapod footprints, there is no demonstrable co-occurrence of invertebrate and tetrapod traces on the same bedding planes. This implies differences in facies and/or taphonomy controlled the distribution of the invertebrate and the vertebrate traces.