CONODONTS IN THE UPPER ORDOVICIAN KEISLEY LIMESTONE OF NORTHERN ENGLAND: TAXONOMY, BIOSTRATIGRAPHICAL SIGNIFICANCE AND BIOGEOGRAPHICAL RELATIONSHIPS

The classical fauna from the Keisley Limestone, one of the first Upper Ordovician conodont faunas described from the UK, is re-evaluated based on relatively abundant collections of well-preserved conodont elements representing 20 multielement taxa. Common taxa are representatives of Amorphognathus duftonus, Birksfeldia circumplicata, Eocarniodus aff. E. gracilis, Hamarodus brevirameus, Scabbardella altipes, Strachanognathus parvus and several coniform species. These occur with more rare specimens of, among others, Aphelognathus nudus, Amorphognathus aff. A. ordovicicus, Dapsilodus mutatus, Icriodella prominens, Icriodella rhodesi sp. nov. and Protopanderodus liripipus. One new species is proposed, and the concepts of Birksfeldia, Gamachignathus, Icriodella and Notiodella are discussed. It is proposed that Birksfeldia is a senior synonym of Gamachignathus and Notiodella a junior synonym of Icriodella. The described Keisley species association, which represents the A. ordovicicus Zone and is of late Katian (Ka4) age, is closely similar to those of the Boda Limestone of south-central Sweden and some other European faunas, but coeval shallow-water faunas from eastern North America differ in several respects from these European faunas. Biostratigraphy and carbon isotope chemostratigraphy indicate that most of the Keisley Limestone is of late Katian age.