Abstract Chitinozoans and graptolites are the main components of preserved Ordovician zooplankton. As with much of the modern plankton, the ‘first-order’ species distributions of Ordovician plankton reflect water masses defined on the basis of sea surface temperatures. For ‘time slices’ of less than a few million years, zooplankton distribution patterns can be used to infer latitudinal sea surface temperature gradients, key palaeoceanographical boundaries and the position of Ordovician climate belts. Here, using two Late Ordovician time intervals – the early Sandbian and Hirnantian – we review how zooplankton distribution patterns identify Late Ordovician cooling and reflect the development of severe icehouse conditions. Supplementary material: Additional information on methods and material is available at: http://www.geolsoc.org.uk/SUP18670

Abstract In an overview of the Silurian of Central Europe, it is important to realize that during this period the study area was spread more widely over the globe than nowadays because at least two oceans were present in the area which have since disappeared. Several palaeocontinents such as Baltica or Gondwana, smaller palaeo-plates such as Avalonia and Far Eastern Avalonia, and Peri-Gondwana terranes such as Perunica, were separated by the Tornquist Sea and the Rheic Ocean. These palaeocontinents were brought together in the present-day configuration by closing of the oceans and the subsequent orogenic collisions, respectively termed the Caledonian and Variscan orogenies. Plate movements before and during the Alpine orogeny also brought pieces of northern Gondwana into the study area. These Proto-Alps are now included in the basement of the Alps and are observable in several tectonic windows (e.g. Carnic Alps).

ABSTRACT Biostratigraphic investigations have been carried out on Ordovician chitinozoans mostly from the Quwarah, Ra’an, Kahfah and Hanadir members of the Qasim Formation in central Saudi Arabia. Among the 96 core samples processed from seven wells, about half of them (from wells Berri-84, Shedgum-239, Ain Dar-196 and Ain Dar-277) provided workable specimens, whereas the other wells, namely Ain Dar-281, Haradh-51 and Abu Jifan-25 were barren. Some chitinozoan species of Baltic affinities (e.g. Laufeldochitina striata ) or of Laurentian affinities (e.g. Lagenochitina cf. pirum ) are present, but most of the recovered chitinozoan species are of northern Gondwana affinities. These chitinozoans allow accurate correlation with the local chitinozoan biozones already established for northern Saudi Arabia. Precise correlation with some of the Ordovician chitinozoan biozones for the northern Gondwana regions can also be proposed (e.g. lower part of the pissotensis biozone). The available samples are not, however, sufficiently closely spaced for characterizing all the Ordovician chitinozoan biozones. It is not yet possible, therefore, to document eventual hiatuses in the Ordovician sedimentary succession of central Saudi Arabia. The investigated samples from the Quwarah, Ra’an, Kahfah and Hanadir members of the Qasim Formation are respectively dated as Ashgill, late Caradoc/earliest Ashgill, Caradoc and Llanvirn. Strata referred to the Sarah Formation are probably of topmost Ashgill age but may range into earliest Llandovery. Several new species have been observed. They are presently kept in open nomenclature until better preserved material is available for proposing well-documented diagnosis.