First Appearance Data of selected acritarch taxa and 1 correlation of Lower and Middle Ordovician Stages 2 3

8 9 First Appearance Data (FADs) of selected, easily recognizable acritarch morphotypes 10 are assessed to determine their potential contribution to correlation of Lower and Middle 11 Ordovician stages and substage divisions along the Gondwanan margin (Perigondwana) and 12 between Perigondwana and other palaeocontinents. The FADs of nineteen genera, species 13 and species groups are recorded throughout their biogeographical ranges. The taxa 14 investigated fall into three groups. Some have FADs at about the same level throughout their 15 biogeographical ranges and are useful for long-distance and intercontinental correlation. 16 Among these are: Coryphidium, Dactylofusa velifera, Peteinosphaeridium and 17 Rhopaliophora in the upper Tremadocian Stage; Arbusculidium filamentosum, Aureotesta 18 clathrata simplex and Coryphidium bohemicum in the lower–middle Floian Stage; 19 Dicrodiacrodium in the upper Floian Stage; Frankea in the Dapingian–lower Darriwilian 20 stages; and Orthosphaeridium spp., with FADs in the Dapingian–lower Darriwilian stages of 21 Perigondwanan regions and at about the same level in Baltica. Other taxa, however, have 22 diachronous (or apparently diachronous) FADs, and this needs to be taken into account when 23 using them for correlation. A second group of genera and species, comprising Striatotheca, 24 the Veryhachium lairdii group and the V. trispinosum group, have a recurring pattern of 25 FADs in the Tremadocian Stage on Avalonia and in South Gondwana and West Gondwana, 26 but in the Floian Stage of South China and East Gondwana. The third group, consisting of 27 Arkonia, Ampullula, Barakella, Dasydorus, Liliosphaeridium and Sacculidium, have FADs 28 that are markedly diachronous throughout their biogeographical ranges, although the global 29 FADs of Arkonia, Ampullula, Liliosphaeridium and Sacculidium are apparently in South 30 China and/or East Gondwana. It is possible that diachronous FADs are only apparent and an 31 artefact of sampling. Nevertheless, an alternative interpretation, suggested by recurring 32