Integrated graptolite, conodont and chitinozoan biostratigraphical data is presented from the Rhuddanian through to lower Sheinwoodian of the Aizpute-41 core, Latvia. Correlation of the biozonation schemes based upon the three groups is achieved from the cyphus through to lowermost riccartonensis graptolite biozones, except for the upper Aeronian and lower Telychian, which lack both chitinozoans and graptolites, and upper lapworthi through to approximately base murchisoni graptolite Biozone, where there is interpreted to be an unconformity. Datum 2 of the Ireviken Event is correlated with a level at the base of or within the murchisoni Biozone. It is possible that the changes in conodont assemblages at Datum 2 on Gotland are the result of an unconformity here. Streptograptus? kaljoi sp. nov., from the lower spiralis graptolite Biozone, is described.

A global Chitinozoan biozonation for the Silurian Period is proposed. Each biozone is an interval range defined by the first occurrence of an index species, selected from well-defined species with a relatively short biostratigraphic range. To be included in the scheme, index species must have been recorded from at least two major Silurian palaeocontinents where usable Chitinozoa assemblages have been studied, that is, Avalonia—Baltica (which had already undergone collision by the Silurian), Laurentia, Gondwana and Southern China. Seventeen biozones are identified with seven in the Llandovery: the fragilis, postrobusta, electa, maennili, alargada, dolioliformis and longicollis biozones; four in the Wenlock: the margaritana, cingulata, pachycephala and lycoperdoides biozones; three in the Ludlow: the elongata, philipi and barrandei biozones; and three in the Přídolí: the kosovensis, elegans and superba biozones. Chronostratigraphic calibration is partly provided by reference to the range of the appropriate Chitinozoa index species in the global stratotype sections and points (GSSP) of the Silurian series, e.g. in Bohemia (Czech Republic) for the Přídolí and the Welsh Borderland in England for the Ludlow and part of the Wenlock. When this information was not available, independent biostratigraphic control was provided by calibration with graptolite biozones or in a few cases, conodont or trilobite biozonal schemes. The index and most characteristic species of each biozone are illustrated and their total stratigraphie range provided.

Integrated graptolite, conodont and chitinozoan biostratigraphical data are presented from the Llandovery and Wenlock of the Kolka-54 core, Latvia. Correlations between graptolite and chitinozoan biozones are consistent with those published from other East Baltic sections and the Welsh Basin. While most correlations between graptolite and conodont biozones agree with those presented in previous studies, there are important exceptions. Significantly, we report here the discovery of Distomodus staurognathoides Biozone conodonts in the lowest Aeronian Demirastrites triangulatus graptolite Biozone. The base of the D. staurognathoides Biozone was previously considered to lie much higher in the Aeronian. Also it is shown that Walliserodus survived the late Wenlock Mulde Event, during which it was considered previously to have become extinct.

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The successful definition of chitinozoan genera depends primarily on the precision of the criteria used. A standardized morphological terminology based upon details from scanning electron microscope observations of the most representative taxa bearing these characters is therefore proposed. The 143 genera, or subgenera, described so far in the literature are reviewed in order to exclude invalid taxa and obvious junior synonyms. Particular attention is paid to preventing the overlap of generic definitions of the 56 genera ultimately retained. A brief account of the diagnostic features and stratigraphic range of selected genera is given, and basic information concerning the type material of these genera is listed. Finally, a suprageneric classification of the whole Chitinozoa group based on diagnostic features whose hierarchy is established on statistical and evolutionary grounds, is given. One new subfamily, Pogonochitininae, three new genera, Baltochitina, Hyalochitina, and Saharochitina, and a new species Baltochitina nolvaki, are defined. The subspecies Fungochitina fungiformis spinifera is elevated to a specific rank.