Calibration of Chitinozoa versus graptolite biozonation in the Wenlock of Builth Wells district (Wales, U.K.), compared with other areas in Avalonia and Baltica

The Wenlock sections around Builth Wells, recently used to establish a revised Wenlock graptolite biozonation have been here studied for Chitinozoa. Fifty-five samples have been collected from four sections containing three to five graptolite biozones; samples were taken from the same slabs of rocks used for the redefinition of the graptolite biozonation. The index species of three globally recognised chitinozoa biozones are present, Margachitina margaritana, Cingulochitina cingulata, and Conochitina pachycephala, together with several other characteristic species. These global biozones can be divided into several local biozones. In levels with the centrifugus and murchisoni graptolite biozones, a local margaritana biozone is defined; within the riccartonensis graptolite biozone two biozones are defined, the mamilla and burdinalensis biozones with a third, the tuba biozone at the top. The cingulata biozone starts at the base of the dubius graptolite biozone; the pachycephala biozone commences halfway up the rigidus graptolite biozone and at the base of the (newly defined) lundgreni graptolite biozone, is the subcyatha biozone. Halfway through the lundgreni graptolite biozone the cribrosa biozone begins and continues into the uppermost samples which are referred to the nassa-ludensis graptolite biozones. In five areas of Avalonia and Baltica a calibration between chitinozoa and graptolite biozonations has been established for the Wenlock: Builth Wells district (U.K.), Brabant Massif (Belgium), Estonia and northern Latvia, Gotland (Sweden) and mainland Sweden. For the chitinozoa a similar succession of nine biozones and two interzones (as the expression of extinction events) is present in at least three of the five areas. From bottom to top: the Wenlock part of the local margaritana biozone coincides with the centrifugus and murchisoni graptolite biozones. The mamilla and burdinalensis biozones are present respectively in the lower and upper part of the riccartonensis graptolite biozone. The lower extinction event is either located in the topmost part of the riccartonensis graptolite biozone (Builth Wells district) or in the dubius graptolite biozone (other areas). The main part of the tuba biozone and the cingulata biozone are found in the dubius graptolite biozone and the lagena and pachycephala biozones in the rigidus graptolite biozone. The subcyatha and the cribrosa biozones are found in the lundgreni graptolite biozone. An interzone and the indecoral/lycoperdoides biozone starts within the nassa biozonal complex. The apparent minor differences between the calibrations of biozonations of the two groups could be explained by differences in sample density, by the discontinuous or condensed sedimentation in the sections or differences in biozone definition. Eight new taxa are described, named Cingulochitina angusta, C. bouniensis, C. burdinalensis, C. dreyensis, C. pitetensis, Eisenackitina anulifera, E. causiata, and E. ithoniensis.