A global Chitinozoa biozonation for the Silurian

Abstract

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.

At what stratigraphical level is the mid Ludfordian (Ludlow, Silurian) positive carbon isotope excursion in the type Ludlow area, Shropshire, England?

“…Ludfordian chitinozoan biostratigraphy and the mid Ludfordian positive carbon isotope excursion Verniers et al (1995), in their 'global Chitinozoa biozonation for the Silurian', recognised two Ludfordian biozones. The lower, Eisenackitina philipi Biozone has its base defined by the FAD of the eponymous species in the Ludlow area.…”

“…As noted above, the base of the E. philipi Biozone was defined in the Ludlow area (Verniers et al 1995). The biozone here encompasses the upper part of the Upper Leintwardine Formation, the entire Lower Whitcliffe Formation and lower part of the Upper Whitcliffe Formation.…”

“…This sample is from 0.23-0.26 m below the base of the Ludlow Bone Bed Member of the Downton Castle Sandstone Formation at Weir Quarry, near Downton ca 5 km W of Ludlow. The identification of E. barrandei was accepted by Verniers et al (1995) in their global biozonation paper. Miller et al (1997) subsequently erected new chitinozoan species from Nantyrhynau Quarry, eastern Wales, more than 30 km WNW of Weir Quarry, in deeper water, turbiditic strata, considered to straddle the Ludlow/Přídolí series boundary and be the lateral equivalent of the upper part of the Upper Whitcliffe Formation and lower Downton Castle Sandstone Formation.…”

Classification of Chitinozoa (Llandoverian, Canada) Using Image Analysis

“…Endemic taxa are useful for paleobiogeographical reconstructions. Biozonations exist for Gondwana [1], north east Laurentia [2] and Baltica [3] during the Ordovician and a global zonation has been proposed for the Silurian [4]. The biological affinities of the Chitinozoa are not known precisely, but most authors do favor the hypothesis of a reproductive stage of Metazoa.…”

“…In consequence, the classification generally in use today is the morphological parataxonomy of Paris [5]. Herein [6] for locations and precise stratigraphic positions), collected throughout "Member 3" and in the lower part of "Member 4 Together with the six morphometric parameters, they define the parameters used to characterize the Chitinozoa shape. Although these parameters should be good general descriptors of the shape and dimensions of Chitinozoa, they do not provide precise data on small variations of the organism's contour.…”