Chitinozoan biostratigraphy of the Silurian Wenlock–Ludlow boundary succession of the Long Mountain, Powys, Wales

Abstract

Systematic collecting through the upper Wenlock (upper Homerian) and lower Ludlow (Gorstian and lowermost Ludfordian) Silurian rock succession of the Long Mountain, Powys, Wales, identifies some 48 chitinozoan species that distinguish four biozones, two subzones and an interregnum. Consideration of the chitinozoan biozones together with those of the graptolites enables a local three-fold subdivision of the late Homerian lundgreni graptolite Biozone, and the distinction of lower and upper intervals for the Gorstian incipiens graptolite Biozone. The base of the Ludlow Series in the Long Mountain more or less equates to the base of the Cingulochitina acme chitinozoan Biozone, although no key chitinozoan first or last appearance datums are associated with the series boundary itself. The new graptolite-chitinozoan biozonation allows enhanced correlation between upper Wenlock and lower Ludlow sedimentary deposits of the Lower Palaeozoic Welsh depositional basin and those of the palaeo-shelf in the stratotype Wenlock and Ludlow areas of Shropshire. Chitinozoans seem affected by the phenomena that caused the late Wenlock 'Mulde extinction' in graptolites but, with the final disappearance of 9 species and re-appearance of 11 species following an interval of overall low diversity, they seem to have suffered less severely than their macro-zooplanktonic contemporaries

Chitinozoans and scolecodonts from the Silurian and Devonian of Japan

“…Unlike the holotype, the specimens illustrated by Laufeld () from the type area of the neotype (Gotland) have large spines that also are much larger than the ones observed from Japan. However, the Japanese specimens are identical to the specimens of Angochitina elongata found in the Long Mountain sections in the UK (Steeman et al, ). Angochitina elongata is also typical of strata of Gorstian Stage in the Type Ludlow area of Shropshire, UK, and is not known to range above the lower Ludfordian in Wales or the middle Ludfordian elsewhere (Figure ).…”

“…Verniers et al () used A. elongata as an index species in their global chitinozoan biozonation for the Silurian, typical of the Gorstian and lower Ludfordian. In summary, based on known occurrences elsewhere, reviewed by Steeman et al (), the First Appearance Datum (FAD) of A. elongata seems to be within the scanicus graptolite biozone with its Last Appearance Datum (LAD) somewhere in the Ludfordian. All this strongly suggests a Ludlow age for the Yoshiki Formation.…”

“…Angochitina elongata is reported to have its lowest occurrence in the Middle Elton Formation of the Type Ludlow area in the Anglo‐Welsh basin, and its highest local occurrence in the Lower Leintwardine Formation ( scanicus to leintwardinensis graptolite biozones, Gorstian and lower Ludfordian) in Sutherland's () monograph of the type region. Steeman et al () also reported the species in more basinal marine facies of the Welsh Basin, in the Long Mountain succession near Welshpool, with a very similar range ( scanicus to leintwardinensis ). In Gotland, the species has been reported to range from the uppermost Klinteberg Formation into the Eke Formation (Laufeld, ), which corresponds to the uppermost Wenlock to the lower Ludfordian according to Calner, Jeppsson, and Munnecke's () summary revision of the stratigraphy of the island.…”

The paleobiogeographical significance of the Silurian and Devonian trilobites of Japan

“…Palynological biozonations based on chitinozoans provide similar resolution (e.g. Steeman et al, ). However, the Japanese succession is so far devoid of graptolites, and biostratigraphically significant chitinozoans have only now been reported (Vandenbroucke et al, 2018).…”

“…Palynological biozonations based on chitinozoans provide similar resolution (e.g. Steeman et al, 2016).…”