Chitinozoan biostratigraphy of the Rheidol Gorge Section, Central Wales, UK: a GSSP replacement candidate for the Rhuddanian–Aeronian boundary

Abstract

As part of an international effort to evaluate potential replacement sections for the current Aeronian GSSP, the stratigraphy of the Rheidol Gorge section (central Wales, UK) is being restudied. A total of 28 samples from the Rhuddanian-Aeronian boundary interval at Rheidol Gorge were studied for chitinozoans. This famous Welsh Basin section, its stratigraphy rooted in the work of O.T. Jones and M. Sudbury, comprises a series of laminated black shales with abundant graptolites, alternating with grey mudstone intervals, deposited in anoxic to dysoxic/oxic seafloor conditions, respectively. The exposed section of c. 20 m spans the middle part of the upper Rhuddanian Pernerograptus revolutus Biozone through the lower Aeronian Neodiplograptus magnus Biozone of the lower to mid-Llandovery Series (Cwmere and Derwenlas formations). Our chitinozoan analysis yielded an assemblage of over 2300 specimens distributed approximately uniformly throughout the entire succession. A total of 16 different taxa are identified, of which 12 remain in open nomenclature, reflecting the poor to moderately intact preservation of the specimens. Based on the presence of key species such as Spinachitina maennili and Conochitina iklaensis, the Rheidol Gorge section is referred to the Spinachitina maennili Biozone. This observation is in perfect alignment with the typical signature of other Rhuddanian-Aeronian boundary successions in the Welsh Basin and in other parts of the world.

Integrated stratigraphical study of the Rhuddanian-Aeronian (Llandovery, Silurian) boundary succession in the Rheidol Gorge, Wales: a proposed Global Stratotype Section and Point for the base of the Aeronian Stage

“…Steeman et al 2016) successions of the Welsh basin. The chitinozoan biostratigraphy of the upper Rhuddanianlower Aeronian interval of the Rheidol Gorge section was fully documented by De Weirdt et al (2019). The 28 samples that were collected as part of that study were integrated with the lithological and graptolite biostratigraphical data documented here.…”

“…1) detailed description of the lithological succession, particularly focusing on sedimentological evidence of occurrence of intervals of gravity-driven sediment transport, and combined sedimentological/ ichnological evidence of the changing redox state of the bottom waters at the time of deposition; 2) systematics and biostratigraphy of the chitinozoan faunas, summarized here and published by De Weirdt et al (2019); and 3) the carbon isotope chemostratigraphy based on analysis of the whole-rock organic matter.…”

“…The chitinozoan work done as part of this study was fully documented by De Weirdt et al (2019) and the illustrated specimens are housed in the collections of the Department of Geology, Ghent University.…”

“…The preliminary results of much of this work were presented by Melchin et al (2018) and the text and illustrations in this paper draw extensively from that report. A separate paper, describing the chitinozoan fauna, was published by De Weirdt et al (2019).…”

Revision of Ordovician chitinozoan Lagenochitina esthonica sensu lato: morphometrics, biostratigraphy and paleobiogeography

“…Chitinozoans are organic-walled microfossils common in Ordovician to Devonian marine sediments. In spite of their disputed biological origin (see Liang et al, 2020a and references therein), chitinozoans are widely used in biostratigraphy of early and middle Paleozoic rocks (e.g., Grahn and Gutiérrez, 2001; Asselin et al, 2004; Vandenbroucke, 2004; Grahn, 2005a; Steemans et al, 2009; Vandenbroucke et al, 2010, 2015; de la Puente and Rubinstein, 2013; Wang et al, 2013; Paris et al, 2015a, b; Al-Shawareb et al, 2017; De Weirdt et al, 2019; Liang et al, 2020b) since the regional and global biozonal schemes were first established around the 1990s (Achab, 1989; Paris, 1990; Nõlvak and Grahn, 1993; Verniers et al, 1995; Paris et al, 2004; Grahn, 2005b, 2006). However, alongside revised genus- and family-level systematics (Paris et al, 1999), and continuously expanding datasets from different regions in recent years, revisions of several well-known taxa are required.…”