Graptolites from the Ludlow Series (Silurian) of Wales and the Welsh borderland

• Main Author: Wilkinson, James
• Format: Text
• Language: unknown
• Published: University of Leicester 2022
• Subjects: Uncategorized; Arctic; Canada
• Online Access: https://dx.doi.org/10.25392/leicester.data.19153022.v1; https://leicester.figshare.com/articles/thesis/Graptolites_from_the_Ludlow_Series_Silurian_of_Wales_and_the_Welsh_borderland/19153022/1

Graptolites form an important component of Silurian faunas in Wales and the Welsh borderland. Thirty-two graptolite species are here identified, from fifteen genera, from the Ludlow Series (Silurian) of Wales and the Welsh borderland, including one species described in open nomenclature. The graptolites reveal important data bearing on the morphological, biostratigraphical and palaeogeographical significance of the Ludlow Series succession. They include: one species of Neodiversograptus, two species of Bohemograptus, two species of Cucullograptus and a further two species of the sub-genus Lobograptus, one species of Crinitograptus, one species of Heisograptus, one species of Monograptus, six species of Pristiograptus, two species of Pseudomonoclimacis, eight species of Saetograptus and a further three species of the sub-genus Colonograptus, one species of Spinograptus and one species questionably assigned to Gothograptus. Further refinement of the species Pristiograptus dubius provided a rationale to include Pristiograptus dubius tumescens and Pristiograptus d. t. minor as junior synonyms.Here, I refine four graptolite biozones and a fifth sub-biozone, within the Ludlow Series of Wales and the Welsh borderland, providing more detail regarding species occurrence and ranges than that which was available to Elles and Wood (1901-1918). Within Wales and the Welsh borderland, the nilssoni Biozone is identified as having a lower and upper sub-division, also described as the nilssoni and progenitor sub-biozones, followed by the scanicus Biozone, the incipiens Biozone and the leintwardinensis Biozone. The final Bohemograptus Proliferation Zone of the Ludlow Series is described from a single locality, south-west of Felindre, Wales (loc. 154, SO 1558 8014), but was not found in the successions studied here.The graptolites include many biostratigraphically short-ranging species which are useful for inter-regional correlation with the Silurian successions of the Lake District, England as well as farther afield including the Czech Republic and the Arctic Islands of Canada: especially Neodiversograptus nilssoni and Saetograptus (Saetograptus) wandalensis characteristic of the nilssoni graptolite Biozone and Cucullograptus progenitor and Saetograptus (Colonograptus) ludensis characteristic of the nilssoni and progenitor sub-biozones.The lithological constraints on Silurian graptolite preservation through Wales and the Welsh borderland are also analysed within the context of sedimentary sorting, bulk grain size and siliciclastic rock classification of the parent rock. Analysis of graptolite “forms” against preservation biases, showed a strong correlation between the preservation of “straight form” and “curved form” graptolites with fine-medium sand-grade rocks. The degree of sedimentary sorting also showed links with graptolite preservation with all “forms” showing a strong affinity with “well sorted” to “moderately sorted” rocks.