Graptolithina

Abstract: Since its designation as the Global Stratotype Section and Point (GSSP) for the base of the Silurian System, the choice of Dob9s Linn, Southern Scotland, has received criticism due to the difficulties of relating its well-constrained graptolite biostratigraphy to shallow-water sequences elsewhere. Kerogen samples from across the Ordovician-Silurian boundary interval at Dob9s Linn have yielded carbon stable-isotope signatures consistent with those recorded elsewhere, in particular showing a clear positive δ 13 C excursion in the terminal Ordovician. The architecture of the δ 13 C curve from Dob9s Linn enables very high-resolution stratigraphic subdivision and direct correlation between the deep water Dob9s Linn section and time-equivalent carbonate shelf deposits. An integrated stratigraphic scheme using isotope stratigraphy and biostratigraphy of graptolites, conodonts and shelly faunas has been constructed. This direct correlation shows that the shallow water successions, including the former stratotype candidate at Anticosti Island, are generally incomplete, with hiatuses related to the rapid sea-level changes during the Hirnantian stage. This confirms and greatly increases the global utility of Dob9s Linn as a boundary stratotype.

Abstract: In the neighbourhood of St. David9s, the Arenig rocks, described by Mr. Hicks in his paper read at the last meeting of the Geological Society*, are the earliest in which Graptolites are known to occur; and yet, when once they appear, so diversified are their forms that these Pembrokeshire rocks are only equalled, in the number and variety of the genera they contain, by the Canadian Graptolite-bearing rocks of equivalent age known as the Quebec Group. In more ancient deposits two species only, belonging to one of the two great sections into which these fossils are divided, have hitherto been detected, viz. Dictyograptus (Dictyonema) socialis , Salter, and Dendrograptus Hallianus , Prout. The former occurring in the lower portion of the Tremadoc rocks of North Wales, and the latter in the equivalent strata (the Potsdam Sandstone of America), it is impossible to say which genus is the earlier, or whether the group is first represented in Britain or in America. Before the discovery, in 1872, of the extensive series of Graptolites which characterize the Lower Arenig rocks of Ramsey Island, the Skiddaw Slates of Cumberland were supposed to be our earliest Graptolite-bearing rocks; but it is now known that the lowest rocks of the Arenig Group exposed in the vicinity of St. David9s, in which Graptolites abound, are of greater age than any part of the Skiddaw Slates yet described; and it is highly probable that they are also older even than the lowest beds of the Quebec Group known to contain

Abstract: PAGE INTRODUCTION V5 General features V5 Outline of classification V6 Historical notes on classification of Graptolithina V6 Classification of Hemichordata V7 Morphological terms applied to Graptolithina and other Hemichordata V8 Glossary of morphological terms V8 Stratigraphical note V12 HEMICHORDATA V12 Phylum Hemichordata Bateson, 1885, emend. Fowler, 1892 V12 ENTERoPNEusTA V13 Class Enteropneusta Gegenbaur, 1870 VB PTEROBRANCHIA V13 Class Pterobranchia Lankester, 1877 VB Morphology V13 Order Rhabdopleurida Fowler, 1892 V14 Order Cephalodiscida Fowler, 1892 V16 PLANCTOSPHAEROIDEA V17 Class Planctosphaeroidea van der Horst, 1936 V17 GRAPTOLITHINA V17 Diagnosis and general features V17 Class Graptolithina Bronn, 1846 V17 Pioneer work on graptolites V17 Techniques V18 Preparation of specimens V18 Illustration V20 Structure and composition of periderm V21 Graptolite affinities V22

Abstract: The Watch Hill Grits, in the lower Ordovician Skiddaw Group of the Lake District, are dated as latest Tremadoc or earliest Arenig (older than the Didymograptus deflexus Biozone) by means of acritarchs and graptolites. This places them lower in the Skiddaw sequence than previous authors supposed, and they are thought to have been thrust southwards over younger Arenig rocks. The interval inferred for the Watch Hill Grits brackets the Tetragraptus approximatus Biozone and its equivalents in Australasia, S China, N America and Scandinavia. The dating of the Watch Hill Grits suggests that equivalents of the Tetragraptus approximatus Biozone are present in N England and that sedimentation continued in the Lake District during a period of global eustatic regression. Other beds in the Lake District and on the Isle of Man have yielded similar acritarch assemblages and are probably correlatives. Five new acritarch taxa are described: Caldariola gen. nov., Acanthodiacrodium? dilatum, Stellechinatum sicaforme and Striatotheca prolixa spp. nov., and Tetraniveum arenigum cumbriense subsp. nov.; one new combination, Caldariola glabra (Martin) comb, nov., is proposed.