R. B. Rickards

Graptolites in British stratigraphy

697 taxa of planktonic graptolites are recorded, and their stratigraphical ranges are given, through 60 biozones and subzones in the Ordovician and Silurian strata of England, Wales and Scotland, in the first such stratigraphical compilation for Great Britain since the synthesis of Elles & Wood (1901–1918).

Fission-track dating of British Ordovician and Silurian stratotypes

Fission-track dating of zircons and apatites from tuffs and bentonites has produced the first isotopic ages for the type sections of the Ordovician and Silurian Systems. In the Ordovician the following ages have been determined: lower Arenig 493 Ma, lower Llanvirn 487 Ma, lower Llandeilo 477 Ma, upper Caradoc 463 Ma and upper Ashgill 434 Ma. In the Silurian, the following: lower Llandovery 437 Ma, lower Wenlock 422 Ma, upper Wenlock 414 Ma and Ludlow 407 Ma. The Ordovician-Silurian boundary is interpreted as occurring at about 436 Ma. Three North American Rocklandian bentonites yielded zircons whose ages average 453 Ma. This is about 10 Ma younger than supposedly correlative units in the British type sections.

Lower Palaeozoic stratigraphy and palaeontology, Karadere–Zirze area, Pontus Mountains, northern Turkey

Lower Palaeozoic rocks in the Karadere-Zirze area, east of Safranbolu (Pontides, northern Turkey), range from Early Ordovician to Silurian. Overlying the probably Tremadoc Bakacak Formation are Aydos Formation quartzites, followed conformably by the Karadere Formation, dated as Early Arenig to Early Llanvirn by means of graptolites which are assigned to seventeen genera and include three new forms: Eoglyptograptus bouceki, Prolasiograptus haplus praecursor and Undulograptus ? mui. Late Arenig trilobites from the Karadere Formation include Bergamia, Cyclopyge, Dionidella ? , Leioshumardia and Seleneceme. In the Limestone Member of the overlying Ketencikdere Formation, uncommon trilobites suggest only a mid- to late Ordovician age, but con-odonts with Colour Alteration Index 5–6 indicate the Amorphognathus tvaerensis Biozone (early Caradoc). Macrofossils are rare in the Siltstone Member, but conodonts from the middle of the unit suggest the highest subzone of the A. tvaerensis Biozone; the youngest visible strata are, on acritarch evidence, at least as high as Caradoc, but the Ashgill is not confirmed and the contact with overlying Silurian rocks is unexposed. The Findikli Formation comprises: a Lower Member, black argillites with Llandovery graptolites and acritarchs; and an Upper Member, grey shales with late Wenlock graptolites, overlain unconformably by Devonian rocks. The succession differs significantly from contemporaneous deposits in southern Turkey and its affinities lie with western Europe, including the Welsh Basin.

The age of the earliest club mosses: the Silurian Baragwanathia flora in Victoria, Australia

It is argued that there is overwhelming evidence from a good graptolite record that the earliest club mosses on Earth were of Gorstian (Ludlow, Silurian) age, and that Baragwanathia longifolia Lang & Cookson and its associated flora persisted through the Přidoli and into the early Devonian, showing some changes during this time.

Definition of the Tremadoc series and the series of the Ordovician System in Britain

The Tremadoc Series and series of the Ordovician System are widely used in international correlations, and this review explains and summarizes how the base of each of these series is recognized in Britain. No type section in which the base of any one of them is displayed has ever been selected and formally designated, hence the basal horizon of each series is a matter of general understanding rather than precise definition. This general understanding is of particular biozones, or of assemblages of fossils and their relative ages. The geographical separation between type sections of series, and differences in facies, complicate the understanding of relative ages of biozones. Brachiopod and trilobite faunas of the Llandeilo, Caradoc and Ashgill Series have been studied in detail in recent years, and biozones proposed. These biozones have proved difficult or impossible to recognize outside their type areas in Britain, and brachiopod–trilobite faunas of the Tremadoc, Arenig and Llanvirn Series are likewise highly provincial. Thus for correlation within and outside Britain attention has been focused on the ‘standard’ graptolite biozones, particularly since such conodont faunas as have been found have not proved useful. Yet there are problems associated with these ‘standard’ biozones; for example, the Tetragraptus approximatus Biozone is unknown in Wales or England, the relationship between the base of the Glyptograptus teretiusculus Biozone and the earliest Llandeilo shelly faunas requires clarification, and at what level the base of the Caradoc Series may lie in the Nemagraptus gracilis Biozone is unknown. Thus to bring precision to definitions of Ordovician Series in Britain the identification of continuous, fossiliferous sequences that will serve as stratotypes, is required. Such a section for the basal portion of the Tremadoc Series has recently been proposed. Systematic studies related to particular biozones are needed, for example early Caradoc and early Ashgill brachiopods, and graptolites of the Didymograptus bifidus and Glyptograptus teretiusculus Biozones. The stratigraphical ranges and abundances of species in particular sections require documentation, so that the type of biozone (assemblage, acme, concurrent-range, etc.) may be recognized. Such work could lead towards chronostratigraphy, the provision of a standard stratigraphical scale in Britain for the Tremadoc Series and Ordovician series.

A new Arenig (Ordovician) graptolite fauna from the Kerman District, east-central Iran

A single slab of silicified silty and cross-bedded mudstone from the Katkoyeh Formation, east-central Iran yields Yutagraptus cf. mantuanus Riva which indicates a late Arenig age. The slab also bears Dictyonema ghodsiae sp. nov., regarded as the only unequivocally planktonic dendroid from post-Tremadoc strata. The sicula is surrounded by at least twelve vesicular bodies which constitute a raft of floats. These are the only certain floats yet identified in the Order Dendroidea, and one of very few cases known in the Class Graptolithina.

The significance of the graptoloid Amphigraptus divergens from the (mid-Rawtheyan) type section of the Ashgill Series, Ordovician

Amphigraptus divergens Lapworth is known to be restricted, in Britain, to the linearis Biozone, and its occurrence in the type Ashgill Series reinforces Rickards’ prior independent attribution of that part of the type Ashgill to the linearis Biozone. The thecal structure revealed in this paper allies A. divergens to the dicellograptids rather than the leptograptids, hence it is placed in the family Dicranograptidae. The evolutionary origin of Amphigraptus is almost certainly rooted in Dicellograptus .

Graptolite ultrastructure; evolution of descriptive and conceptual terminology

Graptolite ultrastructural studies using SEM and TEM are now more than a quarter of a century old and have reached the stage of routine procedure where material of suitable preservation is to hand. (Specimens may be chemically isolated from the rock or, in SEM and STEM studies, may be retained in the matrix.) Description of ultrastructural elements has evolved from the purely empirical to a conceptual/biological classification following (a) the recognition of the role of collagen fibrils in skeletal construction, and (b) the realization that at least two models of skeletal secretion are possible, each essentially opposed to the other. In this paper old terminology is explained with reference to current practice and the latter is tabulated and referred to the increasing number of described ultrastructural elements. Some of the outstanding problems of interpretation are outlined; and a set of concise, in some cases revised, ultrastructural definitions is given together with illustration, where appropriate, of the ultrastructural element defined.