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Graptolites have been collected from sections through Lower Ordovician strata in northern Iran. At the Saluk Mountains, in the Kopet–Dagh region, mudrocks yielded fragmentary tubaria ofRhabdinoporasp. cf.R. flabelliformis, indicating the presence of lower Tremadocian strata there; stratigraphically, they lie between two limestone beds with the euconodontCordylodus lindstromi. At Simeh–Kuh in the eastern Alborz Mountains (Semnan Province), upper Tremadocian – lower Floian strata include laminated dark mudstones that contain restricted graptolite faunas, mainly of small declined didymograptids; these are thought to represent incursions of plankton during periods of marine highstands. The lower major flooding surface in Simeh–Kuh coincides with an invasion of the graptolite biofacies and an incursion ofHunnegraptus? sp.; the second major flooding surface is associated with an incursion ofBaltograptus geometricus. They were most probably synchronous with those in the lower part of theHunnegraptus copiosusBiozone and at the base of theCymatograptus protobalticusBiozone in the of the Tøyen Shale Formation succession of Västergötland, Scandinavia, suggesting that observed characters of sedimentation were eustatically controlled.

The lower Palaeozoic marine succession of NE Vietnam accumulated on the South China plate. Despite historical works dating to French colonial times, the stratigraphy and palaeontology of the succession is poorly constrained. Chief amongst the lower Palaeozoic lithostratigraphical divisions is the Than Sa Formation, a c. 1200 m thick succession of clastic rocks of Cambrian and early Ordovician age. Newly collected graptolites (including Rhabdinopora? sp. and Tetragraptus approximatus) from the upper part of the formation identify strata assignable to the Tremadocian and Floian stages of the Lower Ordovician. The same succession also probably records the Cambrian–Ordovician boundary. Our analysis identifies one or more intervals of graptolite-bearing laminated mudstones in the upper part of the Than Sa Formation that may be widespread in NE Vietnam during the early Tremadocian.

A diverse fauna of dendroid graptolites and graptoloids occurs in the Tremadocian Madaoyu Formation of the Panjiazui section, near Changde, Hunan Province, South China. The dendroids are Callograptus cylindricus, C. ellipsoideus, C. isolateralis, C. salteri, C. suxianensis, C. taitzehoensis, and Dendrograptus yini; the graptoloids are Anisograptus delicatulus, Anisograptus sp., Adelograptus sp., Staurograptus cf. mojotorensis, Rhabdinopora flabelliformis anglica, R. umbellata, Rhabdinopora sp. 1, and Rhabdinopora sp. 2. All taxa are figured and the selected graptoloid taxa are described. Based on this taxonomic revision, the Tremadocian Rhabdinopora flabelliformis anglica and Adelograptus biozones are recognized in the Panjiazui section, allowing correlation with other graptolitic successions worldwide.

Three conodont fossil occurrences stratigraphically above and below a rhyolitic tuff in the Lower Ordovician Crow Formation, southeast Yukon, constrain the tuff to the early Tremadocian Rossodus manitouensis Zone and possibly the Rossodus tenuis Zone. U–Pb dates were obtained from zircon from the tuff that was imaged with cathodoluminescence and chemically abraded to reduce the likelihood of dating grains that contain older components or suffered Pb loss, respectively. Six dates from grains with oscillatory parallel zoning are equivalent, with a weighted mean 206Pb/238U date of 491.04 ± 0.13 Ma. These grains are interpreted as being primary volcanic crystals, and the date is therefore taken as the depositional age of the tuff. Four other dates from grains with sector zoning are slightly older, up to 492.0 Ma, and are interpreted as inherited or recycled from earlier volcanics. The currently defined Cambrian–Ordovician boundary is inferred to be below the tuff and separated from it by at least two conodont biostratigraphic zones that are estimated to span at least 2.25 Ma based on the top of the Rhabdinopora flabelliformis parabola graptolite zone being older than the base of the Rossodus tenuis conodont zone. We interpret the evidence in southeast Yukon to suggest that the global Cambrian–Ordovician boundary is older (>493.3 Ma) than previous estimates of ∼488 and 491 Ma based upon legacy 207Pb/206Pb zircon ages.

On the basis of studies of the graptolite sequence at the Xiaoyangqiao section in the vicinity of Dayangcha Village and at the Qinggouzi and Muxiantougou sections near Hunjiang City, southeastern Jilin, five graptolite zones may be recognized in the ‘Yeli Formation’ in ascending order: 1. Rhabdinopora parabola zone, 2. Anisograptus zone, 3. Psigraptus zone, 4. Callograptus? taizehoensis zone, and 5. Adelograptus-Clonograptus zone. The correlation of each zone with its equivalents in other areas both in China and abroad and the Cambrian-Ordovician boundary are discussed.

Based on a Backscatter Electron (BSE) examination of partially 3-dimensional graptolite specimens of Airograptus from the Tremadocian (Early Ordovician) Yehli Formation at Dayangcha, Baishan of Jilin Province, NE China, the thecal structure and astogenetic aspects of this genus (except for the missing proximal part) are described and illustrated in detail. In the present collection, no furcating but only peltate thecal apertural processes are confirmed. A comparison between the presumably benthic dendroid Airograptus and Rhabdinopora, the earliest planktonic graptolite, indicates that there are remarkable similarities in astogenetic aspects between them, although there are also differences in the development of dissepiments and thecal apertural processes. A possible ancestor-descendent relationship between the two genera is herein proposed.

Two thin volcaniclastic sandstone beds in the Bryn-llin-fawr road section in North Wales overlie an apparent sequence boundary within the uppermost Cambrian Acerocare Zone and are overlain by lowest Ordovician (lower Tremadoc) Rhabdinopora faunas. U–Pb geochronology of zircons from these sandstones yields a maximum Cambrian–Ordovician boundary age of 489±0.6 Ma. This age indicates both that the Tremadoc Series (lowest Ordovician) may be shorter in duration than was previously thought and that the duration of the Middle and Late Cambrian (c. 22 Ma) was much less than that of the Early Cambrian (c. 33 Ma). Cambrian trilobite zones locally had an average duration as short as 1 Ma.

Conical multistiped graptolites (in particular Rhabdinopora) grow in patterns which can be understood using a lateral inhibition model based on diffusion of nutrients or pheromones around the colony. Many aspects of rhabdosome morphology are explained with this model. These include branching and the distribution of branching points, the morphology of bifurcations, the almost constant spacing of stipes, the high rate of branching in early stages, the variation in expansion rate, and regeneration patterns after damage. For the first time, a full three-dimensional computer simulation of graptolite growth has been accomplished, assuming biologically justified processes instead of formal growth rules. The simulations indicate that dissepiments were instrumental in controlling rhabdosome shape, but not sufficient to keep its horizontal section perfectly circular.

The succession of earliest Ordovician planktic graptolites (graptoloids) in 11 key sequences around the world is reviewed and used to assess their distribution in depth facies, in paleolatitude and in time. An ecostratigraphic model shows the pattern of distribution of species across the shore-to-ocean profile in time. Fluctuations in this pattern are related to eustatic change; the Acerocare, Black Mountain, Ceratopyge and possibly, the Peltocare Regressive events are recognized. Surface-water temperature is found to have had little significant effect on distribution of Early Tremadoc species, and once the effects of facies is allowed for, their biostratigraphic utility can be assessed. A global suite of four chronozones is proposed for the Early Tremadoc and a further five zones comprise a provisional suite for the Late Tremadoc. The nine zones enable precise correlation of Tremadoc graptolitic sequences. The zones replace the previously used graptolite assemblages, are correlated with key conodont horizons and are, in upward sequence, the Zones of Rhabdinopora praeparabola, R. flabelliformis parabola, Anisograptus matanensis, R. f. anglica, Adelograptus, Paradelograptus antiquus, Kiaerograptus, Araneograptus murrayi-Araneograptus pulchellus, and Hunnegraptus copiosus.

In Cambro-Ordovician transitional beds of the Hassi R'mel area, northwestern Algeria, three stratigraphically successive acritarch assemblage zones can be recognized: HM/A (uppermost Cambrian), HM/B (basal Tremadoc), and HM/C (lower Tremadoc; with associated graptolites of the Rhabdinopora flabelliformis sensu lato group). Thus, the boundary between assemblages HM/A and HM/B is considered to approximate to the Cambrian-Ordovician boundary in the study area. Six morphologically distinctive acritarch species that facilitate recognition of zonal boundaries are described. One genus and four species are newly instituted: Phenacoon imperfectum gen. et sp. nov., Impluviculus araneus sp. nov., Ladogella saharica sp. nov., and Multiplicisphaeridium attenuatum sp. nov. Revised circumscriptions of two other species—Acanthodiacrodium baculatum and Baltisphaeridium verutum—incorporate morphological details not previously available. P. imperfectum and L. saharica are abundant at the top of assemblage zone HM/A; A. baculatum characterizes the base of assemblage zone HM/B; and B. verutum, I. araneus and M. attenuatum are characteristic components of the lower part of HM/C assemblage zone.

Cambrian–Ordovician boundary interval dendroid graptolites and conodonts occur in continental slope facies in the Taconic allochthon. The upper part of the Hatch Hill Formation has lowest Ordovician (lowest Tremadocian) nematophorous dendroid and lower Fauna B-aspect conodont assemblages with Cordylodus caseyi (emend.) and Iapetognathus preaengensis. Comparable dendroid-conodont faunas occur in Baltoscandia, northeastern China, and western Newfoundland; this suggests that no diachroneity can be demonstrated between the lowest occurrences of Rhabdinopora flabelliformis in a number of faunal provinces.A practical and correlatable Cambrian–Ordovician boundary stratotype horizon defined by the lowest occurrence of a Rhabdinopora flabelliformis assemblage within an interval with lower Fauna B-aspect conodonts is advocated. A biostratigraphic horizon defined by conodonts has far less utility due to strong lithofacies-conodont biofacies linkages, unresolved problems with the species-level taxonomy of cordylodans, and possible diachronous first-appearances of Cordylodus species. Because the areally most extensive Cambrian–Ordovician boundary sequences are siliciclastic-dominated (Avalonian, Baltoscandian, Gondwanan-Hercynian platforms and shale-dominated slope sequences), the lowest local occurrence of R. flabelliformis assemblages provides a practical and traditional definition for the base of the Ordovician System and Tremadocian Series in regions where conodonts are rare or recoverable only with difficulty.Strata correlative with the Cambrian–Tremadocian boundary interval are not represented across most of Laurentia. In this region, the “Sauk III Subsequence” locally has an important unconformity within it, and lower Tremadocian-equivalent rocks can unconformably overlie units as low as the Proterozoic. The earliest Ordovician featured a relatively simple eustatic history characterized by an “early Tremadocian onlap”; no compelling evidence supports a eustatic fall-rise couplet, or “Black Mountain eustatic event,” in this interval. Available stratigraphic information is reinterpreted to demonstrate an initially slow and subsequently higher rate of continued sea level rise in the Cambrian–Ordovician boundary interval.

The Karadere area to the east of Safranbolu in NW Anatolia is one of the very few localities in Turkey where the contact between the Cadomian basement and the Lower Palaeozoic transgressive succession is well exposed. The Ordovician graptolite Rhabdinopora flabelliformis (Eichwald) ssp. was found in the basal part of the Bakacak Formation, indicating an Early to early Late Tremadocian age for the beginning of the Palaeozoic transgression in the Zonguldak terrane. A few metres above this occurrence, another horizon contains Paradelograptus cf. antiquus (T. S. Hall), which mainly ranges into the Late Tremadocian. Higher up in the Ordovician succession, a new graptolite bed confirms an early Darriwilian (Dw1) age for the middle part of the Karadere Formation with the occurrence of the biozonal index Levisograptus austrodentatus (Harris & Keble) and the first record of Tetragraptus cor (Strandmark) in the area. The palaeobiogeographic distribution of these Karadere fossils is in agreement with a peri-Gondwanan affinity of the Zonguldak Terrane of the Pontides, NW Anatolia, during the Early-Middle Ordovician.

The Cambrian–Ordovician Boundary beds at the Tônismägi section in Tallinn, North Estonia, are represented by siltstones and sandstones with interlayers of clay and kerogenous argillites. In this section, three conodont zones –Cordylodus proavus, C. intermediusandC. rotundatus-angulatus– were established. Graptolites of theRhabdinopora flabelliformegroup were found in the upper part of theC. proavusZone. This section demonstrates that in suitable facies, species ofRhabdinoporamay appear earlier than usually considered.

On the basis of studies of the graptolite succession at the Xiaoyangqiao, Qinggouzi and Muxiantogou sec­tions in the vicinity of Dayangcha village and near Hunjiang City in SW Jilin Province four graptolite zo­nes can be recognized in the 'Yeli Formation': 1. Rhabdinopora parabola Zone, 2. Anisograptus Zone, 3. Psigraptus Zone and 4. Adelograptus-Clonograptus Zone. The correlation of each zone with the equiva­lent zone and the Cambrian-Ordovician boundary both in China and in other areas of the world are dis­cussed.

The earliest nematophorid (planktic) dendroid graptolites (Family Anisograptidae) are described and revised from several heterofacial facies sections of the Broom Point Formation, Cow Head Group, western Newfoundland (Broom Point North and St Paul's represent medial to upper continental slope, Martin Point and Green Point distal slope environments). Noting potential conodont recycling in the palaeoslope environments of the proximal Cow Head Group sections, graptolites are the only group of fossils that could not have been subject to post-burial transport. The graptolites belong to an informal group of quadriradiate form-species principally representing two originally related but then diverging phyletic groups: pendent Rhabdinopora Eichwald, 1855 and horizontal to sub-horizontal Staurograptus Emmons, 1855; both being stratigraphically within Assemblage Zone I (emended after Cooper, 1979) of earliest Tremadoc or early Ibexian age (top of Cordylodus intermedius to base of C. lindstromi conodont zones). Assemblage I graptolites offer a higher resolution than coeval conodonts and are a better tool for inter-facies correlations both within the Broom Point Formation and with other graptolitic sequences on a global scale. Form-species of quadriradiate Rhabdinopora rustica (such as ?R. praeparabola and R. parabola) and of Staurograptus dichotomus (e.g. ‘Radiograptus’ or ‘Heterograptus’ flexibilis or ‘Aletograptus’) are interpreted as astogenetic morphs, or as phylogenetically descendant variants of these forms. Upon evidence of late astogenetic insertion of dissepiments in Staurograptus and Anisograptus the names ‘Heterograptus’ and ‘Radiograptus’ are placed into junior synonymy, and ‘Aletograptus’ is regarded a junior synonym of Staurograptus because its post-primary dichotomies are merely delayed distally (or omitted) to be observed exclusively in very mature specimens. The Cow Head Group fauna is compared with earliest nematophorids from Oslo, Norway (Naersnes) and Dayangcha, Jilin, China (Xiaoyangqiao), and a new comprehensive ecostratigraphic zonation for early Tremadoc graptolites is proposed.

The middle Lancefieldian (late Tremadocian) dissepimentous siculate graptolites Dictyonema macgillivrayi T. S. Hall 1897 and D. pulchellum T. S. Hall 1899 possess a biradially symmetrical proximal end development which appears to be fully homologous with Clonograptus. For this reason and because of the apparent absence of bithecae and/or basal attachment structures, these species have to be removed from either Dictyonema or Rhabdinopora. The new genus Araneograptus is introduced for such dissepiment-bearing clonograptids. The mature development of fully cone-shaped rhabdosomes of A. pulchellus is contrasted with the mostly horizontal development of A. macgillivrayi. Possible relations with other late Tremadocian (or early Arenigian) ‘heavy-meshed’ species, such as ‘Dictyonema’ murrayi J. Hall, 1865 are discussed.

The biostratigraphic and depth-related ecostratigraphic significance of a newly discovered graptolite–phyllocarid faunule containing diminutive Rhabdinopora enigma and Anisograptus compactus of early to middle Tremadoc age from the Cooks Brook Formation (Humber Arm Supergroup) is described and discussed. The faunule is identical with the La1 (Lancefieldian) graptolite–phyllocarid-bearing black shale "band" of Victoria, Australia, which was until now only known from two or three outcrops in Victoria and a single bed in northern Norway. This graptolite faunule probably represents a very distal shelf-marginal or open-ocean correlative of the Cap des Rosiers (Quebec) and Green Point (Cow Head Group, Newfoundland) late early Tremadoc anisograptid graptolite biofacies realm.

Summary The fossiliferous Dictyonema Shales of Matane are represented in older collections by a rich graptolite fauna which is here fully described on the basis of material sent to Lapworth at the end of last century, and the Richardson collection and other material from McGill University, Montreal. Included in this fauna is Dictyonema canadense (figured but not described by Lapworth) and four new species of Dictyonema , together with six species and varieties of Anisograptus, Triograptus and Staurograptus, and a species of Didymograptus . From other less fossiliferous localities spread along the south shore of the St. Lawrence River come some of the above, with another new species of Dictyonema , a Bryograptus and a monotypic new genus Radiograptus , intermediate between Anisograptus and Dictyonema . The stratigraphical relations of these various localities are unknown, but the whole constitutes a fauna of Tremadoc age which is assigned either to the highest part of the Dictyonema Shales (of Scandinavia) or the lower part of the Upper Tremadoc. Structurally, all the species of Anisograptus , the Bryograptus and the Staurograptus are shown to possess stolothecae, and bithecae can be identified in most of them; the occurrence of this dendroid branch structure is also believed to characterize Triograptus and Radiograptus . Accordingly, all these genera, together with Clonograptus and Adelograpius , are separated from the Dichograptidae where they were previously placed, and united into a new family, the Anisograptidae, which is transferred to the Dendroidea. No phylogeny can be indicated with any assurance, but the whole group seems to constitute a plexus intermediate between the Dendroidea and the Graptoloidea.