The first chapter of this special issue introduces the proceedings of two workshops concerning the cystbased genus Spiniferites Mantell 1850. The historical background of the cyst-based genus Spiniferites, its closely related genera and the theca-based genus Gonyaulax Diesing 1866 is presented here.

We present a summary of two round-table discussions held during two subsequent workshops in Montreal (Canada) on 16 April 2014 and Ostend (Belgium) on 8 July 2015. Five species of the genus Achomosphaera Evitt 1963 and 33 of the genus Spiniferites Mantell 1850 emend. Sarjeant 1970 occuring in Pliocene to modern sediments are listed and briefly described along with remarks made by workshop participants. In addition, several holotypes and topotypes are reillustrated. Three species previously assigned to Spiniferites are here considered/accepted as belonging to other genera: Impagidinium inaequalis (Wall and Dale in Wall et al. 1973) Londeix et al. 2009, Spiniferites? rubinus (Rossignol 1962 ex Rossignol 1964) Sarjeant 1970, and Thalassiphora balcanica Balteş 1971. This summary forms the basis for a set of papers that follows, where points raised during the workshops are explored in greater detail.

In this article, we are proposing an Identification Key for recognition of Quaternary Spiniferites species and some morphologically close Quaternary taxa of some related genera. We summarize the morphological features of 43 taxa (including three subspecies and one variety) based on the original description of the holotypes and sometimes supplemented by our observations. In addition to the Identification Key, we refer to published illustrations that feature both typical and atypical specimens for each taxon.

The compilation of this key gave us the opportunity to reconsider some taxonomic concepts, which resulted in two new combinations and an emendation: Hafniasphaera granulata (Mao 1989) comb. nov., emend. and Hafniasphaera multisphaera (Price and Pospelova 2014) comb. nov. In addition, we recommend that the names Spiniferites nodosus and Spiniferites pseudofurcatus subsp. obliquus be restricted to their holotype.

The use of dinoflagellate cyst assemblages as a tool for palaeo-environmental reconstructions strongly relies on the robustness of cyst identification and existing information on the distribution of the different species. To this purpose, we propose a functional key for the identification of Pliocene and Quaternary Spiniferites bearing intergonal processes and depict the range of morphological variation of the different species on the basis of new observations from estuarine and coastal regions. Accordingly, the description of Spiniferites mirabilis is emended to include the new subspecies Spiniferites mirabilis subsp. serratus. We also report the occasional presence of intergonal processes in Spiniferites bentorii and Spiniferites belerius. This key aims to facilitate identification of this group of Spiniferites bearing intergonal processes and standardize cyst identification among researchers.

Here we present new observations of Spiniferites alaskensis, a relatively rare species described from the Eemian of the Gulf of Alaska. We show that the species shows a gonyaulacacean tabulation: Po, 4′, 6′, 6c,? s, 6′, 1p, 1′. The surface is finely granulate to scabrate. The species bears characteristic processes: these are exclusively gonal, membranous, perforated and end distally in platforms with stumpy ends. We provide more detail as how this species compares to closely related species belonging to the genus Spiniferites.

The genus Spiniferites currently encompasses 142 dinoflagellate cyst species. Some Spiniferites species are difficult to identify because of an incomplete or doubtful description, and/or substandard iconography. This study re-describes and re-illustrates the Spiniferites holotypes first described by Reid in 1974. It also discusses topotype material from surface sediments recovered from British estuaries, and attempts to provide further constraints on the classification of species in this genus using the geochemical characterization of their cyst walls. Reid described four new Spiniferites species: Spiniferites belerius, Spiniferites delicatus, Spiniferites elongatus and Spiniferites lazus. New photomicrographs are presented here for the holotypes of Spiniferites delicatus and Spiniferites elongatus, and additional morphological observations based on newly processed topotype material are given. The geochemical characterization of the Spiniferites cyst walls showed overall consistency with a carbohydrate-based dinosporin. However, variability in the dinosporins suggests that, in this genus, the cyst wall composition may be species-specific. Analysis of the characteristic spectral regions for unclassified Spiniferites species showed that, in some cases, it may be possible to constrain the likely species affinity using the cyst wall chemistry. However, in most cases, the morphologically unspeciated cysts did not show sufficient similarity to an identified species' cyst wall chemistry to be more conclusive. This could either reflect an intermediate species that cannot be clearly characterized using morphology or dinosporin composition, or it represents a completely different species. In either case, both the morphological and geochemical evaluations highlight the difficulties in classifying species of this genus unequivocally.

We restudied the morphological complex comprising the cyst-based species Spiniferites elongatus/ Spiniferites frigidus/Rottnestia amphicavata. We reviewed existing studies, and acquired new morphometric measurements of recent cysts from across the Northern Hemisphere, scanning electron microscopy (SEM) observations of cysts from Barents Sea surface sediments, and genetic analyses of cysts from the Beaufort Sea. The measurements suggest that populations and morphospecies cannot be distinguished based on morphometric criteria. Furthermore, sequential sediment trap samples from Hudson Bay reveal that morphological variation can occur at the same location over a few weeks, arguing against a uniform morphological response to environmental parameters. The SEM observations reveal a consistent gonyaulacacean tabulation (Po, 4′, 6″, 6c, 5s, *6′″, 1p, 1″″). The small and large subunit ribosomal RNA genes and the internal transcribed spacer sequences obtained from differing cysts from the Beaufort Sea with morphologies attributable to the complex, including forms that correspond to Rottnestia amphicavata, were all identical and conform to those of Gonyaulax elongata from the Orkney Islands. The molecular analyses thus support the conclusion that the morphological variability is not reflected genetically and occurs within one species. Based on arguments against the generic attribution of amphicavata to Rottnestia, the continuum between the extreme ends of the morphological range, and the molecular data, we suggest Rottnestia amphicavata to be conspecific with Spiniferites frigidus, and both to be junior synonyms of Spiniferites elongatus. The morphometric data further indicate that Spiniferites ellipsoideus, an elongate cyst from the Miocene, can also be considered a junior synonym of Spiniferites elongatus. It is recommended to use two informal types in census work (i.e. Spiniferites elongatus – Beaufort morphotype for morphologies formerly assignable to Spiniferites frigidus/Rottnestia amphicavata, and Spiniferites elongatus – Norwegian morphotype for cysts with strongly reduced processes) to separate specimens at both extreme ends of the morphological spectrum from typical specimens of Spiniferites elongatus.

Miocene to modern sediments of the Ponto-Caspian basins and Mediterranean Sea are uniquely distinguished by presence of gonyaulacacean cysts with ellipsoid to cruciform endocysts and highly variable ectocystal features, including pterate (wing-like) and galeate (helmet-like) outer wall layers. The term cruciform is defined to indicate cysts with concave epicyst and hypocyst surfaces and moderate dorsoventral flattening. These features may be morphological responses to salinity stress and some biometrical studies conclude that the cruciform Ponto-Caspian species Spiniferites cruciformis of Wall and Dale 1973 and Pterocysta cruciformis Rochon et al. 2003 are morphotypes of the ellipsoidal-subpentagonal Miocene-Holocene species Galeacysta etrusca Corradini and Biffi 1988. We show that the holotypes of these cruciform and rhomboid-subpentagonal galeate taxa differ in endocyst shape, ectocyst structure and attachment points, process morphology, and sulcal plate expression, and we present new data on morphological variations, modern distribution and ecology. We list multiple criteria for distinguishing these taxa from Paratethyan dinoflagellates with shared features, including Thalassiphora spp., Lophocysta, Romanodinium, and Seriliodinium. Log transforms of endocyst:ectocyst (EN:EC) dimensions cannot fully capture cruciformness or galeate and pterate wall characteristics that distinguish the genera, and at DSDP Site 380, EN:EC values for Pleistocene populations of Spiniferites cruciformis and Galeacysta etrusca are significantly different. Re-examination of the history of studies on the Galeacysta etrusca complex and comparison with new studies of Pleistocene to recent cysts leads to the conclusion there is insufficient evidence to justify combining the cruciform species with Galeacysta etrusca and we provide criteria for distinguishing among the main components of the complex. Using multiple morphological features, it appears there is a replacement of large Palaeogene marine-brackish water camocavate-circumcavate taxa with elliptical endocysts first by the Miocene rhombo-subpentagonal galeate species Galeacysta etrusca and then by the Pliocene – Holocene semi-marine–brackish cruciform species Spiniferites cruciformis and stenohaline Pterocysta cruciformis.

Despite the fact that dinoflagellate cysts of the diverse genus Spiniferites are abundant in coastal and estuarine sediments worldwide, little is known about patterns of their seasonal or annual production. In this paper we review previously published data on Spiniferites cyst fluxes from eight sediment trap time series in estuarine (the Strait of Georgia, Saanich Inlet, Hudson Bay, Omura Bay), coastal (the Santa Barbara Basin, the Arabian Sea), and offshore (off Cape Blanc) environments. This is the first study that provides detailed inter-site comparison of dinoflagellate cysts in sediment traps and analyzes seasonal, annual, and inter-annual cyst production from different geographic regions. We identified that cyst fluxes of all Spiniferites species at a given location increased or decreased simultaneously in all studied sediment trap records. This indicates that different Spiniferites species react in a similar way to local environmental triggers at each site. Average daily total cyst fluxes recorded in the sediment trap time series and in the dated surface sediment samples are greater in coastal and estuarine waters where marine primary productivity is higher. This implies that nutrient availability might be an important factor stimulating Spiniferites production. There is no uniform seasonal pattern in Spiniferites fluxes, but the timing of elevated total Spiniferites fluxes coincided with intervals of local seasonal environmental change at each site. Analyses of all sediment traps revealed that intervals with the highest total Spiniferites fluxes correspond to the timing and intensity of local environmental change at the sea-surface when waters had: minimal turbidity, some water column stability or stratification, availability of nutrients, and sea-ice free conditions. The multi-year trap data record considerable inter-annual variability in Spiniferites fluxes and seasonality when environmental conditions between the years varied. A combination of factors and specific environmental conditions are required to enhance Spiniferites cyst production in each region.

In marine sediments of late Cenozoic age, Spiniferites is a very common genus of dinoflagellate cysts (dinocysts). Despite some taxonomical ambiguities due to large range of morphological variations within given species and convergent morphologies between different species, the establishment of an operational taxonomy permitted to develop a standardized modern database of dinocysts for the midhigh latitudes of the Northern Hemisphere. In the database that includes 1490 surface sediment samples, Spiniferites mirabilis-hyperacanthus, Spiniferites ramosus and Spiniferites elongatus were counted in addition to Spiniferites belerius, Spiniferites bentorii, Spiniferites bulloideus, Spiniferites delicatus, Spiniferites lazus and Spiniferites membranaceus. Among these taxa, Spiniferites mirabilis-hyperacanthus, Spiniferites ramosus, and Spiniferites elongatus are easy to identify and are particularly common. Spiniferites bentorii and Spiniferites delicatus also are morphologically distinct and occur in relatively high percentages in many samples. Spiniferites lazus and Spiniferites membranaceus also bear distinctive features, but occur only in a few samples. The identification of other taxa (Spiniferites belerius, Spiniferites bulloideus, notably) may be equivocal and their reported distribution has to be used with caution. The spatial distribution of Spiniferites species, with emphasis on the five most common taxa, is documented here with reference to hydrography (salinity and temperature in winter and summer, sea ice cover), primary productivity and geographical setting (bathymetry, distance to the coastline). The results demonstrate distinct ecological affinities for Spiniferites elongatus, which has an Arctic-subarctic distribution and appears abundant in low productivity environments characterized by winter sea ice and large temperature contrast between winter and summer. Spiniferites mirabilis-hyperacanthus, which occurs in warm temperate water sites, is more abundant in high salinity environments. It shares its environmental domain with Spiniferites bentorii, which appears to have a narrower distribution towards the warm and high salinity end of the Spiniferites mirabilis-hyperacanthus distribution. In contrast, Spiniferites delicatus, which occurs in warm-temperate to tropical environments, shows preference for relatively low salinity and low seasonal contrasts of temperature. Spiniferites ramosus exhibits a particularly wide distribution that overlaps both cold and warm Spiniferites taxa. Its cosmopolitan occurrence and its long-ranging biostratigraphical distribution suggest a high plasticity of the species and/or cooccurrence of several cryptic species. Hence, whereas Spiniferites elongatus and Spiniferites mirabilishyperacanthus are useful palaeoecological indicators despite their large morphological variability, Spiniferites ramosus is a taxon with an unconstrained ecological significance.

A new quantitative biostratigraphical method, based on an ‘index of stratigraphical abundance’ (ISA), has been used to refine the stratigraphical range of 50 dinoflagellate cyst taxa. Most of these correspond to Quaternary representatives of Achomosphaera or Spiniferites. Some extinct taxa are also discussed when their morphology is close to that of recent species (e.g. Spiniferites lenzii, S. twistringiensis and S. pseudofurcatus), as well as some species possibly related to the motile Gonyaulax spinifera complex. The ISAs of S. mirabilis and S. hyperacanthus are similar, as well as for S. elongatus and S. sphaericum. Although initially presumed as indicating morphotypes of a same taxon, it seems rather that the ISAs similarites must be interpreted as an indication of close ecological preferences. The emergence of forms of Spiniferites with low to absent septa appears relatively late (compared to the range of the genus), but the first occurrence of intergonal processes is relatively early, indicating that it is probably a character deeply rooted into the generic genome. The previously calculated phylogeny of some Gonyaulax motile cells shows a different tree pattern than the stratigraphical first appearance succession of their presumed cyst equivalents.

Dinoflagellates encompass two taxonomic systems (dual taxonomy) reflected by separate traditions of nomenclature: one based mainly on living motile stages, and the other mainly on fossil cysts (dual nomenclature). Modern cysts may therefore bear two names if their life cycle is known. There have been attempts to rationalize this duality, but at species and genus level this has been largely unfruitful. New and continuing developments call for a renewed evaluation of this duality: (1) the elucidation of multiple new cyst–motile stage relationships creating overlaps between cyst-based and motile-based systems, and (2) the advent of DNA sequence-based phylogenies, revealing evolutionary patterns (underlying the phenotypic differences) that disagree with trees obtained from the study of fossil cysts. We examine the background of dual nomenclature and discuss the implications of new advances in molecular phylogeny for dual taxonomy as well as briefly review earlier attempts to unite cyst/fossil and motile-/living-based nomenclatures. From this basis, we explore routes for bringing the separate taxonomic systems closer together. Our rationale for doing this lies in the challenges facing communication between the biologists and geologists who work on these different life cycle stages. These challenges encompass taxonomic issues, nomenclature, evolutionary interpretations, and the nature of what we perceive as a species. We use the motile/cyst pair Gonyaulax/Spiniferites as our example, as these, and related genera, provide a useful model for illustrating the difficulties in bridging the gap between biology and palaeontology because they are numerous, with regard to both species and specimens, and are ubiquitous in both time and space.