Dinoflagellate Cyst Genus Research Articles

The Neogene record of the dinoflagellate cyst genus Trinovantedinium Reid 1977 in the tropical Americas

The genus Trinovantedinium Reid 1977 comprises eleven NeogeneQuaternary and one Paleogene dinoflagellate cyst taxa that have been successfully used for biostratigraphy. Combining new and published records of Trinovantedinium species in the Neogene of the tropical Americas, we report preliminary stratigraphic ranges for T. applanatum (AquitanianRecent), T. ferugnomatum (BurdigalianZanclean), T. glorianum (BurdigalianMessinian), T. papula (AquitanianZanclean), T. uitpensis (AquitanianTortonian), T. variabile (AquitanianRecent) and T.? xylochoporum (AquitanianMessinian) in the region. Although the stratigraphic ranges of T. applanatum, T. variabile and T.? xylochoporum in the tropical Americas seem to be coetaneous with previous records elsewhere, the first appearance datums of T. ferugnomatum, T. glorianum and T. papula seem to be at least ∼2–3 Myr. older in the tropical Americas. In contrast, only the last appearance datum of T. glorianum seems to be ∼4 Myr. younger in the Northern Hemisphere. Our results demonstrate that the Cenozoic marine palynological record of the tropical Americas is therefore key to discriminate dinoflagellate cyst stratigraphic ranges across latitudes.

A review of the Jurassic dinoflagellate cyst genus Gonyaulacysta Deflandre 1964 emend. nov.

The Middle–Late Jurassic gonyaulacacean dinoflagellate cyst genus Gonyaulacysta is characterised by an epicyst which is around twice the length of the hypocyst. The sulcus is L-type and may be slightly sigmoidal in shape, but is never S-type sensu stricto. The forms with slightly sigmoidal L-type sulcuses may have developed into species with S-type sulcuses. Gonyaulacysta jurassica, the holotype of which is the nomenclatural type of the genus, had a global distribution. A total of 151 species have been assigned to Gonyaulacysta although 126 of these have been transferred to other genera. Prior to this contribution, 25 species were accepted in the current Lentin and Williams Index. This is herein reduced to eight and 18 species are transferred to other genera. The accepted species are: Gonyaulacysta adecta stat. nov., emend. nov.; Gonyaulacysta australica comb. nov., emend. nov.; Gonyaulacysta ceratophora; Gonyaulacysta desmos stat. nov., emend. nov.; Gonyaulacysta dualis emend. nov.; Gonyaulacysta fenestrata emend. nov.; Gonyaulacysta jurassica emend. nov.; and Gonyaulacysta longicornis stat. nov., emend. nov. These species form a closely related plexus, and are distinguished on differences in cavation style, form of sutural crests/ridges and size of the apical horn. All except Gonyaulacysta australica are reliable marker taxa. Our main taxonomic proposals involve the elevation of all subspecies and varieties of species here retained in Gonyaulacysta to species rank, or their synonymisation. This avoids cumbersome infraspecific names. Gonyaulacysta exhibits substantial provincialism; for example Gonyaulacysta dualis is confined to the Oxfordian–Kimmeridgian of the Boreal Realm. Gonyaulacysta adecta, Gonyaulacysta desmos and Gonyaulacysta longicornis are present in the Bathonian to Oxfordian of Laurasia and surrounding areas. Three species, Gonyaulacysta australica, Gonyaulacysta ceratophora and Gonyaulacysta fenestrata, are restricted to the Oxfordian to Tithonian of Australasia. Gonyaulacysta adecta and the cosmopolitan Gonyaulacysta jurassica both exhibit overall size increases throughout the Bathonian to Kimmeridgian of Europe.

Revision of the endemic dinoflagellate cyst genus Pontiadinium Stover & Evitt, 1978 from Lake Pannon and the Paratethys realm (Late Miocene–Early Pliocene, Central Europe)

The biota of the brackish-water Lake Pannon in the Pannonian Basin is characterised by remarkable endemism due to the isolated evolution of the lake for 8 myr after the last Miocene marine connection ceased (∼11.6 Ma). A conspicuous feature of this endemism is the large, probably ecophenotypic variation in the morphology of brackish-water dinoflagellate cysts that challenges taxonomy and complicates biostratigraphical and ecological interpretations. We conclude that a widely debated Lake Pannon genus, Pontiadinium, includes several proximate dinoflagellate cyst species with prominent apical and antapical protuberances, and we show how the genus differs from the morphologically similar gonyaulacacean cyst genera Impagidinium, Leptodinium, Cribroperidinium and Komewuia. The generic description of Pontiadinium is emended together with the species descriptions of Pontiadinium inequicornutum, Pontiadinium obesum and Pontiadinium pecsvaradanesis. A new species is described as Pontiadinium szentaiae sp. nov. from Našice (northern Croatia) that is characterised by unique trabeculate sutural septa formed from a beaded tegillum supported by columellae or rod-like luxuriae. The dinoflagellate cyst assemblages of the long-lived brackish-water Lake Pannon clearly demonstrate that dinoflagellate cysts in low-salinity, isolated epicontinental seas display greater morphological plasticity than their normal-marine relatives. The development of an antapical horn appears to be a previously undocumented example of phenotypic morphological features that developed in response to subnormal salinities within at least two dinoflagellate cyst genera endemic to Lake Pannon and the Post-Paratethyan seas of the Ponto-Caspian realm. This ecophenotypic variation resulted in a higher level of morphological adaptation, leading to the evolutionary development of new dinoflagellate cyst species and genera.

Hiddenocysta matsuokae gen. et sp. nov. from the Holocene of Vancouver Island, British Columbia, Canada

A new dinoflagellate cyst genus and species are described here as Hiddenocysta gen. nov. and Hiddenocysta matsuokae sp. nov. from Holocene sediments in a core from the west coast of Vancouver Island (British Columbia, Canada). The genus Hiddenocysta encompasses spherical to ovoid skolochorate cysts, characterized by a gonyaulacoid plate pattern and a 2P precingular archeopyle. The species H. matsuokae is characterized by a granular wall and slender trifurcate processes with heavily perforated process bases. Two end members are described here based on process morphology and number of processes (formas 1 and 2). Cyst wall chemistry is analyzed using micro-Fourier transform infrared (FTIR) spectroscopy and reveals a unique dinosporin composition consistent with a gonyaulacoid autotrophic feeding strategy.

New Paleocene species and biostratigraphic relevance of the genus Spiniferites across the North Sea and Norwegian Sea

Industrial palynological studies of the Paleocene of North Sea and Norwegian Sea basins over several decades have led to the consistent recognition of six distinctive species of organic-walled dinoflagellate cyst genus Spiniferites. These species, however, have not previously been formally described, a situation that we remedy herein. Hence, we propose the new species Spiniferites fentonii, Spiniferites rhomboideus, Spiniferites truncatus, Spiniferites sabulus, Spiniferites kirschii and Spiniferites dolichoidus. These new species have distinctive morphological features that separate them, from other species, both of Spiniferites and other genera. Some of the new species are very useful stratigraphic markers across north-western Europe. The material described is from the early Thanetian Lista Formation and a late Danian interval including the basal Maureen Formation and parts of the Våle Formation. The new species have not been observed in the Selandian (between the Danian and Thanetian), and Spiniferites diversity during that stage seems to have been lower in general across the study area; this was probably related to regional environmental changes. The stratigraphic range of the new species is used in combination with other known markers to increase stratigraphic resolution across the region.

A review of the Sentusidinium complex of dinoflagellate cysts

The Jurassic to Neogene (Miocene) dinoflagellate cyst genus Sentusidinium has a relatively simple overall morphology. This genus, together with Batiacasphaera, Kallosphaeridium and Pentafidia, comprises the Sentusidinium complex. This is distinct from the superficially similar laterally asymmetrical and subspheroidal/lenticular Cyclonephelium complex. The genus Sentusidinium is an acavate, subcircular, proximate to proximochorate, sexiform gonyaulacacean genus with an apical archaeopyle and typically low relief ornamentation. Since the erection of Sentusidinium in 1978, three similar genera have been established, which we consider to be taxonomic junior synonyms of that genus: Barbatacysta, Escharisphaeridia and Pilosidinium. However, we deem the Early Cretaceous to Miocene genera Batiacasphaera, Kallosphaeridium and Pentafidia are deemed to be separate from Sentusidinium. We refine the definition of the Early Cretaceous to Miocene genus Batiacasphaera to circumscribe cysts with a reticulate to rugulate autophragm and an apical archaeopyle with a free operculum. By contrast, Kallosphaeridium has a ventrally attached apical archaeopyle with five plates that can be interpreted as type (4A1I)@ or type (5A)@; it also has a small operculum relative to the overall cyst diameter. The six accepted Kallosphaeridium species are confined to the Palaeogene. The Australian genus Pentafidia is unusual in appearing to only have five precingular plates; this comprises two species from the Jurassic–Cretaceous transition of Western Australia. Therefore, we emend Sentusidinium to restrict it to acavate, proximate or proximochorate dinoflagellate cysts with an autophragm devoid of, or covered with, highly variable, non-linear ornamentation and a type (tA) apical archaeopyle. Occasionally the elements of ornamentation may be connected, but rarely is a cingulum indicated, and the tabulation is never clearly evident. A kalyptra may be occasionally present. The operculum is free. Following a comprehensive literature review, we accept 17 species in Batiacasphaera. In Kallosphaeridium we accept six species confidently and consider six species to be problematical. We list 38 (34 accepted and four problematical) species of Sentusidinium. Kallosphaeridium? helbyi is here transferred to Cyclonephelium without question. The species Batiacasphaera angularis is occasionally tabulate and hence we transfer it, with question, to Meiourgonyaulax. The Sentusidinium complex is clearly polyphyletic, and all genera considered herein belong to the order Gonyaulacales. Batiacasphaera, Kallosphaeridium and Pentafidia cannot be confidently assigned to a family, whereas Sentusidinium belongs to the Gonyaulacaceae. The number of species within the complex has been reduced from 137 to 68; furthermore, all infraspecific taxa have been eliminated.

Marine palynostratigraphy and new species from the Cape Roberts drill-holes, Victoria land basin, Antarctica

Three research wells were cored near Cape Roberts, McMurdo Sound, during the summers of 1997, 1998 and 1999. Rich marine palynomorph assemblages were obtained from the early Miocene and Oligocene sediments, and almost immediately described at a reconnaissance level by the original on-site science team. In these studies, a number of undescribed species were reported, most strikingly of the distinctive dinoflagellate cyst genus Lejeunecysta. This report continues that work with the description of six new species: ?Cymatiosphaera robertsii, Lejeunecysta acuminata, Lejeunecysta attenuata, Lejeunecysta katatonos, Lejeunecysta rotunda, and Lejeunecysta striata. A new zonation comprising five marine palynomorph biozones is proposed, largely based upon the new taxa. The taxon ranges are derived from earlier published ranges with some minor updates.

Batiacasphaera bergenensis and Lavradosphaera elongata — New dinoflagellate cyst and acritarch species from the Miocene of the Iceland Sea (ODP Hole 907A)

A detailed palynological investigation of the almost continuous middle through upper Miocene sediment sequence of ODP Hole 907A in the Iceland Sea revealed the presence of a new species of the dinoflagellate cyst genus Batiacasphaera, and a new species of the acritarch genus Lavradosphaera. Batiacasphaera bergenensis sp. nov. and Lavradosphaera elongata sp. nov. are both morphologically distinctive and have well-defined stratigraphic range tops that are independently constrained by the pristine paleomagnetic record of Hole 907A. Both species disappeared within a narrow interval across the middle to late Miocene boundary, when small-scale glaciations on Greenland were large enough to reach sea level. The distinct morphology of the proposed species and their highest occurrence in this critical interval highlights their potential for future biostratigraphic application in the Miocene of the high northern latitudes, an area important for understanding the Late Cenozoic transition into a bipolar glaciated world.

The Late Cretaceous dinoflagellate cyst Manumiella — Biostratigraphy, systematics, and palaeoecological signals in Antarctica

This study of the Maastrichtian (latest Cretaceous, 71–65 Ma) species of Manumiella Bujak and Davies 1983 in the James Ross Basin, Antarctic Peninsula is focussed on the biostratigraphical and palaeoecological significance of this peridinioid dinoflagellate cyst genus, in particular with reference to oceanic changes associated with the Cretaceous/Tertiary (KT) boundary. New palynological analyses throughout the upper part of the latest Maastrichtian López de Bertodano Formation on Seymour Island has resulted in the emendation of Manumiella to include the presence of a mesophragm, and the formal description of Manumiella bertodano sp. nov. A morphological continuum exists between species of Manumiella, but the apparent biostratigraphical significance of the end-member morphologies emphasises the need for consistent and distinct species concepts. Consequently, a new botanical key has been devised based on the morphological features of Manumiella bertodano sp. nov., Manumiella conorata [Stover, L.E., 1973. Palaeocene and Eocene species of Deflandrea (Dinophyceae) in Victorian coastal and offshore basins, Australia. In: Glover, J.E., Playford, G. (Eds.), Mesozoic and Cainozoic Palynology: Essays in Honour of Isabel Cookson. Geological Society of Australia, Special Publication vol. 4, 167–188.] Bujak and Davies 1983, Manumiella druggii [Stover, L.E., 1973. Palaeocene and Eocene species of Deflandrea (Dinophyceae) in Victorian coastal and offshore basins, Australia. In: Glover, J.E., Playford, G. (Eds.), Mesozoic and Cainozoic Palynology: Essays in Honour of Isabel Cookson. Geological Society of Australia, Special Publication vol. 4, 167–188.] Bujak and Davies 1983, Manumiella seelandica (Lange, D., 1969. Mikroplankton aus dem Fischton von Stevns-Klint auf Seeland. Beiträge zur Meereskunde, 24–25, 110–121.) Bujak and Davies emend. Firth 1987 and Manumiella seymourensis Askin 1999. Manumiella druggii and Manumiella seelandica are retained as separate species. The key has been successfully tested on the well-preserved dinoflagellate cyst assemblages from the uppermost López de Bertodano Formation on Seymour Island, resulting in a clear biostratigraphy based on an existing preliminary scheme. Manumiella seymourensis is prominent in the lowermost part of the succession, and the uppermost part of the range of this species overlaps with that of Manumiella bertodano sp. nov. The latter species ranges into the latest Maastrichtian, where it co-occurs with Manumiella conorata and Manumiella seelandica. Manumiella druggii is confined to the uppermost part of the succession examined. Abundance spikes of Manumiella immediately prior to the KT boundary in this succession have also been recognised globally, and may be related to short-term regressions and/or ocean cooling before the KT event.

Arcticacysta: A new organic-walled dinoflagellate cyst genus from the early Miocene? of the central Arctic Ocean

The Cenozoic sedimentary record of the central Arctic Ocean recovered during the Arctic Coring Expedition (ACEX) Integrated Ocean Drilling Program (IODP) Expedition 302 contains a new dinoflagellate cyst genus, Arcticacysta, assignable to the Peridiniales. Two different species are here formally described, Arcticacysta backmanii and A. moraniae gen. et sp. nov. Both species have a combination archeopyle, formed by the loss of several plates. In A. backmanii the archeopyle involves three apical (2’-4’) and three intercalary (1a-3a) plates. In A. moraniae the archeopyle is formed by the loss of four apical (1’-4’) and three intercalary (1a-3a) plates. The species differ in size and in the wall structure. A. backmanii has a microreticulate wall and is relatively large, whereas A. moraniae is smaller and with spinose wall. Their lowest occurrence in the ACEX core lies at the base of subunit 1/5 (core M0002A 46X, 113-114 cm, 198.70 meters composite depth, mcd). Morphologically closely related taxa, potentially re/presenting other species of Arcticacysta, have been previously recovered from the late Oligocene and early Miocene sediments of the Norwegian-Greenland Seas.

Occurrence of dinoflagellate cyst genera Wanaea and Komewuia in Upper Jurassic strata, coastal Tanzania

Species of the dinoflagellate cyst genera Komewuia and Wanaea are well represented in surface and subsurface, Upper Jurassic strata of coastal Tanzania. One new species of Wanaea, W. tendaguruensis, is described from the Trigonia smeei Bed of the Tendaguru Formation in the Mandawa Basin; it appears to be a useful index of the Kimmeridgian. Wanaea clathrata, W. spectabilis, and W. talea are rare to common constituents of the Oxfordian strata of the middle part of the Bagamoyo Formation in the Ruvu Basin. Komewuia glabra, characteristic of the Kimmeridgian-Tithonian in the southern hemisphere, is recorded from the offshore Kipatimu Formation and from the Trigonia smeei Bed. The Kipatimu Formation was previously dated as Early Cretaceous, but is now considered Late Jurassic due to its content of K. glabra. The informally designated Komewuia sp. A occurs sporadically in the Nerinea Bed (Oxfordian-Kimmeridgian) of the Tendaguru Formation. The species documented herein have short stratigraphic ranges in the Upper Jurassic of coastal Tanzania and are thus potentially useful for regional biozonation and correlation.

New insights on the Paleogene dinoflagellate cyst genera Enneadocysta and Licracysta gen. nov. based on material from offshore eastern Canada and southern Argentina

Enneadocysta is a biostratigraphically important Paleogene dinoflagellate cyst genus. Its original interpretation as a partiform gonyaulacoid (thus belonging to the gonyaulacalean suborder Cladopyxiineae) was based on the presence of two antapical processes. Another Paleogene genus, Areosphaeridium , is similar, at least superficially, to Enneadocysta , but has a single antapical process, placing it in the sexiform gonyaulacalean suborder Gonyaulacineae. The morphology of a new species of Enneadocysta from offshore eastern Canada, Enneadocysta magna , shows that the two antapical processes of the genus (unlike processes reflecting other plate series) are penitabular, not mesotabular, and indicates that the genus is gonyaulacinean, not cladopyxiinean. This new interpretation, as well as new material from southern Argentina, confirms the generic assignment of the Southern Hemisphere species Enneadocysta dictyostila (emended) and a new species, Enneadocysta brevistila . A new genus and species, Licracysta corymbus , from offshore eastern Canada is intermediate in morphology between Enneadocysta and Cleistosphaeridium and strongly suggests an assignment for all three genera to the gonyaulacinean family Areoligeraceae. The new combination Licracysta ? semicirculata is questionably proposed, and the descriptive terms licrate, dolabrate, entire clypeate, ragged clypeate, intratabular, mesotabular, obtabular, contabular and nontabular are either proposed as new or reviewed.

Palynodinium minus sp. nov., a new dinoflagellate cyst from the Cretaceous–Paleogene transition in New Zealand; its significance and palaeoecology

This is the first record of a representative of the organic-walled dinoflagellate cyst genus Palynodinium from the New Zealand region. Previously, Palynodinium grallator has been reported from two sites in offshore Tasmania during Ocean Drilling Program, Leg 189. The new species Palynodinium minus is found consistently in latest Maastrichtian? and earliest Danian strata (latest Haumurian–earliest Teurian) in two sections located along the mid-Waipara and Grey rivers, North Canterbury, South Island. It occurs in a narrow stratigraphic interval at the Cretaceous–Paleogene transition and is potentially a stratigraphically important index species in the region. Its range coincides with that of the global earliest Danian index fossil Carpatella cornuta and a basal Danian acme interval of Trithyrodinium evittii, and is correlated to ca. 65.5–65.2 Ma. Its highest occurrence is at the base of an acme interval of Palaeoperidinium pyrophorum. It has only been recorded from sections composed of siliciclastic shelf sediments, suggesting that it might have had a palaeoecological preference for relatively near-shore environments.

Belowicysta nom. nov., a new name for the Jurassic dinoflagellate cyst Belowia Riding & Helby, 2001

The Jurassic dinoflagellate cyst genus Belowia Riding & Helby, 2001 is a junior homonym of Belowia Moquin-Tandon, 1849, a genus of the Chenopodiaceae. The new generic name Belowicysta is proposed here to replace Belowia Riding & Helby, 2001.

Fostericysta Riding nom. nov. (Division Dinoflagellata)

AbstractThe Jurassic dinoflagellate cyst genus Fosteria Riding & Helby 2001 is a later (junior) homonym of Fosteria Molseed 1968, a genus of the Iridaceae. The generic name Fostericysta is proposed here to replace Fosteria Riding & Helby 2001.

NEW DINOFLAGELLATE CYST AND INCERTAE SEDIS TAXA FROM THE PLIOCENE OF NORTHERN BELGIUM, SOUTHERN NORTH SEA BASIN

New dinoflagellate cyst taxa have been recovered from Pliocene deposits in the Antwerp area of Belgium, where a detailed analysis of the Kattendijk and Lillo formations has yielded a diverse dinoflagellate record for the southern margin of the North Sea Basin. Four new species and two new genera of dinoflagellate cysts are recognized: the gonyaulacaceans Spiniferites coniconcavus n. sp. and Pyxidinopsis braboi n. sp., the goniodomacean Desotodinium wrennii n. gen. and sp., and the protoperidiniacean Scaldecysta doelensis n. gen. and sp. The protoperidiniacean genus Barssidinium Lentin, Fensome, and Williams, 1994 has been emended from new observations on the tabulation of Barssidinium pliocenicum (Head, 1993) Head, 1994a emend. The excellent preservation of the Belgian material accounts for these new observations. Barssidinium wrennii Lentin, Fensome, and Williams, 1994 is considered a taxonomic junior synonym of Barssidinium pliocenicum (Head, 1993) Head, 1994a, which thus becomes the correct name for the type of the genus. The new marine incertae sedis palynomorph Waaslandia geminifera n. gen. and sp. is also proposed.

A reappraisal of the dinoflagellate cyst genus Montanarocysta

A reappraisal of the dinoflagellate cyst genus Montanarocysta

A reappraisal of the dinoflagellate cyst genus Montanarocysta

The dinocyst genus Montanarocysta Corradini is emended, based on re-examination of the holotype, the paratype and well-preserved specimens of the type species, Montanarocysta aemiliana Corradini. This re-examination shows that M. aemiliana has a sexiform gonyaulacacean tabulation incompletely delineated by processes and septa, a large untabulated ventral area, an apical archeopyle type (tA)a with well-developed precingular accessory sutures, and a simple adnate operculum attached to the sulcal as and precingular plates 1ʺ and 6ʺ. Maghrebinia chleuh Below and Maghrebinia mirabilis (Below) Masure are here transferred to Montanarocysta, since they have closely similar morphology to M. aemiliana. Previously Atopodinium Drugg has been considered as taxonomic senior synonym of Maghrebinia Below and Bejuia Stover Williams, and the species in these two genera have been transferred to Atopodinium. From the present analyses, Atopodinium is separated from Montanarocysta on the ornamentation: Atopodinium has a more subdued ornamentation, consisting of ridges, folds, grana and gemmae; whereas Montanarocysta is characterised by processes and septa. Maghrebinia is herein retained as separate genus, which has a ceratocoryacean tabulation and contains the single species Maghrebinia perforata Below.

Cold marine indicators of the late Quaternary: the new dinoflagellate cyst genus Islandinium and related morphotypes

AbstractRound, brown, spiny dinoflagellate cysts characterise many modern high‐latitude assemblages. Abundance is often highest where summer sea‐surface temperatures seldom exceed 7 °C and where winter sea‐surface temperatures are around 0 °C, making this morphological group important for reconstructing cold intervals within marine Quaternary sequences. Our analysis of modern sediments from the Beaufort Sea of Arctic Canada, the Kara and Laptev seas of Arctic Russia, and across the Arctic Ocean, allows us to recognise the new cyst genus Islandinium along with the extant species Islandinium minutum (Harland and Reid in Harland et al., 1980) new combination (basionym: Multispinula? minuta), Islandinium? cezare (de Vernal et al., 1989 ex de Vernal in Rochon et al., 1999) new status and combination (basionym: Multispinula? minuta var. cezare) sensu lato, and Echinidinium karaense new species. Of these, the generotype Islandinium minutum is a well‐known but morphologically problematic species. We have re‐examined the type material from the Beaufort Sea and studied specimens from across the Arctic, and our observations clarify ambiguities in the original description of this species. The archeopyle of Islandinium minutum results from the loss of three apical plates, an unusual style among peridiniphycidean dinoflagellates. The asymmetrical location of these plates around the apex is distinctive, and probably contributed to earlier misunderstandings of the archeopyle. Previous attributions to Multispinula? and Algidasphaeridium? are unsustainable. Maps showing the distribution of Islandinium minutum are given for the Northern Hemisphere and show this species to be polar to north‐temperate, whereas Islandinium? cezare s.l. and Echinidinium karaense appear to be more restricted to polar environments. Copyright © 2001 John Wiley & Sons, Ltd.

GEONETTIA WALTONENSIS,A NEW GONIODOMACEAN DINOFLAGELLATE FROM THE PLIOCENE OF THE NORTH ATLANTIC REGION, AND ITS EVOLUTIONARY IMPLICATIONS

Abstract A new species of the unusual dinoflagellate cyst genus Geonettia de Verteuil and Norris, 1996a is here described from the Pliocene of the western North Atlantic and eastern England. Geonettia waltonensis new species is only the second species to be formally described for this genus, whose type, G. clineae de Verteuil and Norris, 1996a, has a range of Miocene through Pliocene. Geonettia is a gonyaulacalean, goniodomacean genus of the subfamily Pyrodinioideae and is closely related to Eocladopyxis Morgenroth, 1966 and Capisocysta Warny and Wrenn, 1997, also found in the Cenozoic. However, Geonettia is the only known dinoflagellate cyst genus to have plates that dissociate extensively on both epi- and hypocyst during excystment. Geonettia waltonensis has this style of excystment, but its hypocystal tabulation is more akin to Capisocysta lata Head, 1998a than to G. clineae. Comparison of tabulation and other morphological features suggests that during the late Miocene, Capisocysta lata evolved from G...