New dinocyst taxa from the Eocene of the North Sea

Dinoflagellate cysts (dinocysts) are abundant in the Eocene of the North Sea and provide a high-resolution biostratigraphic zonation. Twelve species are erected to accommodate zonal markers that have not been previously described. These are Areosphaeridium ebdonii, Areosphaeridium michoudii, Cerebrocysta magna, Diphyes brevispinum, Diphyes pseudoficusoides, Hystrichosphaeropsis costae, Hystrichostrogylon clausenii, Membranilarnacia compressa, Phthanoperidinium clithridium, Phthanoperidinium distinctum, Phthanoperidinium powellii and Phthanoperidinium regalis.


INTRODUCTION
Bujak  defined eight dinocyst zones and 23 subzones for the Eocene of the North Sea based on last occurrence or abundance events (Fig. l), which were observed in more than 150 wells examined from Quadrant 3 in the north to Quadrants 21 and 22 in the south. Many of the zonal markers also occur in northwest European stratotypes and reference sections, permitting correlation with standard nannoplankton (NP) zonal assignments made for these sections, of significance because few Eocene NP index species occur in North Sea wells.
Mudge  defined five North Sea Eocene stratigraphic sequences and eight subsequences bounded by high gamma log peaks, based on the examination of over 750 released wells from Quadrant 3 to Quadrants 21 and 22. They demonstrated that each sequence and subsequence boundary consistently falls within the same dinocyst subzone, giving a high degree of confidence that these boundaries represent time lines.
Twelve dinocyst taxa used to define the zones and subzones erected by  have not been previously described, although some have been informally illustrated from northwest Germany, Belgium and Denmark. The purpose of this paper is to formally describe these taxa in order to provide the taxonomic framework needed for the North Sea Eocene dinocyst scheme proposed by . hypocyst; areas between process bases smooth, or indistinctly ornamented, or finely and irregularly granulate. Processes less than 1 5 p m in length. Process stems solid, with proximal fibrous ends expanded slightly and merging smoothly with the autophragm surface. Process stems expanded distally into vasiform or infundibular, fenestrate to reticulate process platforms with irregular to circular outlines. On some specimens the vasiform or infundibular distal terminations have irregular or entire margins. The processes reflect a paratabulation of 4', 6", Oc, 5"', lp, l"", Os, with the hypocystal paratabulation following the standard sexiform pattern. Paracingulum typically expressed by the absence of processes, but one or two narrow paracingular processes may be present. Parasulcal processes absent although a medial parasutural notch is frequently discernible. Archeopyle apical; operculum tetraplacoid, contiguous. Type locality and stratum. Locality: North Sea, UK22/14-1 well. Stratum: section at 8020 ft, Early Eocene (Ypresian Diagnosis. Cysts skolochorate with subspherical bodies comprising an autophragm. Central body diameter typically greater than 50pm. Body typically with 17 intratabular processes, ten on the epicyst and seven on the hypocyst; areas between process bases smooth, or indistinctly ornamented, or finely and irregularly granulate. Process stems solid, with proximal fibrous ends expanded slightly and merging smoothly with the autophragm surface. Process stems expanded distally, and forming recurved or straight distal bifurcations. The distal margin of the bifurcations are digitate or bear completely or partially developed fenestrations which are restricted to a band lying along the bifurcations. The processes reflect a paratabulation of 4', 6 , Oc, 5"', lp, l"", Os, with the hypocystal paratabulation following the standard sexiform pattern. Paracingulum typically expressed by the absence of processes. Parasulcal processes absent although a medial parasutural notch is frequently discernible. Archeopyle apical; op rc 1-um tetraplacoid, contiguous.

Remarks.
A. rnichoudii is distinguished from all other species of Areosphaeridiurn by its distally bifid processes which lack fenestrate or reticulate process platforms. The process terminations on A . michoudii are similar to those on several species of Enneadocysta, including E. arcuata (Eaton) Stover & Williams and E. rnulticorni~ta (Eaton) Stover & Williams which are common in the North Sea. A . michoudii differs from all species of Enneadocysta in having a sexiform hypocystal paratabulation reflected by a processes on paraplates 5"', l p and 1"". Species of Enneadocysta have a partiform hypocystal partabulation reflected by processes on paraplates 6 , 2 and ps.
(Plate 2, figs 10-11) Derivation of name. Latin, magnus, large, with reference to the large cyst size. Diagnosis. Autocyst spherical to ovoidal. without apical, antapical or other projections, except for an apparently random ornament of low crests which give the cyst a cerebral appearance. The crests occasionally suggest a paratabular arrangement. Cyst diameter always exceeding 40 pni. Archeopyle formed by the loss of precingular paraplate 3".  Bujak (1980) for specimens present in the Barton Beds of the Hampshire Basin, with the type species, C. bartonensis Bujak, having a cyst diameter ranging from 24-28 p m (Plate 2, fig. 12). C. bartonensis is persistently present in fairly low numbers in the North Sea where it has its highest occurrence in Bartonian Subzone E7a of . C. magna is identical to C. barfonensis in all respects except for its larger size, and there is no indication that the two species intergrade. C. rnagna has a more restricted stratigraphic range in the North Sea, having its highest occurrence at the top of Lutetian Subzone E4c of .
(Plate 2, figs 4-6)  (Cookson) Loeblich & Loeblich. D. nudurn does not possess processes except for the large antapical process, whereas D. ficusoides has processes that typically vary in length between 11-15pm. D. brevispinurn has a more restricted North Sea stratigraphic range than D. ficusoides which ranges up into Lutetian Subzone E4d of . D. nudurn is rare in North Sea wells (Bujak, pers. obs.), and has approximately the same stratigraphic range as D. breuispinunz.

Derivation of name.
Greek, pseudos, false, plus ficusoides, with reference to the similarity to, but distinction from Diphyes ficusoides.
Diagnosis. Cyst subspherical, skolochorate and biphragmal, endophragm and periphragm adpressed except beneath the processes. Periphragm smooth to chagrinate, forming numerous nontabular processes. Processes greater than 10 p m in length; processes other than the antapical process tubular, tapering from broad circular bases to narrow necks before ending in expanded, distally aculeate or entire terminations. Antapical process expanded, having a maximum breadth of between 1 0 p m and 1 5 p m , and tapering distally to a constricted termination that is typically open. Archeopyle apical; operculum tetraplacoid, contiguous. parasutural crests, and in having processes that are absent or restricted to the paracingular region and which are much shorter than those on R. borussica (typical process length on R. borussica is greater than 10pm). H. costae differes from Hystrichosphaeropsis? rectangularis Bujak in having less well-developed parasutural crests.
H. costae is considered to be conspecific with Hystrichosphaeropsis sp. 1, which Heilmann-Clausen & Costa (1989) described from the Wursterheide research well, northwest Germany, within the Glinde Formation, assigned to NP14 in their table 1. This informal species was also described from both the German Wursterheide research well and the Hinge section in Denmark as Hystrichosphaeropsis sp. by Heilmann-Clausen (1993). Heilmann-Clausen documented a morphological series from R. borussica to Hystrichosphaeropsis sp. and suggested that Hystrichosphaeropsis sp. was descended from R. borussica.
Subzone E4d of . D. ficusoides and D. pseudoficusoides have their first North Sea occurrences in the early Ypresian, whereas D. colligerurn ranges down in the Palaeocene in the North Sea Basin.

Explanation of Plate 3
Magnification bar represents 20 p m for all specimens . Figs 1-4: Hystrichosphaeropsis costae sp. nov. Figs 1-3. Holotype. UK22/14-1, 8020 ft; fig. 1, lower surface; fig. 2, optical section; fig. 3, upper surface. Fig. 4. Paratype. UK22/14-1, 8020 ft. Remarks. P. clithridium is distinguished from all other species of Phthanoperidiniurn except P. regalis by the formation of an archeopyle involving the loss of paraplates 2a and 4". The mode of archeopyle formation is identical in P. clithridium and P. regalis which differ in their ornament. P. clithridiurn does not possess the penitabular ornament present on P. regalis and has less well-defined parasutural ornament. Rare specimens intergrade between these taxa, but the majority of specimens comprise distinct morphological populations, which have different palaeogeographic distributions in the North Sea (Bujak, pers. obs.). P, clithridiurn has not been recorded outside the North Sea region.
The mode of archeopyle formation is identical in P. regalis and P. clithridiurn which differ in their ornament. P. clithridiurn does not possess the penitabular ornament present on P. regalis and has less well-defined parasutural crests. Rare specimens intergrade between these taxa, but the majority of specimens assigned to the two taxa comprise populations with distinct palaeogeographic distributions in the North Sea (Bujak, pers. obs.). P. regalis has not been recorded outside the North Sea region.