Some Middle Jurassic dinocysts from the Brent Group of the northern North Sea

Selected dinocyst taxa from the Brent Group equivalent (Bathonian-Aalenian) of the northern North Sea are described. The diagnosis of Dissiliodinium (Drugg 1978) is emended. Two new species: Dissiliodinium willei and Durotrigia aspera are introduced. In addition, several new taxa are informally described and illustrated.


INTRODUCTION
This paper describes some initial results of palynological analysis of conventional core samples from the Brent Group (Aalenian-Bathonian) and lower Humber Group (Bathonian-Early Callovian). All material is from wells in blocks 9/8a and 9/9b in the UK Sector of the northern North Sea (Fig. 1). Several hundred samples have been analysed and many previously undescribed marine species were recorded. Selected dinocyst taxa are described in the following systematic section. All of these forms have stratigraphic potential.
All type and illustrated material are housed in the collection of BP Exploration Ltd, 301 St Wolfard and Van Erve 198 1. Remarks. Smelror and Aarhus (1989) emended the diagnosis of Crussolia to incorporate the morphological features of their new species C. dalei, which is a tabulate variety. This is slightly contentious, because many genera are currently separated on the basis of the presence or absence of tabulation (e.g.) Spiniferites vs. Achomosphaeru, Lagenadinium vs. Stephenelytron, Pareodinia vs. Purugonyaulacysta and Dissiliodinium vs. Durotrigia). If this precedent were to be followed, then a new genus, incorporating cavate pareodinioid dinocysts with reflected tabulation, would have to be erected.
We agree with Smelror and Aarhus (op. cit.) that Crussoliu never has a 51 archaeopyle as indicated in the original diagnosis of Wolfard and Van Erve (1 981 ). We would, however, interpret the true archaeopyle structure as variable intercalary (type 1-31) rather than type 31. Aarhus et ul., 1989(P1.3, Figs. 4,5, 7 & 8) 1989 Crussolia sp. Smelror and Aarhus, p.44, FigsSC, D. 1989 Crussoliu perireticulutu Aarhus rt ul., p.46, Figs 5a-i. Description. Proximate pareodinioid dinocyst with a pyriform outline and of intermediate to large size (60-100 micrometers). Cysts circumcavate with thicker, scabrate endophragm and thinner, smoother periphragm. The periphragm is modified into an imperfect reticulum that may be perforate or vacuolate. There are generally no indications of tabulation other than the archaeopyle. Some specimens have a larger, more regular reticulate ornament along the cingulum. Occurrence. This species has been observed in sediments of Bathonian to Middle Oxfordian age. There is some doubt as to whether the younger records are in situ or reworked, because it is rare and inconsistent in the Oxfordian and Callovian. C . per-ireticulafa has also been recorded from Callovian sediments of the Norwegian Sector of the North Sea (Aarhus e f al. 1989) and from Late Bathonian to Callovian aged sediments of Kong Karls Land and Franz Josef Land . There are no published records of this species from onshore UK.

Ci-ussolia perireticulutu
Genus DissiIiodinium Drugg 1978 emend. Type species. Dissiliodinium globulum Drugg 1978, p.68, p1.4, Emended diagnosis. Proximate subspherical gonyaulacoid dinocyst of intermediate tolarge size (SO-1 lOmicrometers).The autophragm is smooth, or more normally ornamented by nontabular features of low relief. The archaeopyle is variable precingular (type 1 P-5P), opercular pieces normally free. Apical plates and plate 6" remain attached to hypocyst. Epicystal tabulation derived from the outline of archaeopyle indicate formula; 4, Oa, 6". Faint parasutures delineating the cingulum may rarely be observed. Hypocyst lacks any indication of tabulation. Remarks. Dissiliodinium is originally described with a type 4P-SParchaeopyle. The generic diagnosis is emended to include the variable 1P-SP archaeopyle and the absence of dorsal intercalary plates on the epicyst. Comparison. Dissiliodinium differs from Durotrigia Bailey (1987)  Description. The cyst outline is often modified by one or two strong, sub-parallel folds. The autophragm is ornamented by low relief features which vary in complexity, from isolated low rnoundsorgranules, tomore elongate and branching vermiculae, which may develop into an incomplete reticulum. This variation in ornament is often seen on a single specimen. Tabulation is generally indicated by the archaeopyle only, but in some rare forms with stronger ornament, individual elements may merge to form partial cingular parasutures. The archaeopyle is formed by the loss of up to five precingular plates. Size intermediate, generally around 70 micrometers in length and 50 micrometers width. Comparison. Dissiliodinium willei sp.nov. is distinguished from other species of the genus by the style of ornament. The low undulose vermiculae and reticulate areas are coarser and distributed in a more open pattern than in D . globulum (Drugg 1978).
Occurrence. This species has been observed in sediments ranging in age from Early Bajocian to Callovian. It is often seen in flood abundance in early Middle Jurassic semi-marine sediments from the North Sea and adjacent areas. It appears however, to be a rare form in southern England (pers. obs).
Genus Durorrigia Bailey 1987  Intratabular areas exhibit a coarse ornamentation of isolated or coalescing granules and vermiculae. The para sutural ridges reflect a gonyaulacoid tabulation (4,2a, 6", 6c, 6", lp, 1 "", ?5s, fpa). The autophragm is relatively thick (1-2 micrometers) and the archaeopyle is formed by the loss of between one and five precingular plates. Excystment begins dorsally at the 3" position and progresses ventrally in both directions. . Cysts suturocavate and epicomucavate, with thicker endophragm and thinner, more delicate periphragm. Gonyaulacoid tabulation formula (4', Oa, 6", 6c, 6"", lp, l"", ?s, fpa) reflected by distinct parasutural crests (formed by the periphragm only) which are perforate and distally denticulate. The crests average 5 micrometers in height and have a tent-like structure, formed by separation of the two wall layers. Intratabular areas have a smooth to scabrate surface, with irregularly scattered granules or venucae. The periphragm also forms a small blunt apical horn, which may also be perforate. The archaeopyle is precingular, type P (3" only). Remarks. Gonyaulacysta sp. 1. differs from G . pectinigera (Gocht) Fensome 1979 in having perforate, rather than U Fig. 3 cionyuulucysta sp.  fig.4). Description. Subpentagonal gonyaulacoid dinocyst of intermediate size (60-80 micrometers). Cysts suturocavate, with wall layers appressed over intratabular areas, separating adjacent to plate boundaries and forming tent-like crests. The crests are proximally peforate to vacuolate and distally denticulate. Many of the larger vacuoles are interrupted by the line of contact between endophragm and periphragm and form broad arches along both flanks of the crest margins. Some vacuoles merge to form heart-shaped arches (see text fig.5). A short apical horn is apparently formed by both wall layers. The tabulation is gonyaulacoid, although the exact fodula is uncertain. The archaeopyle is precingular, type P (3" only). Remarks. This is a very distinctive species, but only two Explanation of Plate 2 All x800 approx .  Figs 1-6,9. Drrotrigia aspera sp. nov. 1,2. Holotype. Dorsal view, note the coarse intratabular ornament and the nature of the parasutural features (arrowed). UKCS Well 9/9b-4z, 12314' (slide l), EF E61/2; 3,6. Ventral and dorsal foci on specimen with a short apical horn. UKCS Well 9/9b-42, 12331' (slide l), EF H60/3; 4 , 5 . Ventral and dorsal views of specimen with a 2P archaeopyle. UKCS Well 9/8a-7, 12763' (slide 3). EF N60; 9. Ventral view UKCS Well 9/9b-4z, 12314 (slide l), EF E61/2.  specimens have been observed to date (both of which are damaged). It is readily identified by the unique structure of the suturocavate crests which are perforate and vacuolate. Gonyaulucystucentriconnata Riding 1983 is also suturocavate, but the periphragm is entire. Gonyuulacysta sp.2 is related to Gonyuulacysta pectinigera (Gocht 1970 Description. Small to intermediate sized proximate dinocyst with a short apical horn and an ellipsoidal outline. Gonyaulacoid tabulationexpressedby low parasuturalridges. The autophragm is smooth to finely scabrate. The archaeopyle is intercalary, but the exact type is uncertain. Size range: 45-60 micrometers. Remarks. This species is distinguished by its small size, the nature of the parasutural features and by the ellipsoidal shape (which is caused by the reduced (or even absent) apical horn). The type species, P. calloviensis has a longer horn and stronger sutural crests supported by short gonal spines, which gives this species a more polygonal outline. Paragonyaulacysta retiphragmata Dorhofer and Davies 1980 has a finely reticulate autophragm and puctate parasutural crests. Paragonyaulacysta capillosa (Brideaux and Fisher) Stover and Evitt 1978 has a strongly tripartite outline and a covering of fine short hairs. P. borealis (Brideaux and Fisher) Stover and Evitt 1978 is larger, with a thin wall, very indistinct tabulation and a much longer apical horn. This taxon is provisionally placed in Paragonyaulacysta because of uncertainty as to the number of intercalary plates.