Two new Early Cretaceous dinocyst species from the Central North Sea Basin

Two new species of dinocyst, Cerbia monilis and Hapsocysta susanae are described from the Lower Cretaceous of the Central North Sea Basin. The first ranges across the Aptian/Albian boundary and the latter is restricted to the Early to Middle Albian interval; both are valuable index taxa in this area. Hapsocysta susanae is remarkably similar to cysts ‘without walls’ described from the Late Oligocene and Early Miocene, and detailed comparisons are made. The ranges of the two species described here are illustrated against regional lithostratigraphic and biostratigraphic schemes.


INTRODUCTION
The Lower Cretaceous sections of the United Kingdom Sector of the Central North Sea Basin are generally shaley, but with coeval turbidites that can be allocated to specific depositional sequences (Jeremiah, 2000); palynofloras are typically very rich and very diverse throughout. A recent, detailed palynological analysis of sections in many Central North Sea wells has allowed the development of a detailed and robust palynofloral subdivision of the entire Lower Cretaceous (Duxbury, 2001).
Analysis of Late Aptian and Albian sections in the Moray Firth area, quadrants 13, 14 and 20 ( Fig. 1), has allowed the recognition of two further age-significant dinocyst species, in addition to those described in Duxbury (2001). These are Cerbia monilis Duxbury, n. sp. and Hapsocysta susanae Duxbury n. sp., and the present work is concerned with the morphological description and discussion of these taxa.
The upper part of the scheme defined in Duxbury (2001) is included in Figure 2, together with some lithostratigraphic and current new species data. Lithostratigraphy is essentially that defined by Johnson & Lott (1993), although the ages of some of the units have been modified (Fig. 2), based on these observations. These modifications include the restriction of the Captain Sandstone Member to the latest Aptian and earliest Albian, younger than the LK30 (nutfieldensis) marine flooding event (Jeremiah, 2000;Duxbury, 2001) and the placing of the Rødby/ Carrack formational boundary at a mid-Early Albian level. Johnson & Lott (1993) placed the base of the Captain Sandstone Member towards the base of the Early Aptian, and they had the Rødby/Carrack formational boundary at the top of the Early Albian.
Both new species are valuable stratigraphic indices, since they have very restricted ranges, and each can be common. The availability of core and sidewall core material in the study area has been valuable, particularly in resolving the range bases of these taxa.
Cerbia monilis is restricted to the Aptian/Albian boundary interval, zones LKP31 and LKP32 of Duxbury (2001), jacobi to tardefurcata ammonite zones. In the Moray Firth, its inception is within the Wick Sandstone Formation, Captain Sandstone Member and Upper Britannia Sandstone Formation, and its extinction is within the Carrack Formation (Johnson & Lott, 1993).
Hapsocysta susanae is restricted to the Early to Middle Albian, palyzones LKP33.1 to LKP34 of Duxbury (2001), tardefurcata to loricatus ammonite zones. Its inception is within the upper Carrack Formation, and its extinction is within the lower Rødby Formation (Johnson & Lott, 1993).

SYSTEMATIC DESCRIPTIONS
The following section includes brief descriptions of two new species, Cerbia monilis n. sp. and Hapsocysta susanae n. sp.; these taxa are treated alphabetically.
Figured specimens of these taxa, including holotypes and paratypes, are held at the Natural History Museum, Cromwell Road, London SW7 5BD. Natural History Museum reference numbers (F.D. numbers) are included for figured specimens in plate and figure captions.
It therefore appears that the genus Tenua Eisenack, 1958 is essentially as defined by that author, as 'Thin-walled, oval shells without fields or tabulation, which are comparatively thickly set with short, solid, rod-like spines' (translation by Norris & Sarjeant, 1965), although this was felt actively misleading by Sarjeant (1985). It may be added that there is only rarely some penitabular alignment of spines, and that the mid-dorsal and mid-ventral areas are essentially devoid of ornament. Otherwise, Tenua is dissimilar to the clearly paratabulate genus Cerbia.
The genus Cerbia Below, 1981 has therefore been retained here, as in Duxbury (2001).

Diagnosis.
A thin-walled, proximate dinoflagellate cyst with an angular outline. A pronounced, rounded apical horn and low lateral and antapical projections are present. Short spines are aligned very largely along epicystal and hypocystal parasutures, although occasionally these also may occur along paracingular margins. These spines are variable in their morphology, often distally bifurcate and sometimes laterally joined at their distal extremes. Spines at the apex anastomose into a fibrous network. Paratabulation appears to be typical for the genus. The archaeopyle is apical, with a zig-zag margin; the operculum is almost always detached. Remarks. Cerbia monilis n. sp. differs from all other species of this genus in its very distinct cyst morphology, particularly an angularity of outline caused by the presence of apical, lateral and antapical projections, and the characteristic cyst folding. The latter feature is almost exclusively restricted to parasutural areas, so that parasutural spines typically run along the folds.
Folding imparts an essentially triangular shape to the hypocyst, causes some constriction of the archaeopyle and emphasizes the height of the dorsal epicyst. In addition, although some dorso-ventral flattening is observed, it appears to be significantly less pronounced than in other species of this genus.
The operculum is almost always detached, and only one specimen with an attached (but displaced) operculum was observed in the study wells (paratype, Pl.1, fig. 12).
This very distinctive species is very short-ranging, restricted to the Aptian/Albian boundary interval, zones LKP31 and LKP32 of Duxbury (2001) (jacobi to tardefurcata ammonite zones), where it can be common. In the study area, its inception is within the Wick Sandstone Formation, Captain Sandstone Member and Upper Britannia Sandstone Formation, and its extinction is within the Carrack Formation.

Diagnosis. A spheroidal dinocyst species which consists mainly
of an open parasutural network. Individual parasutures are V-shaped in cross-section, composed of a thickened, linear base and two smooth, ribbon-like flanges. A gonyaulacacean paratabulation of the form acl., aa, 4#, 6##, as, 6c, pras, ras, las, 6###, rs, ls, ps, 1p, 1#### is outlined by trifurcation of the parasutures. The width of individual parasutures varies between 0.8 µm and 2.8 µm, with the more slender ones bordering the smaller paraplates. A short apical projection, composed entirely of parasutural material, is observed in lateral orientation, and a small, very thin-walled, wrinkled endocyst is observed in the minority of specimens.
Paratype. Plate 1, fig. 4; fig. 4.  Remarks. This species has been placed in Hapsocysta Davey, 1979 because it is essentially an open meshwork, composed of narrow strands, outlining the paratabulation and surrounding an endocyst. Unfortunately, the paratabulation of the genus Hapsocysta has not been formally reported, and no direct comparisons may be made. However, Davey (1979) described, 'large, basically pentagonal, pre-and postcingular, and antapical areas, elongate paracingular areas and small, subcircular parasulcal areas'. An endocyst is observed only in the minority of specimens of Hapsocysta susanae, and no clear attachment to the parasutural framework has been discerned. The absence of an endocyst from the majority of specimens may be due to its easy detachment and small size; it is presumably easily detached and typically lost through the wide mesh of the encircling framework.

Locality
Close comparisons may be made between H. susanae and the type species, Hapsocysta peridictya (Eisenack & Cookson, 1960) Davey, 1979, which occurs at the same stratigraphic level in the study area (although the last species has a significantly longer range). Specimens of H. peridictya (Eisenack & Cookson, 1960) Davey, 1979 illustrated and described by Morgan (1980) are particularly reminiscent of H. susanae, including specimens which, 'lack an endophragm'.
H. susanae differs from H. peridictya (Eisenack & Cookson, 1960) Davey, 1979 in having a more rigid parasutural framework with broader individual elements, in its significantly smaller overall size and in the smaller size of the endocyst relative to the parasutural network. The last feature may account for the characteristic loss of the endocyst in H. susanae.
In addition, H. susanae has an apical projection, a feature absent from H. peridictya (Eisenack & Cookson, 1960) Davey, 1979, and no specimens of H. susanae have yet displayed, 'a continuous ectophragm joining the trabeculae', as described for some specimens of H. peridictya by Morgan (1980).
The structure of H. susanae is very reminiscent of the genera Evittosphaerula Manum, 1979 and Chaenosphaerula Damassa, 1997. These are monotypic, morphologically very similar, and the type species, Evittosphaerula paratabulata Manum, 1979 and Chaenosphaerula magnifica Damassa, 1997 were described from the Lower Miocene and Upper Oligocene respectively. However, H. susanae differs from both of these in possessing an apical projection and an endocyst (although this is usually lost).
The structure of H. susanae also bears some resemblance to the genus Cannosphaeropsis Wetzel, 1933, although the  trabecular network of the latter genus bears short, distally bi-or trifurcate spines (see Duxbury, 1980;Marheinecke, 1992), a feature absent from H. susanae.
The stratigraphic range of H. susanae is short, restricted to the Early to Middle Albian, palyzones LKP33.1 to LKP34 of Duxbury (2001), where it can be common. In the study area, its inception is within the upper Carrack Formation, and its extinction is within the lower Rødby Formation.