Rotosphaeropsis thula (Davey 1982) comb. nov., emend.: a dinoflagellate cyst from the Upper Jurassic-Lower Cretaceous of England

The Upper Jurassic-Lower Cretaceous dinoflagellate cyst Cannosphaeropsis thula Davey 1982 does not conform to the morphological parameters of the genus Cannosphaeropsis O. Wetzel 1933 emend. Duxbury 1980. The species also exhibits no indications of belonging to the Spiniferites complex of Evitt (1985). The species is transferred to Rotosphaeropsis Davey 1987 by virtue of its shape, archaeopyle type, processes and trabeculae and the specific diagnosis is emended in the light of this reassessment.


INTRODUCTION
During the course of separate palynological investigations into the Upper Jurassic and Lower Cretaceous strata of eastern England, both authors encountered large numbers of the dinoflagellate cyst Cannosphaeropsis thula Davey 1982. This species was previously placed in open nomenclature, as Cannosphaeropsis sp. A Davey 1979, and is of stratigraphical significance, ranging from the Upper Kimmeridgian (sensu anglico) to the Upper Ryazanian in north-west Europe (Davey, 1979;Duxbury, 1977Duxbury, , 1978Riding, 1984).
The morphology of this taxon does not accord with that of the genus Cannosphaeropsis 0. Wetzel 1933 emend. Duxbury 1980, principally because the paratabulation of C. thula differs markedly from that of Spiniferites Mantell 1850 emend. Sarjeant 1970 and also it lacks processes of Spiniferites-type. Cannosphaeropsis is a member of the Spiniferites complex within the informal category of Gs-cysts of Evitt (1985). The Spiniferites complex contains genera with widely differing morphologies, yet a distinctive series of constant features which strongly suggest a close biological relationship. The most important of these features are: trifurcate gonal and bifurcate intergonal processes (the primary furcations pointing to the centres of adjacent paraplates); a strongly sigmoidal (S-type of Evitt, 1985) parasulcus lacking intraparasulcal parasutures and normally, a single paraplate precingular archaeopyle (see Evitt, 1985 fig. 5.8B). Cannosphaeropsis thula Davey 1982, while sharing certain morphological features with Cannosphaeropsis, has a longitudinal parasulcus (Ltype of Evitt, 1985 -see Figure 1 and Plate I, fig. 1) and lacks Spiniferites-type processes. The purpose of this short communication is to transfer C. thula to Rotosphaeropsis Davey 1987, which is consistent with the morphology of this species, and to make certain minor emendations to the specific diagnosis.  width to the processes, but may broaden markedly and bear distally low, irregularly-spaced spines or denticles. Paratabulation sexiform gonyaulacacean, indicated by trabeculae, Kofoidian formula: 4', 6 , ?6c, 6"', lp,l"", 5s. Paracingular and parasulcal trabeculae are rarely discernible and the former may be partially suppressed. Parasulcus longitudinal (L-type of Evitt, 1985 Remarks. This species has been transferred to Rotosphaeropsis Davey 1987 as it exhibits distal trabeculae surmounting irregularly-spaced parasutural processes, a precingular archaeopyle and lacks Spiniferites-type processes. The basionym genus, Cannosphaeropsis, although superficially similar in terms of basic cyst organisation, shape and archaeopyle type, is inappropriate as it differs markedly in crucial details of paratabulation and morphology. R. thula comb. nov., emend. nov. has a longitudinal parasulcus; Cannosphaeropsis, by virtue of its position within the Spiniferites complex, has a sigmoidal ventral area. Certain topological paratabulation differences associated with L-type and S-type parasulci therefore exist between R. thula and Cannosphaeropsis,' for example the former has a subparallel sided 6 paraplate whereas the equivalent in the latter is pseudotriangular (see Evitt, 1985 fig. 5.8B). In addition, R. thula exhibits intraparasulcal trabeculae (Pl. 1, fig. l), features not associated with Spiniferites and its allies. Rotosphaeropsis thula lacks the characteristic (of Cannosphaeropsis) triradiate branches at gonal points and the paired branches located intergonally on the trabeculae (see Plate 1, fig.  10). Cannosphaeropsis exhibits a much reduced number of gonal processes and entirely lacks intergonal shafts, a situation differing markedly with the full complement of gonal processes together with the numerous intergonals of R. thula. The distally flared processes and thickened trabeculae of R. thula are unknown in Cannosphaeropsis.

SYSTEMATIC SECTION
The processes of R. thula are very variable in length. The paratabulation is readily discernible in forms with low parasutural processes although morphotypes with longer shafts tend to be distorted making the paratabulation difficult to interpret. Davey (1982) commented that the number of processes is greater in specimens from the Upper Kimmeridgian to lowermost Portlandian.
Rotosphaeropsis thula differs from R. munu, the type species, in that the cyst body of the former is less elongate; R. munu has considerably more numerous processes, which are consistently longer than those of R. thula. MediaUdistal cross linkage of processes, in addition to the trabeculae, are characteristic of R. munu but are not exhibited by R. thula. Because of the relatively long and complex processes of R. munu, the trabeculae are invariably distorted and the detailed paratabulation cannot be established.
Stratigraphical Distribution and Geographical Occurrence. The total range of R. thula onshore is Upper Kimmeridgian (sensu anglico) , Pectinatites hudlestoni Zone to Upper Ryazanian, Surites stenomphalus Zone; it is most common (up to 5% of the dinoflagellate cyst assemblage) from the Portlandian, Galbanites kerberus Zone to the Lower Ryazanian, H. kochi Zone. R. thula has been reported from England and Denmark and occurs extensively in the North Sea Basin (personal observations).