Archaeopyle variability in Early Cretaceous dinocysts of the partiform gonyaulacoid genus Druggidium Habib

Scanning electron microscope (S.E.M.) observation techniques have enabled paratabulation formulae to be elucidated (in both Kofoidian and Taylor-Evitt notation) for the partiform gonyaulacoid dinocyst genus Druggidium Habib. Archaeopyle variability within the species of this genus is highlighted, further emphasising the uncertainties concerning the taxonomic significance of archaeopyle type. The genus Druggidium is therefore emended, as is the species D. jubatum Duxbury. One new species, D. augustum, is described.


INTRODUCTION
The Early Cretaceous dinocyst genus Druggidium was erected by Habib (1973) for three species of small proximate dinocysts present in core samples collected by the Deep Sea Drilling Project from Site 105 in the western North Atlantic. The three species described were D. apicopaucium (the type species), D. deflandrei and D. rhabdoreticulatum, ranging from the Berriasian to the Hauterivian. Subsequently, Davey (1974) illustrated a further species of this genus from the Barremian of Speeton, Yorkshire. This form was described by Duxbury (1980) as D. jubatum on examination of further material from Speeton. Lejeune-Carpentier & Siiijeant (1983) transferred two species from Phanerodinium into the genus Druggidium as D. fourmarierii and D. laeve. However, the attribution of these species is questionable for reasons that will be discussed later. A new species referable to Druggidium has recently been discovered and is described herein.
The nature of the archaeopyle in Druggidium was defined as 2P, (originally paraplates 2" and 3" were believed to be involved, now 3" and 4" have been shown to be the paraplates concernedsee Habib & Drugg, 1983, p. 634). However, studies conducted on Western European material from the Boreal Province indicate that the formation of a 2P archaeopyle, with the opercular pieces adherent to the paracingulum is only one of a range of archaeopyle types found within the genus. Evitt (1985, p. 145) lists several genera whose individual species have been found to exhibit archaeopyle variability. Mehrotra & Sarjeant (1984) reported archaeopyle variability within a single species, Dingodinium cerviculum.
Throughout this paper, Kofoidian nomenclature has been employed, Taylor-Evitt equivalents being given in square brackets [ ] where applicable.

STRATIGRAPHIC CORRELATION OF SAMPLES
Samples examined in this study came from three localities: 1. The Speeton Clay series of Speeton, E. Yorkshire (see Rawson & Mutterlose, 1984). 2. The Otto-Gott brickpit, N.E. of Sarstedt, near Hannover, West Germany (see Mutterlose, 1984 Gallois, 1972). Specimens of Druggidium have been encountered in samples from all three localities, accounting for up to 8% of the dinocyst floras. The species identified are D. jubatum, D. augustum sp. nov. and D . cf. apicopaucicum (rare). Peak abundances of this genus are found within the Blatterton facies (Bed 100 at Gott). This facies comprises black, finely laminated claystones which have been reliably correlated using cephalopods with the LB1 beds of the Speeton Clay succession (Rawson & Mutterlose, 1983). It was from the top of the LB1 beds that Duxbury (1980) originally described D. jubatum.
The correlation with the "Snettisham Clay" of the Hunstanton borehole is less well defined and is the subject of current research but preliminary work indicates that the clays between 130ft and 150ft may correlate with the Blatterton.
The present examination of species of Druggidium has shown that only a relatively low percentage of cysts of this genus exhibit an excystment structure. Of specimens of D. jubatum, 32% displayed archaeopyles, the figure being 27% for D. augustum sp. nov. The few specimens of D. cf. apicopaucicum studied yielded no information on archaeopyle type. Despite this, the relatively high abundance of the first two species has enabled a significant number of excysted specimens to be examined.
operculum free, compound biplacoid type involving 3'" 4i] and 4"[4u] see (Pl. 1, fig. 9). Of these types, the second one is by far the most commonly observed (60% of specimens). The third type (although it is the type described in the generic diagnosis) is the least common of the three (15%) and may be a modification of the second type caused by processing techniques.

SYSTEMATIC DESCRIPTIONS
Remarks. Evidence from S.E.M. observations upon species of this genus reveal the presence of distinct anterior intercalaries associated with seven precingular paraplates. Thus, the genus should be placed in the Microdinium complex of Gp-cysts rather than the (i) Type p Phanerodinium complex (Evitt, 1985, p. 215-6).
The transferals of Phanerodinium fourmarierii Lejeune-Carpentier and P. laeve Lejeune-Carpentier to Druggidium (Lejeune-Carpentier & Sarjeant, 1983) are questionable. P. laeve is a species based on one very poorly known specimen. The diagnostic apical and ventral areas are not visible in the flint matrix and the specimen displays no archaeopyle. The paratabulation formula indicates no anterior intercalaries, a feature now known to characterise the genus. It is recommended that P. laeve is retained in Phanerodinium and the species name suppressed, being used to refer only to the holotype.
The characters displayed by Phanerodinium fourmarierii do not unequivocally argue for a transferral to Druggidium. The arrangement of the apical paraplates on the paratype is clearly dissimilar to that now known for the latter genus as it possesses only one anterior intercalary paraplate and a much reduced second precingular. The ventral paratabulation of this species is also poorly known, a posterior intercalary paraplate was "presumed but could not be confirmed" (Lejeune-Carpentier & Sarjeant, 1983, p. 3). Thus P. fourmarierii should be retained in Phanerodinium until more details of its paratabulation are known. In addition to the above differences, it is of importance to note that all of the positively identified species of Druggidium are restricted to the pre-Aptian Early Cretaceous whereas P. fourmarierii and P. laeve were described from the Senonian. The present state of knowledge indicates that this genus is of excellent stratigraphic value, the individual species being of restricted range and of moderate abundance.
Druggidium augustum sp. nov. has been found in samples of latest Hauterivian to latest Early Barremian at Gott (Harding & Mutterlose, in press), Speeton and Hunstanton.  showing flagellar scar (X3500). Remarks. This species bears some resemblance to Druggidium deflandrei (Millioud) Habib. The electron micrograph of this species illustrated by Habib (1973, pl. 3, fig. 4) shows an alveolate surface sculpture very like that of D. sp. However, although the dimensions of the present specimens agree with those determined from the electron micrograph, they do not agree with the dimensions stated in the species descriptions or with those determinable from the light micrographs (the magnification of which seems to be erroneous). The present specimens compared with the description of D. deflandrei are smaller, less anteroposteriorly elongated and have a higher C.I. (cingulum index).
This species is a good marker for the latest Hauterivian to very earliest Barremian at Gott and Speeton.