Observations on the Jurassic dinoflagellate cyst Nannoceratopsis ambonis Drugg, 1978

The Lower to Middle Jurassic dinoflagellate cyst Nannoceratopsis ambonis Drugg, 1978 is here divided into two species, Nannoceratopsis ambonis Drugg, 1978 emend. nov. and N. dictyambonis sp. nov. on the basis of differing saggital band morphologies. The two species do not appear to intergrade morphologically and have differing stratigraphical ranges, N. ambonis emend. nov. being found from the Upper Pliensbachian (spinatum Zone) to the Upper Bajocian and N. dictyambonis sp. nov. being restricted to the uppermost Toarcian (moorei Subzone: levesquei Zone) – Lower Bajocian (laeviuscula Zone) interval.


INTRODUCTION
described three species of the Jurassic dinoflagellate cyst genus Nannoceratopsis Deflandre from material collected in Europe -N . ambonis, N . plegas and N . triceras. N . plegas is similar in overall morphology to N . gracilis Alberti, 1961 but has a more elongate dorsal antapical horn which imparts a deep, sickle-shaped concavity to the antapex. It is a rare species which seems to be confined to the Aalenian of Europe (Drugg op. cit.,p. 71) and possibly is a variety of N . gracilis.
N . triceras is a distinctive taxon because it is the only species of Nannoceratopsis known to possess three antapical horns. It is never found in large numbers, ye? is persistent from the Lower Toarcian (tenuicostatum Zone) to the Lower Bajocian (laeviuscufa Zone) in England (Riding, 1983;Woollam & Riding, 1983).
N . ambonis was erected by Drugg ( o p . cit.) to encompass forms similar in outline and overall morphology to N . gracilis, but differing in having smooth to very finely granulate autophragm and prominently thickened saggital bands, although the original description included forms with both solid and reticulate saggital areas (Drugg o p . cit.,p. 70,pl. 6 ,. During investigations into the stratigraphical distributions of dinoflagellate cysts from the Jurassic of Britain, the author has observed many specimens of N . ambonis and it has become clear that the species, as described by Drugg, includes two distinct forms; those with thickened solid saggital bands, and forms with distinctly reticulate thickened saggital areas. In the populations o f N . ambonis (sensu lato) studied, these two forms do not appear to intergrade morphologically with each other and, in Europe, have differing stratigraphical ranges. The morphotypes with solid unbroken saggital bands are found in strata of Upper Pliensbachian (spinaturn Zone) to Upper Bajocian age, whereas those with reticulate saggital bands have a more restricted range, occurring from the Upper Toarcian (moorei Subzone ; levesquei Zone) to the Lower Bajocian (laevicuscula Zone). The latter morphotype is an especially good marker for the uppermost Toarcian, the Aalenian and the Lower Bajocian in England. When it is found near the Lower/ Middle Jurassic boundary with other dinoflagellate cyst taxa of restricted occurrence such as Scriniocussis weberi Gocht, 1964 (which becomes extinct at the top of the Aalenian Stage) or species of Parvocysta Bjaerke, 1980 (which appear to die out at the top of the Aalenian opalinurn Zone), reasonably fine biostratigraphical resolution may be achieved.

1983, pl. 2 , figs. 4 and 5.
horn is normally the longest. The autophragm is smooth with the exception of the saggital area. The saggital band is prominently thickened and is reticulate, usually possessing a single row (rarely two) of lacunae of variable length. Archaeopyle cingular (Pie1 & Evitt, 1980) Remarks. Members of the N . gracilis Alberti,19hllN. senex van Helden, 1977 complex often have a reticulate autophragm, but this reticulation is present over the entire cyst surface, not confined to the saggital area as in N . dictyambonis sp. nov. The lacunae present on the autophragm of N. gracilislsenex are also much smaller in diameter than those in the thickened saggital area of N . dictyambonis sp. nov. This species differs from N. ambonis emend. nov. by having a reticulate saggital band. The overwhelming majority of specimens referable to N . dictyambonis sp. nov. have a single row of lacunae on the saggital band, specimens with two parallel rows (e.g. PI. 1, fig. 5) represent an extremely rare variety of this species. The saggital parasuture is presumed to be coincident with the thin tract between the two rows of lacunae in these forms (see Fig. 1C). Conversely the position of the saggital parasuture in forms of N. dictyambonis sp. nov. with a single row of lacunae is not clear. Stratigraphical range. Upper Toarcian (moorei Subzone; levesquei Zone) to Lower Bajocian (laeviuscula Zone).

DISCUSSION
Piel & Evitt, 1980, in a superbly illustrated account, figure S E M photomicrographs of N . gracilis and N . senex which clearly show, for the first time, the complete paratabulation pattern of this genus. They also illustrate transversely striate saggital bands with a centrally positioned saggital parasuture. Piel & Evitt (op. cit.) interpreted the transverse striations on the saggital band as cyst-equivalents of megacytic zones (areas of thecal growth) in living dinoflagellates. The functional significance of the thickened saggital bands (= megacytic zones) of Nannoceratopsis ambonis emend. nov. and N. dictyambonis sp. nov. is not clear; possibly they imparted added mechanical strength to the theca, and thus to the cyst.
Nannoceratopsis ambonis emend. nov. and N . dictyambonis sp. nov. are distinguished by differing saggital band morphologies. The two species do not appear to intergrade morphologically and have different stratigraphical ranges in Europe.
The distinctive morphology and restricted stratigraphical range of N . dictyambonis sp. nov. makes it an excellent marker within the uppermost Toarcian to Lower Bajocian interval in Europe.