Rhombodinium spinula (Bujak, 1979, p.313, pl.2, figs.3–9; text-fig.8E) Williams et al., 2015, p.311. Holotype: Bujak, 1979, pl.2, figs.3–5; Bujak et al., 1980, pl.15, fig.6. Originally Gochtodinium, subsequently Wetzeliella, thirdly (and now) Rhombodinium. N.I.A. Age: middle Eocene (see Aubry, 1986).

Original diagnosis: Bujak 1979, p. 313: Gochtodinium spinula
Pericyst ambitus rhomboidal. Apical and left antapical horns rounded to conical, right antapical horn strongly reduced or absent, pericingular horns tapering, rectangular or flared. Pericyst breadth equal to or greater than length. Endocyst ambitus rounded rhomboidal. Periphragm laevigate to chagrinate with mostly short (less than 3 µm), hollow, distally evexate or bulbous processes. Longer processes (up to 5µm) sometimes present on horns and paracingulum. Process distribution random or penitabular with bare pandasutural areas. Pericingulum delimited by 2 folds and sometimes by alignment of processes. Endophragm with strong granulation apically, antapically and laterally, and weaker granulation on the remaining ambital periphery and paracingulum. Paratabulation lacking or weakly indicated by process arrangement. When evident, paratabulation is peridinioid with the fourth precingular paraplate 4" broader than the second intercalary 2a. Archeopyle quadra intercalary, soleiform, resulting from the displacement of paraplate 2a, trapezoidal with side Q3 longer than side Q1. Peri-operculum and endoperculum adjacent and remaining attached to the cyst along side Q1.

Dimensions: Pericyst length 90-120µm, breadth 95-130µm. Endocyst length 75-95µm, breadth 75-100µm. Maximum process length on a specimen less than 5µm. (n = 10).

Remarks: Bujak 1979, p. 313
Several species of Rhombodinium, Wetzeliella and Wilsonidium have been described with short processes or large granules on the periphragm. Rhombodinium glabrum granulatum (Wilson) Lentin and Williams from the Upper Eocene of New Zealand has a coarsely granulate periphragm and a shape similar to that of G. spinula. The endocyst of R. glabrum granulatum is circular in outline and thick-walled whereas that of G. spinula is rhomboidal and thin.Wilsonidium ornatum (Wilson) Lentin and Williams from the Lower Eocene of New Zealand is similar to G. spinula, but the ornament of the New Zealand species consists of short, rectangular pillars, usually capped by a fairly stout fibrous membrane. This membrane is absent from G. spinula. Wilsonidium tabulatum (Wilson) Lentin and Williams from the Upper Eocene of New Zealand bears short spines, but these are restricted to parasutural rows. and the pericyst shape is strongly pentagonal with 5 well-developed horns. The 3 New Zealand species discussed above also differ from G. spinula in endoarcheopyle-periarcheopyle relationship. In G. spinula, the two archeopyles are adjacent, have the same size and shape and are attached anteriorly along side Q1. In the New Zealand species, the endoarcheopyle does not underlie the peri-archeopyle anteriorly, so that the 2 opercula are not in overall contact and do not have the same shape. Wetzeliella irtyschensis Alberti from the Lower Oligocene of northern Russia differs from G. spinula in pericyst outline. having 2 distinct antapical horns and a longer than broad pericyst. Wilsonidium lineidentatum (Deflandre and Cookson) Lentin and Williams from the Eocene of Australia, which was erected on a single specimen. is also similar to G. spinula. The Australian species differs in having pericyst spines restricted to the parasutures. The spines on G. spinula are penitabular with bare pandasutural zones, except where the spines are aligned along the anterior and posterior pericingular margins.

Distribution: Upper Eocene, southern England (this paper) .