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Trithyrodinium maculatum
Figures 7.14, 7.15, 7.18, 7.19
Derivation of name. From maculate, meaning spotted or stained, with reference to the distinctive surface markings.
Designation of holotype. Figure 7.18. Sample MR3. Slide MR3(A). Coordinates S32/3. Location, Melton Ross Quarry, Lincolnshire, UK. Stratum, Bed F, Flixton Member, Welton Chalk Formation. The specimen is curated in the MPK collection of the British Geological Survey, Keyworth, Nottingham, UK, specimen number MPK 14662.
Diagnosis. A species of Trithyrodinium possessing an ornament of ring-shaped indentations on the endophragm and periphragm.
Description. Small to intermediate-sized, spheroidal to ovoidal-shaped, circumcavate, peridinioid dinoflagellate cysts. The endophragm (c. 1 μm thick) and periphragm (c. 0.5 μm thick) are smooth to finely-granular, both possessing regular ornament elements that are circular to sub-circular, ring-shaped indentations (c. 3–6 μm diameter), which are at least half the thickness of the surrounding wall areas. Cavation is c. 2–3 μm wide at the lateral margins and up to 10 μm in the antapical region. An archaeopyle involving three anterior intercalary plates, type 3I(1–3a), is formed in the endocyst, with operculum plates attached or detached. The 2a intercalary plate is isodeltaform, hexa-type (Fig. 7.19; cf. text-figure 6 in Bujak and Davies 1983). The periarchaeopyle type has not been confirmed. There is no evidence of tabulation, other than that indicated by the archaeopyle.
Dimensions (length × width of the endocyst). Holotype, 39.6 × 42.5 μm. Other specimens (5 measured): length 38.0 (50.0) 66.9 μm, width 39.5 (45.6) 55.3 μm.
Discussion. Only six specimens of this taxon have been recorded to date. However, its distinctive morphology makes it easy to identify and warrants the erection of a formal species. The endocyst archaeopyle type is consistent with the genus Trithyrodinium but an outer wall layer ( periphragm) has only been observed on two specimens, including the holotype. The circular indentations on the cyst walls are ringshaped, as opposed to the whole area within the circle being thinned. The ring shapes are reminiscent of those attributed to impressions made by coccoliths on the surface of specimens of an alga (Campenia sp.) by Prauss (2012b, figs 10A, 11H). However, the T. maculatum specimens occur in assemblages of abundant dinoflagellate cysts, with other taxa not exhibiting comparable markings. The markings are a key identifying feature but could be a preservation artefact. This type of surface ornament has not previously been reported on other species of P-cysts, including those belonging to the genus Trithyrodinium. In the G-cyst species Apteodinium maculatum, comparable ring-shaped thinning surrounds small thickened circular areas (Eisenack and Cookson 1960).
Stratigraphic range/occurrence. At Melton Ross, Bed B (sample -12) to Bed F (sample -3).
Derivation of name. From maculate, meaning spotted or stained, with reference to the distinctive surface markings.
Designation of holotype. Figure 7.18. Sample MR3. Slide MR3(A). Coordinates S32/3. Location, Melton Ross Quarry, Lincolnshire, UK. Stratum, Bed F, Flixton Member, Welton Chalk Formation. The specimen is curated in the MPK collection of the British Geological Survey, Keyworth, Nottingham, UK, specimen number MPK 14662.
Diagnosis. A species of Trithyrodinium possessing an ornament of ring-shaped indentations on the endophragm and periphragm.
Description. Small to intermediate-sized, spheroidal to ovoidal-shaped, circumcavate, peridinioid dinoflagellate cysts. The endophragm (c. 1 μm thick) and periphragm (c. 0.5 μm thick) are smooth to finely-granular, both possessing regular ornament elements that are circular to sub-circular, ring-shaped indentations (c. 3–6 μm diameter), which are at least half the thickness of the surrounding wall areas. Cavation is c. 2–3 μm wide at the lateral margins and up to 10 μm in the antapical region. An archaeopyle involving three anterior intercalary plates, type 3I(1–3a), is formed in the endocyst, with operculum plates attached or detached. The 2a intercalary plate is isodeltaform, hexa-type (Fig. 7.19; cf. text-figure 6 in Bujak and Davies 1983). The periarchaeopyle type has not been confirmed. There is no evidence of tabulation, other than that indicated by the archaeopyle.
Dimensions (length × width of the endocyst). Holotype, 39.6 × 42.5 μm. Other specimens (5 measured): length 38.0 (50.0) 66.9 μm, width 39.5 (45.6) 55.3 μm.
Discussion. Only six specimens of this taxon have been recorded to date. However, its distinctive morphology makes it easy to identify and warrants the erection of a formal species. The endocyst archaeopyle type is consistent with the genus Trithyrodinium but an outer wall layer ( periphragm) has only been observed on two specimens, including the holotype. The circular indentations on the cyst walls are ringshaped, as opposed to the whole area within the circle being thinned. The ring shapes are reminiscent of those attributed to impressions made by coccoliths on the surface of specimens of an alga (Campenia sp.) by Prauss (2012b, figs 10A, 11H). However, the T. maculatum specimens occur in assemblages of abundant dinoflagellate cysts, with other taxa not exhibiting comparable markings. The markings are a key identifying feature but could be a preservation artefact. This type of surface ornament has not previously been reported on other species of P-cysts, including those belonging to the genus Trithyrodinium. In the G-cyst species Apteodinium maculatum, comparable ring-shaped thinning surrounds small thickened circular areas (Eisenack and Cookson 1960).
Stratigraphic range/occurrence. At Melton Ross, Bed B (sample -12) to Bed F (sample -3).