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Operculodinium plenum
Derivation of name. From the Latin plenus, meaning plenty in reference to the numerous non-tabular natures of the processes.
Holotype. Plate 2, figure 1; from a core sample at 2115.55m, well 25/7-5; Cyst diameter =31um; processes =2.5um.
Type locality. Norwegian North Sea Quad 25 – Alvheim Field, Lista Formation.
Diagnosis. Circular to sub-circular proximochorate cyst with numerous short, solid, non-tabular processes that are capitate distally; surface ornamentation microgranulate to microreticulate (LM), with perforations and thick fibrous exine structures forming the processes visible under a scanning electron microscope; archeopyle is precingular (plate 3ʺ), with a frequently free operculum.
Description. Cyst circular to sub-circular, proximochorate, with a thick wall composed of two closely adpressed layers: a microgranulate to microreticulate periphragm, and a thicker, less ornamented endophragm. Under a light microscope (LM), the cyst surface appears microgranulate to microreticulate. Scanning electron microscopy (SEM) reveals numerous irregular surface striations and perforations, which are randomly distributed and lack any distinct organizational pattern. Exhibit numerous, non-tabular processes (c. 6–12 per pre- and post-cingular plate), short (up to 4 lm in length), solid, and fibrous in appearance. They are uniformly distributed across the cyst surface, with slightly expanded proximal bases and distally capitate terminations. The processes appear to be formed through the fusion of fibrous exine elements, resulting in robust, slightly striate projections. Archeopyle is precingular, formed by the loss of plate 3ʺ, with the operculum frequently free or detached.
Dimensions. Overall diameter of the cyst body 26 (31.5) 35um, length of processes 1 (2.5) 4um. Number of specimens measured =18.
Comparison. Operculodinium plenum sp. nov. is distinguished by its exceptionally short, solid, and striate processes. It resembles Operculodinium centrocarpum (Deflandre and Cookson 1955) Wall 1967in general size and shape, but is readily differentiated by its significantly shorter processes. As noted by Matsuoka et al. (1997), the surface of O. centrocarpum is microreticulate, with coarser ornamentation around the base of the processes features absent in O. plenum sp. nov., which instead exhibits numerous irregular surface striations and perforations (see Plate SEM-D). Operculodinium bellulum Islam 1983also bears short processes, but these are distally acuminate or aculeate, differing from the capitate tips of O. plenum sp. nov. Operculodinium nanaconulum Islam 1983possesses very short, conical, hollow processes with broad bases and pointed apices. Operculodinium nitidum Islam 1983is characterized by short, broad-based processes that often coalesce. Operculodinium piaseckii Strauss and Lund 1992differs by having short, subconical processes. Of these species, typically only O. centrocarpum co-occurs with O. plenus sp. nov. in the Lista and Heimdal formations.
Holotype. Plate 2, figure 1; from a core sample at 2115.55m, well 25/7-5; Cyst diameter =31um; processes =2.5um.
Type locality. Norwegian North Sea Quad 25 – Alvheim Field, Lista Formation.
Diagnosis. Circular to sub-circular proximochorate cyst with numerous short, solid, non-tabular processes that are capitate distally; surface ornamentation microgranulate to microreticulate (LM), with perforations and thick fibrous exine structures forming the processes visible under a scanning electron microscope; archeopyle is precingular (plate 3ʺ), with a frequently free operculum.
Description. Cyst circular to sub-circular, proximochorate, with a thick wall composed of two closely adpressed layers: a microgranulate to microreticulate periphragm, and a thicker, less ornamented endophragm. Under a light microscope (LM), the cyst surface appears microgranulate to microreticulate. Scanning electron microscopy (SEM) reveals numerous irregular surface striations and perforations, which are randomly distributed and lack any distinct organizational pattern. Exhibit numerous, non-tabular processes (c. 6–12 per pre- and post-cingular plate), short (up to 4 lm in length), solid, and fibrous in appearance. They are uniformly distributed across the cyst surface, with slightly expanded proximal bases and distally capitate terminations. The processes appear to be formed through the fusion of fibrous exine elements, resulting in robust, slightly striate projections. Archeopyle is precingular, formed by the loss of plate 3ʺ, with the operculum frequently free or detached.
Dimensions. Overall diameter of the cyst body 26 (31.5) 35um, length of processes 1 (2.5) 4um. Number of specimens measured =18.
Comparison. Operculodinium plenum sp. nov. is distinguished by its exceptionally short, solid, and striate processes. It resembles Operculodinium centrocarpum (Deflandre and Cookson 1955) Wall 1967in general size and shape, but is readily differentiated by its significantly shorter processes. As noted by Matsuoka et al. (1997), the surface of O. centrocarpum is microreticulate, with coarser ornamentation around the base of the processes features absent in O. plenum sp. nov., which instead exhibits numerous irregular surface striations and perforations (see Plate SEM-D). Operculodinium bellulum Islam 1983also bears short processes, but these are distally acuminate or aculeate, differing from the capitate tips of O. plenum sp. nov. Operculodinium nanaconulum Islam 1983possesses very short, conical, hollow processes with broad bases and pointed apices. Operculodinium nitidum Islam 1983is characterized by short, broad-based processes that often coalesce. Operculodinium piaseckii Strauss and Lund 1992differs by having short, subconical processes. Of these species, typically only O. centrocarpum co-occurs with O. plenus sp. nov. in the Lista and Heimdal formations.