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Dimidium
From Williams et al., 2017:
[Dimidium, Pearce, 2010, p. 56–57.
Type species: Gonyaulacysta striata, Clarke and Verdier, 1967 (pl.4, figs.11–13)]
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Original description: [Pearce, 2010]:
Diagnosis:
Proximate dinoflagellate cysts with an epicystal archaeopyle, parasutural crests not (or rarely) exceeding ¼ body width, clearly expressing an exsert first apical homologue (*1'), L-Type and sexiform paraplate patterns and slight dextral torsion.
Description:
Medium-sized sub-spheroidal to oblate proximate dinoflagellate cyst lacking apical or antapical horns. The wall is single-layered and comprised of an autophragm that forms parasutural crests. The paratabulation is indicated by the low parasutural crests, less than ¼ body diameter in height, which conform to the standard gonyaulacoid formula 4', 6", 6c, 5–6s (as, ras, rs, ls, ps), 6'", 1p, 1"" (Fig. 4), and display an L-Type ventral pattern, and sexiform hypocystal paraplate arrangement. Slight dextral torsion exists such that the vertical midlines through the 3" and 4'" paraplates are not in-line, and the 3"/4" suture bisects the right lateral margin of the 5'" paraplate. The archaeopyle is polyplacoid epitractal (Type tAtP), the operculum is occasionally adnate ventrally, but more usually detached and contiguous.
Comparison:
Dimidium most closely resembles Heslertonia Sarjeant, 1966 but differs in possessing parasutural crests that are much lower than ¼ body width. Ornamentation height (i.e. processes or crests) is probably an unsuitable generic level discriminator but the precedent stems back to an original difference between Pterodinium Eisenack, 1958 and Leptodinium Klement, 1960. However, Heslertonia is classified in the Order Gonyaulacales, but unassigned to a suborder or family (Fensome et al., 1993) since by implication the ventral and hyposomal paraplate configuration have not been determined. In contrast, Dimidium gen. nov. is clearly L-Type and sexiform with a slight dextral torsion. Since the orientation of the paraplates is well understood in Dimidium gen. nov., it is considered grounds to treat it distinct from Heslertonia, in the same way that Stover & Evitt (1978) erected Impagidinium when the paratabulation of Pterodinium was unknown. Differs from Ctenidodinium Deflandre, 1939 in lacking anterior intercalary paraplates and from Dichadogonyaulax Sarjeant, 1966 in lacking a preapical structure (pr) separating the second from the fourth apical homologues. It differs from Dinopterygium Deflandre, 1935 by possessing a sexiform rather than quinqueform hypocystal paraplate pattern and in the position of the principal archaeopyle suture that is immediately anterior to the paracingulum rather than within the paracingulum. It differs from species of Impagidinium Stover & Evitt, 1978, Leptodinium Klement, 1960 and Pterodinium Eisenack, 1958 primarily by possessing an epicystal archaeopyle.
[Dimidium, Pearce, 2010, p. 56–57.
Type species: Gonyaulacysta striata, Clarke and Verdier, 1967 (pl.4, figs.11–13)]
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Original description: [Pearce, 2010]:
Diagnosis:
Proximate dinoflagellate cysts with an epicystal archaeopyle, parasutural crests not (or rarely) exceeding ¼ body width, clearly expressing an exsert first apical homologue (*1'), L-Type and sexiform paraplate patterns and slight dextral torsion.
Description:
Medium-sized sub-spheroidal to oblate proximate dinoflagellate cyst lacking apical or antapical horns. The wall is single-layered and comprised of an autophragm that forms parasutural crests. The paratabulation is indicated by the low parasutural crests, less than ¼ body diameter in height, which conform to the standard gonyaulacoid formula 4', 6", 6c, 5–6s (as, ras, rs, ls, ps), 6'", 1p, 1"" (Fig. 4), and display an L-Type ventral pattern, and sexiform hypocystal paraplate arrangement. Slight dextral torsion exists such that the vertical midlines through the 3" and 4'" paraplates are not in-line, and the 3"/4" suture bisects the right lateral margin of the 5'" paraplate. The archaeopyle is polyplacoid epitractal (Type tAtP), the operculum is occasionally adnate ventrally, but more usually detached and contiguous.
Comparison:
Dimidium most closely resembles Heslertonia Sarjeant, 1966 but differs in possessing parasutural crests that are much lower than ¼ body width. Ornamentation height (i.e. processes or crests) is probably an unsuitable generic level discriminator but the precedent stems back to an original difference between Pterodinium Eisenack, 1958 and Leptodinium Klement, 1960. However, Heslertonia is classified in the Order Gonyaulacales, but unassigned to a suborder or family (Fensome et al., 1993) since by implication the ventral and hyposomal paraplate configuration have not been determined. In contrast, Dimidium gen. nov. is clearly L-Type and sexiform with a slight dextral torsion. Since the orientation of the paraplates is well understood in Dimidium gen. nov., it is considered grounds to treat it distinct from Heslertonia, in the same way that Stover & Evitt (1978) erected Impagidinium when the paratabulation of Pterodinium was unknown. Differs from Ctenidodinium Deflandre, 1939 in lacking anterior intercalary paraplates and from Dichadogonyaulax Sarjeant, 1966 in lacking a preapical structure (pr) separating the second from the fourth apical homologues. It differs from Dinopterygium Deflandre, 1935 by possessing a sexiform rather than quinqueform hypocystal paraplate pattern and in the position of the principal archaeopyle suture that is immediately anterior to the paracingulum rather than within the paracingulum. It differs from species of Impagidinium Stover & Evitt, 1978, Leptodinium Klement, 1960 and Pterodinium Eisenack, 1958 primarily by possessing an epicystal archaeopyle.