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Trithyrodinium zakkii
Trithyrodinium zakkii sp. nov.; Pearce, M. A. et al. 2019
Holotype: Pearce, M. A. et al. 2019, Plate IV, 1–3
Age:
Original description: Pearce, M. A. et al. 2019: Trithyrodinium zakkii sp. nov.
Diagnosis:
A species of Trithyrodinium possessing short, minutely bifurcating spines on the endophragm.
Description:
Intermediate to large (when complete) bicavate peridinioid cyst
Wall: two-layered composed of a relatively thick and fibrous to spongy endophragm (~4 μm thick), and a thin and smooth periphragm (<1 μm) that is rarely attached. From
the endophragm, densely arranged solid, non-tabular spines occur that are around 2–9 μm in length (although consistent on individual specimens) and flared to very weakly bifurcate distally.
Periphragm: when present, is longitudinally elongate, forming a broad-based and blunt apical horn, and two broad-based and blunt antapical horns (the right antapical horn being shorter), and being relatively loose fitting elsewhere.
Tabulation: no evidence of tabulation, other than indicated by the archaeopyle.
Archaeopyle: endoarchaeopyle is intercalary Type 3I (1a–3a) and the endoperculum is detached. Too few specimens have been observed to identify the nature of the periarchaeopyle. The cingulum and sulcus are not clearly expressed.
Dimensions:
Endocyst (width x length) = 53(65)75 × 61(68.3)77 μm, maximum spine length = 2(4.3)9 μm.
Number of specimens measured: 23.
Complete specimen from a Heidrun: endocyst (width × length): 63 × 66 μm, pericyst (width × length): 74 × 130 μm, spine length (max.) 9 μm.
two specimens from the 6406/3–6 well: maximum length of the antapical horns were 10 and 12 μm
Remarks:
We have opted to use the term periphragm for the outer wall layer of T. zakkii sp. nov. rather than ectophragm. It is clear that spines on the endocyst do not support the outer wall layer, particularly in the vicinity of the apical horn (Plate VI, figs. 1–6) or antapical horns (Plate IV, figs. 7–9; Plate V, figs. 4–6, Plate VI, figs. 3–5) and are obviously not an integral part of the wall design.
Trithyrodinium zakkii sp. nov. is well known to biostratigraphers working o material from the North Sea and Norwegian–Greenland seas, and has been reported under the informal names Trithyrodinium “echinatum”, T. “hirsutum”, T. “spinosum”, or Trithyrodinium sp. (spines).
Affinities:
Trithyrodinium zakkii sp. nov. differs from Trithyrodinium suspectum (Manum and Cookson, 1964) Davey, 1969, to which it is most similar, by possessing spines. Trivalvadinium Islam, 1983 also possesses a Type 3I(1a–3a) archaeopyle and non-tabular spines. It was the synopsis of Stover and Williams (1987) and the formal emended diagnosis of Khowaja-Ateequzzaman and Garg (1995) that improved the original description by stating that the spines are formed from the periphragm. Despite occurring in the Middle Eocene, Trivalvadinium formosum Islam, 1983 also differs in possessing a rounded pentagonal ambitus, and tubular spines that are much less densely distributed.
Holotype: Pearce, M. A. et al. 2019, Plate IV, 1–3
Age:
Original description: Pearce, M. A. et al. 2019: Trithyrodinium zakkii sp. nov.
Diagnosis:
A species of Trithyrodinium possessing short, minutely bifurcating spines on the endophragm.
Description:
Intermediate to large (when complete) bicavate peridinioid cyst
Wall: two-layered composed of a relatively thick and fibrous to spongy endophragm (~4 μm thick), and a thin and smooth periphragm (<1 μm) that is rarely attached. From
the endophragm, densely arranged solid, non-tabular spines occur that are around 2–9 μm in length (although consistent on individual specimens) and flared to very weakly bifurcate distally.
Periphragm: when present, is longitudinally elongate, forming a broad-based and blunt apical horn, and two broad-based and blunt antapical horns (the right antapical horn being shorter), and being relatively loose fitting elsewhere.
Tabulation: no evidence of tabulation, other than indicated by the archaeopyle.
Archaeopyle: endoarchaeopyle is intercalary Type 3I (1a–3a) and the endoperculum is detached. Too few specimens have been observed to identify the nature of the periarchaeopyle. The cingulum and sulcus are not clearly expressed.
Dimensions:
Endocyst (width x length) = 53(65)75 × 61(68.3)77 μm, maximum spine length = 2(4.3)9 μm.
Number of specimens measured: 23.
Complete specimen from a Heidrun: endocyst (width × length): 63 × 66 μm, pericyst (width × length): 74 × 130 μm, spine length (max.) 9 μm.
two specimens from the 6406/3–6 well: maximum length of the antapical horns were 10 and 12 μm
Remarks:
We have opted to use the term periphragm for the outer wall layer of T. zakkii sp. nov. rather than ectophragm. It is clear that spines on the endocyst do not support the outer wall layer, particularly in the vicinity of the apical horn (Plate VI, figs. 1–6) or antapical horns (Plate IV, figs. 7–9; Plate V, figs. 4–6, Plate VI, figs. 3–5) and are obviously not an integral part of the wall design.
Trithyrodinium zakkii sp. nov. is well known to biostratigraphers working o material from the North Sea and Norwegian–Greenland seas, and has been reported under the informal names Trithyrodinium “echinatum”, T. “hirsutum”, T. “spinosum”, or Trithyrodinium sp. (spines).
Affinities:
Trithyrodinium zakkii sp. nov. differs from Trithyrodinium suspectum (Manum and Cookson, 1964) Davey, 1969, to which it is most similar, by possessing spines. Trivalvadinium Islam, 1983 also possesses a Type 3I(1a–3a) archaeopyle and non-tabular spines. It was the synopsis of Stover and Williams (1987) and the formal emended diagnosis of Khowaja-Ateequzzaman and Garg (1995) that improved the original description by stating that the spines are formed from the periphragm. Despite occurring in the Middle Eocene, Trivalvadinium formosum Islam, 1983 also differs in possessing a rounded pentagonal ambitus, and tubular spines that are much less densely distributed.