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Amphigymnium cooksoniae
From Fensome et al., 2019:
Amphigymnium cooksoniae, (Boltenhagen, 1977, p.75–76, pl.11, figs.5a–c,6–7) Lentin and Vozzhennikova, 1990, p.30.
Holotype: Boltenhagen, 1977, pl.11, figs.5a–c; Lentin and Vozzhennikova, 1990, Appendix A, fig.40.
Originally Dinogymnium, subsequently (and now) Amphigymnium.
Age: Campanian.
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Original description: [Boltenhagen, 1977] (translated from French):
Dinogymnium cooksonae nov. sp.
DIAGNOSIS
Elongated theca, divided by a wide circular cingulum into an ogival epithecium and hypothecium of approximately equal dimensions; punctate or microgranular and inframicroperforated membrane. Absence of ribs.
DESCRIPTION
This species has an elongated theca such that the length-to-width ratio is between 1.7 and 1.9. A circular cingulum, whose width is 1/5 to 1/6 of the length of the polar axis, slightly constricts and divides the theca into two roughly symmetrical dome-shaped parts. However, one part is slightly shorter than the other. A tear below one of the poles, observed in two specimens, suggests an apical archaeopyle. Under these conditions, the epitheca would be slightly smaller than the hypotheca. The punctate theca membrane at 320x magnification appears microgranular and submicroperforated at 800x magnification. Since the size of the omental elements is on the order of a few fractions of a micron, their distribution cannot be determined.
The theca bears irregular, mainly longitudinal folds, but lacks true ribs.
DIMENSIONS
Holotype: theca L x W = 48 x 25 μ
cingulum W = 8-10 μ
Paratypes: theca L x W = 50 x 26 μ; 50 x 29 μ
cingulum W = 9-11 μ, 8-10 μ
coef. (CI) = 46
NOTE: The width of the theca is taken at the midpoint of the cingulum.
COMPARISON
The described species is distinguished from other known related species by the absence of ribs, and the ogival epitheca and hypotheca, which are symmetrical relative to the very broad cingulum.
This latter characteristic brings it closer to Dinogymnium (Gymnodinium) laticinctum DEFL. 1943 (p. 502), and the elongated shape of the theca is reminiscent of Dinogymnium (Gymnodinium) digitus DEFL 1935 (p. 166). Furthermore, Dinogymnium (Gymnodinium) sp. A, presented by W.S. Drugg in 1967 (p. 12, PI. I, Fig. 3), bears a very close resemblance to the described species.
Stratigraphic and Geographic Position
Upper Cretaceous: Campanian, Gabon (Port-Gentil).
Amphigymnium cooksoniae, (Boltenhagen, 1977, p.75–76, pl.11, figs.5a–c,6–7) Lentin and Vozzhennikova, 1990, p.30.
Holotype: Boltenhagen, 1977, pl.11, figs.5a–c; Lentin and Vozzhennikova, 1990, Appendix A, fig.40.
Originally Dinogymnium, subsequently (and now) Amphigymnium.
Age: Campanian.
---------------------------------------------------------------------------------------------------
Original description: [Boltenhagen, 1977] (translated from French):
Dinogymnium cooksonae nov. sp.
DIAGNOSIS
Elongated theca, divided by a wide circular cingulum into an ogival epithecium and hypothecium of approximately equal dimensions; punctate or microgranular and inframicroperforated membrane. Absence of ribs.
DESCRIPTION
This species has an elongated theca such that the length-to-width ratio is between 1.7 and 1.9. A circular cingulum, whose width is 1/5 to 1/6 of the length of the polar axis, slightly constricts and divides the theca into two roughly symmetrical dome-shaped parts. However, one part is slightly shorter than the other. A tear below one of the poles, observed in two specimens, suggests an apical archaeopyle. Under these conditions, the epitheca would be slightly smaller than the hypotheca. The punctate theca membrane at 320x magnification appears microgranular and submicroperforated at 800x magnification. Since the size of the omental elements is on the order of a few fractions of a micron, their distribution cannot be determined.
The theca bears irregular, mainly longitudinal folds, but lacks true ribs.
DIMENSIONS
Holotype: theca L x W = 48 x 25 μ
cingulum W = 8-10 μ
Paratypes: theca L x W = 50 x 26 μ; 50 x 29 μ
cingulum W = 9-11 μ, 8-10 μ
coef. (CI) = 46
NOTE: The width of the theca is taken at the midpoint of the cingulum.
COMPARISON
The described species is distinguished from other known related species by the absence of ribs, and the ogival epitheca and hypotheca, which are symmetrical relative to the very broad cingulum.
This latter characteristic brings it closer to Dinogymnium (Gymnodinium) laticinctum DEFL. 1943 (p. 502), and the elongated shape of the theca is reminiscent of Dinogymnium (Gymnodinium) digitus DEFL 1935 (p. 166). Furthermore, Dinogymnium (Gymnodinium) sp. A, presented by W.S. Drugg in 1967 (p. 12, PI. I, Fig. 3), bears a very close resemblance to the described species.
Stratigraphic and Geographic Position
Upper Cretaceous: Campanian, Gabon (Port-Gentil).