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Dimidium striata
From Fensome et al., 2019:
Dimidium striata (Clarke and Verdier, 1967, p.31, pl.4, figs.11–13; pl.5, fig.15; text-fig.12) Pearce, 2010, p.57. Holotype: Clarke and Verdier, 1967, pl.4, figs.11–13, Jan du Chêne et al., 1986a, pl.88, figs.9–11. Originally Gonyaulacysta, subsequently Leptodinium, thirdly Impagidinium, fourthly Pterodinium, fifthly (and now) Dimidium. Age: Santonian.
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Original diagnosis: Clarke and Verdier, 1967, p.31: Gonyaulacysta striata
A species of Gonyaulacysta having no apical horn but possessing thin, often striate ledges.
Description: Clarke and Verdier, 1067, p.31: Gonyaulacysta striata
Outline circular or oval without an apical horn. Tabulation is distinct and conforms to the Gonyaulacysta pattern. Girdle strongly displaced, sometimes more than two girdle widths. Cingular plates present. Longitudinal furrow straight. Plates smooth, bordered by ledges up to 5 µm high with a non-protruding support at their junctions. These ledges are thin, often transparent and generally vertically striate. The precingular archaeopyle is not an obvious feature of most specimens.
Dimensions: Holotype: Overall length 66 µm, overall breadth 55 µm, height of ledges (max.) 11 µm, width of girdle 5 µm. Range: Overall length 40-66 µm, overall breadth 40-55 µm, height of ledges (max.) 4-11 µm, width of girdle 4-6 µm.
Affinities:
Clarke and Verdier, 1967, p.31: Gonyaulacysta striata
G. striata differs from G. tenuiceras Eisenack in the absence of an apical horn and sculpture on the plates. Gonyaulax freaki Sarjeant differs in having only five postcingular plates and smooth or perforate ledges. G. striata differs from G. ambigua (Defl.) in having higher ledges and from G. pachyderma (Defl.) in the absence of denticulate ledges.
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Comments Pearce, 2010:
Holotype. Gonyaulacysta striata Clarke & Verdier, 1967, plate 4, figs 11–13.
Locality and horizon: Culver Cliff, Isle of Wight, UK; Sample CV26, Marsupites testudinarius Zone (upper Santonian).
Dimensions. Type material of Clarke & Verdier (1967), holotype: overall (w/l): 55 66 μm, height of crests (max.): 11 μm; range: overall (w/l): 40–55 40–66 μm, height of crests (max.): 4–11 μm.
Trunch borehole material, range: central body (w/l) 28(41.3)50 41(43)46 μm, crest height (max) 1(1.7)2 μm, 7 specimens measured.
Stratigraphic range. Type section, as restudied by Prince et al. (1999): Broadstairs Chalk to Newhaven Chalk, high Micraster coranguinum Zone (high middle Santonian) to the Offaster pilula Zone (lower Campanian).
Trunch borehole material: Burnham–Flamborough Chalk (undifferentiated), high Micraster coranguinum Zone (high middle Santonian) to the high Gonioteuthis quadrata Zone (high lower Campanian; Fig. 2).
Remarks. Clarke & Verdier (1967: 31–32) first described this species as: ‘A species of Gonyaulacysta having no apical horn but possessing thin, often striate ledges . . . plates smooth, bordered by ledges up to 5 μm high . . . the precingular archaeopyle is not an obvious feature’. In the measurement of an undisclosed number of specimens, the range of crest height was stated to be 4–11 μm, exceeding that stated in the description, and in the holotype the maximum crest height of 11 μm is less than ¼ the shortest dimension of the central body (i.e. breadth 55 μm). Sarjeant (1969: 12) transferred the form to Leptodinium striatum without discussion, and emended the generic description: ‘. . . height of crests always less (and typically markedly less) than ¼ of shell width. A precingular single-plate archaeopyle . . . may not be present’. A range of surface ornamentation was accepted (but he did not specify striate!). It is unknown which criteria Sarjeant used to justify the transfer, but it may be partly due to the relative height of the parasutural crests of Pterodinium, which, according to the basic description of Eisenack (1958: 395), were described only as ‘broad’. Yun (1981) emended the generic description of Pterodinium by quantifying crest height as ¼ of the body size, which are uniform in height on an individual specimen, and then proposed the combination Pterodinium striatum based on crest heights of 10–17 μm. Yun’s maximum crest height exceeds ¼ of the largest body dimension (51 μm); however, it is not known if this is typical since the shortest crest height is also less than ¼ the shortest body diameter (42 μm). The paratabulation was described with the formula: 3#, 1a, 6$, 6c, 5%, 1p, 1$$; but critically the archaeopyle type was not recognized. From the same material, Yun (1981) also described a very similar new species Heslertonia regula, for otherwise identical species with an epicystal archaeopyle, possessing occasionally striate crests (stated to be up 13 μm high). The average maximum crest height from 18 specimens of 9.7 μm is less than ¼ the average shortest
Dimidium striata (Clarke and Verdier, 1967, p.31, pl.4, figs.11–13; pl.5, fig.15; text-fig.12) Pearce, 2010, p.57. Holotype: Clarke and Verdier, 1967, pl.4, figs.11–13, Jan du Chêne et al., 1986a, pl.88, figs.9–11. Originally Gonyaulacysta, subsequently Leptodinium, thirdly Impagidinium, fourthly Pterodinium, fifthly (and now) Dimidium. Age: Santonian.
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Original diagnosis: Clarke and Verdier, 1967, p.31: Gonyaulacysta striata
A species of Gonyaulacysta having no apical horn but possessing thin, often striate ledges.
Description: Clarke and Verdier, 1067, p.31: Gonyaulacysta striata
Outline circular or oval without an apical horn. Tabulation is distinct and conforms to the Gonyaulacysta pattern. Girdle strongly displaced, sometimes more than two girdle widths. Cingular plates present. Longitudinal furrow straight. Plates smooth, bordered by ledges up to 5 µm high with a non-protruding support at their junctions. These ledges are thin, often transparent and generally vertically striate. The precingular archaeopyle is not an obvious feature of most specimens.
Dimensions: Holotype: Overall length 66 µm, overall breadth 55 µm, height of ledges (max.) 11 µm, width of girdle 5 µm. Range: Overall length 40-66 µm, overall breadth 40-55 µm, height of ledges (max.) 4-11 µm, width of girdle 4-6 µm.
Affinities:
Clarke and Verdier, 1967, p.31: Gonyaulacysta striata
G. striata differs from G. tenuiceras Eisenack in the absence of an apical horn and sculpture on the plates. Gonyaulax freaki Sarjeant differs in having only five postcingular plates and smooth or perforate ledges. G. striata differs from G. ambigua (Defl.) in having higher ledges and from G. pachyderma (Defl.) in the absence of denticulate ledges.
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Comments Pearce, 2010:
Holotype. Gonyaulacysta striata Clarke & Verdier, 1967, plate 4, figs 11–13.
Locality and horizon: Culver Cliff, Isle of Wight, UK; Sample CV26, Marsupites testudinarius Zone (upper Santonian).
Dimensions. Type material of Clarke & Verdier (1967), holotype: overall (w/l): 55 66 μm, height of crests (max.): 11 μm; range: overall (w/l): 40–55 40–66 μm, height of crests (max.): 4–11 μm.
Trunch borehole material, range: central body (w/l) 28(41.3)50 41(43)46 μm, crest height (max) 1(1.7)2 μm, 7 specimens measured.
Stratigraphic range. Type section, as restudied by Prince et al. (1999): Broadstairs Chalk to Newhaven Chalk, high Micraster coranguinum Zone (high middle Santonian) to the Offaster pilula Zone (lower Campanian).
Trunch borehole material: Burnham–Flamborough Chalk (undifferentiated), high Micraster coranguinum Zone (high middle Santonian) to the high Gonioteuthis quadrata Zone (high lower Campanian; Fig. 2).
Remarks. Clarke & Verdier (1967: 31–32) first described this species as: ‘A species of Gonyaulacysta having no apical horn but possessing thin, often striate ledges . . . plates smooth, bordered by ledges up to 5 μm high . . . the precingular archaeopyle is not an obvious feature’. In the measurement of an undisclosed number of specimens, the range of crest height was stated to be 4–11 μm, exceeding that stated in the description, and in the holotype the maximum crest height of 11 μm is less than ¼ the shortest dimension of the central body (i.e. breadth 55 μm). Sarjeant (1969: 12) transferred the form to Leptodinium striatum without discussion, and emended the generic description: ‘. . . height of crests always less (and typically markedly less) than ¼ of shell width. A precingular single-plate archaeopyle . . . may not be present’. A range of surface ornamentation was accepted (but he did not specify striate!). It is unknown which criteria Sarjeant used to justify the transfer, but it may be partly due to the relative height of the parasutural crests of Pterodinium, which, according to the basic description of Eisenack (1958: 395), were described only as ‘broad’. Yun (1981) emended the generic description of Pterodinium by quantifying crest height as ¼ of the body size, which are uniform in height on an individual specimen, and then proposed the combination Pterodinium striatum based on crest heights of 10–17 μm. Yun’s maximum crest height exceeds ¼ of the largest body dimension (51 μm); however, it is not known if this is typical since the shortest crest height is also less than ¼ the shortest body diameter (42 μm). The paratabulation was described with the formula: 3#, 1a, 6$, 6c, 5%, 1p, 1$$; but critically the archaeopyle type was not recognized. From the same material, Yun (1981) also described a very similar new species Heslertonia regula, for otherwise identical species with an epicystal archaeopyle, possessing occasionally striate crests (stated to be up 13 μm high). The average maximum crest height from 18 specimens of 9.7 μm is less than ¼ the average shortest