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Alterbidinium dictyotum
From Fensome et al., 2019:
Alterbidinium dictyotum, Harker and Sarjeant in Harker et al., 1990, p.105–106, pl.8, figs.1–5; text-fig.21 ex Harker and Sarjeant, 1991, p.709.
Holotype: Harker et al., 1990, pl.8, fig.1; text-fig.21. This name was not validly published in Harker et al. (1990) since the lodgement of the holotype was not cited (ICN Article 40.7).
Age: early Campanian.
Locus typicus: Gerald, Saskatchewan, Canada
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Original description: [Harker, Sarjeant and Caldwell, 1990, p. 105]:
Diagnosis:
Bicavate cysts having a subpentagonal ambitus. Apical pericoel larger than antapical; ambital pericoel meagerly developed or lacking. Periblast thin-walled, reticulate, prolonged into a single long, tapering apical horn and two unequal antapical horns (a long tapering left and a very short, rounded right horn). Greatest width in the cingular region; epitract slightly larger than hypotract. Endoblast ovoidal, thin-walled, smooth to finely granular, breadth slightly greater than length. A narrow, slightly laevorotatory cingulum is indicated by low, narrow thickenings of the periphragm; very low, dorsal parasutural crests (delimiting paraplates 1a, 2a, 3a, 3", 4", 5") are developed on the epitracts of some specimens. Peripyle intercalary (1, 2a), standard hexagonal shape, the operculum usually remaining in place.
Description:
The reticulation of the periphragm is the most distinctive feature of this species; it is present all over the periblast except on the distal two-thirds of the apical horn and the distal one-third of the left antapical horn, where the periphragm is laevigate. The lumina are rounded to polygonal, 0.5-2.0 µm in diameter; the muri are 0.2-0.5 µm wide, 0.2 µm high.
Both endophragm and periphragm are thin (less than 0.5 µm) and specimens do not take stain readily.
The tapering apical horn has a blunt or indented distal termination. The left antapical horn is sharply conical, though frequently folded against the main body of the cyst (Pl. 8, Fig. 3), whereas the right is merely a small, rounded outbulge. The endoblast is generally ovoidal in shape, though sometimes developing slight protuberances at apex and antapex.
Parasutural ridges are best developed on the dorsal surface of the epitract, but can be seen clearly only in scanning electron microscope examination (Pl. 8, Fig. 2); they consist of low, narrow thickenings of the periphragm outlining the dorsal intercalary and precingular paraplates (la, 2a, 3a, 3", 4", 5").
The cingulum is well defined by two low, parallel ridges; it is only slightly laevorotatory, sulcal displacement being less than half the cingulum"s width. The peripyle is formed by opening of the second anterior intercalary paraplate (2a), which usually remains in place and indeed is markedly displaced in only one specimen observed (Pl. 8, Fig. 4.).
Dimensions:
Holotype: overall length 111 µm, breadth 68 µm, endoblast length 52 µm, breadth 61 µm, length of apical horn 12 µm, left antapical horn 19 µm, right antapical horn 1 µm, cingulum width 5 µm, transverse peripyle index 0.36, transverse peripyle ratio 0.56.
Range of 53 measurable specimens: periblast length 55 µm, mean 84 µm; breadth 32-69 µm, mean 58 µm; endoblast length 35-52 µm, mean 44 µm; breadth 33-65 µm, mean 54 µm; length of apical horn 9-12 µm, mean 10 µm; left antapical horn 5-19 µm, mean 12 µm; right antapical horn 0.5-5 µm, mean 2 µm; cingulum width 4-7 µm, mean 6 µm; transverse peripyle index 0.26-0.48, mean 0.37; transverse peripyle ratio 0.36-0.93, mean 0.58.
Affinities:
A. dictyotum is similar in general features to Alterbidinium? distinctum (Wilson 1967) Lentin and Williams 1985, but differs from it, and from all other species of Alterbidinium in having a reticulate periphragm.
Alterbidinium dictyotum, Harker and Sarjeant in Harker et al., 1990, p.105–106, pl.8, figs.1–5; text-fig.21 ex Harker and Sarjeant, 1991, p.709.
Holotype: Harker et al., 1990, pl.8, fig.1; text-fig.21. This name was not validly published in Harker et al. (1990) since the lodgement of the holotype was not cited (ICN Article 40.7).
Age: early Campanian.
Locus typicus: Gerald, Saskatchewan, Canada
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Original description: [Harker, Sarjeant and Caldwell, 1990, p. 105]:
Diagnosis:
Bicavate cysts having a subpentagonal ambitus. Apical pericoel larger than antapical; ambital pericoel meagerly developed or lacking. Periblast thin-walled, reticulate, prolonged into a single long, tapering apical horn and two unequal antapical horns (a long tapering left and a very short, rounded right horn). Greatest width in the cingular region; epitract slightly larger than hypotract. Endoblast ovoidal, thin-walled, smooth to finely granular, breadth slightly greater than length. A narrow, slightly laevorotatory cingulum is indicated by low, narrow thickenings of the periphragm; very low, dorsal parasutural crests (delimiting paraplates 1a, 2a, 3a, 3", 4", 5") are developed on the epitracts of some specimens. Peripyle intercalary (1, 2a), standard hexagonal shape, the operculum usually remaining in place.
Description:
The reticulation of the periphragm is the most distinctive feature of this species; it is present all over the periblast except on the distal two-thirds of the apical horn and the distal one-third of the left antapical horn, where the periphragm is laevigate. The lumina are rounded to polygonal, 0.5-2.0 µm in diameter; the muri are 0.2-0.5 µm wide, 0.2 µm high.
Both endophragm and periphragm are thin (less than 0.5 µm) and specimens do not take stain readily.
The tapering apical horn has a blunt or indented distal termination. The left antapical horn is sharply conical, though frequently folded against the main body of the cyst (Pl. 8, Fig. 3), whereas the right is merely a small, rounded outbulge. The endoblast is generally ovoidal in shape, though sometimes developing slight protuberances at apex and antapex.
Parasutural ridges are best developed on the dorsal surface of the epitract, but can be seen clearly only in scanning electron microscope examination (Pl. 8, Fig. 2); they consist of low, narrow thickenings of the periphragm outlining the dorsal intercalary and precingular paraplates (la, 2a, 3a, 3", 4", 5").
The cingulum is well defined by two low, parallel ridges; it is only slightly laevorotatory, sulcal displacement being less than half the cingulum"s width. The peripyle is formed by opening of the second anterior intercalary paraplate (2a), which usually remains in place and indeed is markedly displaced in only one specimen observed (Pl. 8, Fig. 4.).
Dimensions:
Holotype: overall length 111 µm, breadth 68 µm, endoblast length 52 µm, breadth 61 µm, length of apical horn 12 µm, left antapical horn 19 µm, right antapical horn 1 µm, cingulum width 5 µm, transverse peripyle index 0.36, transverse peripyle ratio 0.56.
Range of 53 measurable specimens: periblast length 55 µm, mean 84 µm; breadth 32-69 µm, mean 58 µm; endoblast length 35-52 µm, mean 44 µm; breadth 33-65 µm, mean 54 µm; length of apical horn 9-12 µm, mean 10 µm; left antapical horn 5-19 µm, mean 12 µm; right antapical horn 0.5-5 µm, mean 2 µm; cingulum width 4-7 µm, mean 6 µm; transverse peripyle index 0.26-0.48, mean 0.37; transverse peripyle ratio 0.36-0.93, mean 0.58.
Affinities:
A. dictyotum is similar in general features to Alterbidinium? distinctum (Wilson 1967) Lentin and Williams 1985, but differs from it, and from all other species of Alterbidinium in having a reticulate periphragm.