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Arkellidinium triapertum
Arkellidinium triapertum, Beju, 1979
Originally Arkellidinium, subsequently Sirmiodiniopsis.
Tax. jr. synonym of Sirmiodiniopsis orbis Drugg, 1978, according to Riley and Fenton, 1982.
Holotype: Beju, 1979, fig,1, no.1; fig.2A
Locus typicus: Woodham Pit, Aylesbury, Buckinghamshire, England
Stratum typicum: Peltoceras althea zone, Oxford Clay, Late Callovian
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Original description: [Beju, 1979, p. 4]:
Description:
Cavate cysts dorso-ventrally flattened with subcircular to rounded polyhedral outline; endophragm subcircular. Apical horn absent or vaguely indicated by a low protuberance. Epicyst/hypocyst ratio approximately 2/3. Both wall surfaces smooth to finely punctate. Pericoel well developed, largest on the hypocyst, especially around the antapex, diminishing gradually toward the apex, absent or extremely reduced on the operculum. The two walls are in contact on most of the dorsal and ventral surfaces of the endophragm.
Peritabulation not observed.
Pericingulum distinct, slightly helicoidal, demarcated by low, narrow, sometimes discontinuous folds or ridges. Pericingular tabulation not observed.
Perisulcus weakly indicated by interruption of the pericingulum and a faint longitudinal invagination, extending shortly on both epi- and hypocysts.
Archeopyle apical, formed by detachment of the apex. Operculum attached in well preserved specimens or completely detached, apparently formed by four apical plates (type tA in Evitt 1967).
A sulcal indentation is frequently visible.
The periphragm consistently presents two additional ventral apertures located laterally symmetrical with respect to the longitudinal axis of the cyst. Antapical apertures ovoidal to subtriangular, with the base externally oriented; maximum diameter 12-18 µm.
Dimensions (15 specimens):
Overall length 55-65 µm; overall breadth 55-65 µm.
Affinities: (p.4-5):
The consistent combination of an apical archeopyle with the two ventral antapical orifices in the periphragm renders this dinocyst highly distinctive. It differs from Sirmiodinium Alberti, emend. Warren (1973), which has a gonyaulacystacean paratabulation and compound archeopyle involving the apical and the third precingular paraplates. The genera Scriniodinium Klement (1957) and Endoscrinium (Klement, 1960) Vozzhennikova (1967) both have a precingular archeopyle, while Lithodinia Eisenack, emend. Gocht (1975) is acavate. The genera Ascodinium Cookson and Eisenack (1960), Ovoidinium Davey, emend. Lentin and Williams (1975), and Hexagonifera Cookson and Eisenack (1961, 1962) differ from Arkellidinium by their peridinoid nature, chiefly by having combination archeopyles formed by detachment of the apical and anterior intercalary plates. The genus Senonisphaera Clarke and Verdier (1967), especially its type species S. protusa Clarke and Verdier (1967), has prominent apical and antapical horns and a tendency of forming two lateral horns.
The forms described by Gocht (1970) as Gen. et sp. indet. 1 appear to have a morphology similar to Arkellidinium triapertum, but it is not evident if the German specimens are triaperturate. Hexagonifera jurassica Gitmez and Sarjeant (1972) and Lithodinia staffiensis (Gitmez) Gocht (1975) also appear to be closely related to this genus.
Originally Arkellidinium, subsequently Sirmiodiniopsis.
Tax. jr. synonym of Sirmiodiniopsis orbis Drugg, 1978, according to Riley and Fenton, 1982.
Holotype: Beju, 1979, fig,1, no.1; fig.2A
Locus typicus: Woodham Pit, Aylesbury, Buckinghamshire, England
Stratum typicum: Peltoceras althea zone, Oxford Clay, Late Callovian
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Original description: [Beju, 1979, p. 4]:
Description:
Cavate cysts dorso-ventrally flattened with subcircular to rounded polyhedral outline; endophragm subcircular. Apical horn absent or vaguely indicated by a low protuberance. Epicyst/hypocyst ratio approximately 2/3. Both wall surfaces smooth to finely punctate. Pericoel well developed, largest on the hypocyst, especially around the antapex, diminishing gradually toward the apex, absent or extremely reduced on the operculum. The two walls are in contact on most of the dorsal and ventral surfaces of the endophragm.
Peritabulation not observed.
Pericingulum distinct, slightly helicoidal, demarcated by low, narrow, sometimes discontinuous folds or ridges. Pericingular tabulation not observed.
Perisulcus weakly indicated by interruption of the pericingulum and a faint longitudinal invagination, extending shortly on both epi- and hypocysts.
Archeopyle apical, formed by detachment of the apex. Operculum attached in well preserved specimens or completely detached, apparently formed by four apical plates (type tA in Evitt 1967).
A sulcal indentation is frequently visible.
The periphragm consistently presents two additional ventral apertures located laterally symmetrical with respect to the longitudinal axis of the cyst. Antapical apertures ovoidal to subtriangular, with the base externally oriented; maximum diameter 12-18 µm.
Dimensions (15 specimens):
Overall length 55-65 µm; overall breadth 55-65 µm.
Affinities: (p.4-5):
The consistent combination of an apical archeopyle with the two ventral antapical orifices in the periphragm renders this dinocyst highly distinctive. It differs from Sirmiodinium Alberti, emend. Warren (1973), which has a gonyaulacystacean paratabulation and compound archeopyle involving the apical and the third precingular paraplates. The genera Scriniodinium Klement (1957) and Endoscrinium (Klement, 1960) Vozzhennikova (1967) both have a precingular archeopyle, while Lithodinia Eisenack, emend. Gocht (1975) is acavate. The genera Ascodinium Cookson and Eisenack (1960), Ovoidinium Davey, emend. Lentin and Williams (1975), and Hexagonifera Cookson and Eisenack (1961, 1962) differ from Arkellidinium by their peridinoid nature, chiefly by having combination archeopyles formed by detachment of the apical and anterior intercalary plates. The genus Senonisphaera Clarke and Verdier (1967), especially its type species S. protusa Clarke and Verdier (1967), has prominent apical and antapical horns and a tendency of forming two lateral horns.
The forms described by Gocht (1970) as Gen. et sp. indet. 1 appear to have a morphology similar to Arkellidinium triapertum, but it is not evident if the German specimens are triaperturate. Hexagonifera jurassica Gitmez and Sarjeant (1972) and Lithodinia staffiensis (Gitmez) Gocht (1975) also appear to be closely related to this genus.