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Charlesdowniea limitata
Charlesdowniea limitata, Stover and Hardenbol, 1994, p.34–35, pl.10, figs.70a–c,71a–c.
NOW Michouxdinium. Originally Charlesdowniea, subsequently (and now) Michouxdinium.
Holotype: Stover and Hardenbol, 1994, pl.10, figs.70a–c.
Locality: Both specimens are from Boom Composite Section at 11.20 meters, Steendorp locality.
Occurrence: Most Boom Formation occurrences of Charlesdowniea limitata are in the Terhagen Member between 10.71 and 14.47 meters in the Boom Composite Section. A single specimen was recorded from the Belsele-Waas Member at the 0.50 meter level.
Age: Rupelian.
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Original description: [Stover and Hardenbol, 1994, p.34–35]:
Description:
Cysts proximate, circumcavate and strongly compressed dorso-ventrally and with subquadragular to pentagular outlines.
Apical, two lateral and left antapical horns moderately to well developed; left antapical horn near longitudinal midline. Right antapical horn poorly developed or absent and when present, it is relatively close to the left antapical horn.
Endocyst circular or nearly so in outline, periphery close to the pericyst except at the bases of the horns, hence the lateral pericoel is narrow. The endocyst does not protrude into the horns.
Periphragm smooth; endophragm smooth or faintly ornamented and may have granulate-like thickenings in the cardinal positions.
Processes narrow, generally hollow and are mainly penitabular although scattered intratabular processes present on some paraplates. The fine, aculeate tips of the marginal processes on each paraplate are connected by thin trabeculae thereby forming penitabular process complexes which simulate the shapes of the paraplates.
Pandasutural areas essentially free of processes and intratabular processes are without trabecular connections.
Paratabulation is peridiniacean, formula: 4", 3a, 7", ?5c, 5""", 2"""; epicystal paratabulation styles ortho ventrally and quadra dorsally. A middorsal intercalary archeopyle has a monoplacoid (paraplate 2a), four-sided operculum with narrowly rounded posterior corners; the perioperculum is attached anteriorly and the endoperculum presumably has the same shape as the perioperculum.
Dimensions:
large; periphragm length and width 102 to 118 µm, endophragm length and width 75 to 85 µm, process length 3.5 to 5 µm.
Affinities:
Charlesdowniea limitata differs from C. aculeata (Michoux 1988) comb. nov. by having a substantially reduced or absent right antapical horn; when both antapical horns are developed the distance between them is considerably less than that on specimens of C. aculeata, owing mainly to the positioning of the left antapical horn on C. Iimitata. This horn is much closer to the longitudinal midline of C. limitata cysts. Also the right antapical horn is better developed on C. aculeata. Michoux (1985) stated that the endocysts of C. aculeata may protrude into the horns; this condition was not observed among specimens of C. limitata. On most other species of Charlesdowniea the processes either support a more
or less continuous ectophragmal covering over each paraplate, as in C. coleothrypta (Williams and Downie in Davey et al. 1966) Lentin and Vozzhennikova 1989, or the penitabular and intratabular processes are joined distally by an ectophragmal reticulum as in C. tenuivirgula (Williams and Downie in Davey et al. 1966) Lentin and Vozzhennikova 1989 and C. variabilis (Bujak in Bujak et al. 1980) Lentin and Vozzhennikova 1989.
NOW Michouxdinium. Originally Charlesdowniea, subsequently (and now) Michouxdinium.
Holotype: Stover and Hardenbol, 1994, pl.10, figs.70a–c.
Locality: Both specimens are from Boom Composite Section at 11.20 meters, Steendorp locality.
Occurrence: Most Boom Formation occurrences of Charlesdowniea limitata are in the Terhagen Member between 10.71 and 14.47 meters in the Boom Composite Section. A single specimen was recorded from the Belsele-Waas Member at the 0.50 meter level.
Age: Rupelian.
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Original description: [Stover and Hardenbol, 1994, p.34–35]:
Description:
Cysts proximate, circumcavate and strongly compressed dorso-ventrally and with subquadragular to pentagular outlines.
Apical, two lateral and left antapical horns moderately to well developed; left antapical horn near longitudinal midline. Right antapical horn poorly developed or absent and when present, it is relatively close to the left antapical horn.
Endocyst circular or nearly so in outline, periphery close to the pericyst except at the bases of the horns, hence the lateral pericoel is narrow. The endocyst does not protrude into the horns.
Periphragm smooth; endophragm smooth or faintly ornamented and may have granulate-like thickenings in the cardinal positions.
Processes narrow, generally hollow and are mainly penitabular although scattered intratabular processes present on some paraplates. The fine, aculeate tips of the marginal processes on each paraplate are connected by thin trabeculae thereby forming penitabular process complexes which simulate the shapes of the paraplates.
Pandasutural areas essentially free of processes and intratabular processes are without trabecular connections.
Paratabulation is peridiniacean, formula: 4", 3a, 7", ?5c, 5""", 2"""; epicystal paratabulation styles ortho ventrally and quadra dorsally. A middorsal intercalary archeopyle has a monoplacoid (paraplate 2a), four-sided operculum with narrowly rounded posterior corners; the perioperculum is attached anteriorly and the endoperculum presumably has the same shape as the perioperculum.
Dimensions:
large; periphragm length and width 102 to 118 µm, endophragm length and width 75 to 85 µm, process length 3.5 to 5 µm.
Affinities:
Charlesdowniea limitata differs from C. aculeata (Michoux 1988) comb. nov. by having a substantially reduced or absent right antapical horn; when both antapical horns are developed the distance between them is considerably less than that on specimens of C. aculeata, owing mainly to the positioning of the left antapical horn on C. Iimitata. This horn is much closer to the longitudinal midline of C. limitata cysts. Also the right antapical horn is better developed on C. aculeata. Michoux (1985) stated that the endocysts of C. aculeata may protrude into the horns; this condition was not observed among specimens of C. limitata. On most other species of Charlesdowniea the processes either support a more
or less continuous ectophragmal covering over each paraplate, as in C. coleothrypta (Williams and Downie in Davey et al. 1966) Lentin and Vozzhennikova 1989, or the penitabular and intratabular processes are joined distally by an ectophragmal reticulum as in C. tenuivirgula (Williams and Downie in Davey et al. 1966) Lentin and Vozzhennikova 1989 and C. variabilis (Bujak in Bujak et al. 1980) Lentin and Vozzhennikova 1989.