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Michouxdinium limitatum
Michouxdinium limitatum (Stover and Hardenbol, 1994, p.34–35, pl.10, figs.70a–c,71a–c) Williams et al., 2015, p.307.
Originally Charlesdowniea, subsequently (and now) Michouxdinium.
Holotype: Stover and Hardenbol, 1994, pl.10, figs.70a–c.
Age: Rupelian.
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. Size: large; periphragm length and width 102 to 118µm,
endophragm length and width 75 to 85µm, process length 3.5 to 5µm.
Comparison
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.
Types
Holotype: Plate 10, figures 70a-c. Paratype: Plate 10, figures 71a-c. 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.
Originally Charlesdowniea, subsequently (and now) Michouxdinium.
Holotype: Stover and Hardenbol, 1994, pl.10, figs.70a–c.
Age: Rupelian.
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. Size: large; periphragm length and width 102 to 118µm,
endophragm length and width 75 to 85µm, process length 3.5 to 5µm.
Comparison
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.
Types
Holotype: Plate 10, figures 70a-c. Paratype: Plate 10, figures 71a-c. 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.