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Isabelidinium foucheri
Isabelidinium foucheri Schioler, 1992
Holotype: Schioler, 1992, pl.1, figs.6-7
Locus typicus: Arnager Limestone Formation, Arnager Harbour, Bornholm, Denmark
Stratum typicum: Coniacian
Original diagnosis: Schioler, 1992, p. 12, 16
A relatively large species of Isahelidinium that has slender epi- and hypopericysts which are sharply delimited from the rounded, almost bulging central area of the pericyst. The epipericyst is elongate, subtriangular in ambitus, without pronounced shoulders. Periarchaeopyle rounded steno-deltaform with TAI 0.5 or higher. Endocyst in contact with pericyst laterally and dorsoventrally. The periphragm is psilate to scabrate, but may possess discrete granules on the apical and antapical areas.
Original description: Schioler, 1992, p. 16, 20
Cavate peridinoid cyst. The central part of the periphragm, which is occupied by the
endocyst, is + 1.5 x wider than the cavate portlons of the cyst. The margins of the free part of the periphragm, close to their contact with the endocyst, are subparallel. The apical part of the epipericyst tapers gently to a blunt-pointed horn, giving the epipericyst the overall shape of an elongate triangle. The epipericyst has no (or only weakly developed) shoulders. The lateral margins of the hypopericyst are parallel from their contact with the endocyst, until the left side tapers into an acute antapical horn. The endocyst is subcircular and often slightly broader than long. and is capped posteriorly by a mucoid layer or mesophragm which extends 3-5 Ám into the hypopericoel, giving the impression of a thickened posterior part of the endophragm. Some of the specimens observed have lost their endocyst, due to damage of the periphragm (Plate 1, 8). Paracingulum not present, but indicated by a periphragmal fold on a few specimens. Parasulcal depression sometimes present on the hypopericyst. The periarchaeopyle is of type 1(2a) and gently rounded steno-deltaform with TAI often above 0.5. The TAI-value is relatively high for the genus, and results from the slenderness of the upper part of the pericyst. Perioperculum free. Endoarchaeopyle type not determined. Apart from the periarchaeopyle, no other indications of paratabulation were observed.
Dimensions: holotype range
length of pericyst: 120 µm 110-131 µm
width of pericyst: 49 µm 38-53 µm
length of endocyst: 39 µm 30-51 µm
width of endocyst: 46 µm 34-50 µm
Specimens measured: 35
Affinities:
Schioler, 1992, p. 20: The strong morphological contrast between the slender peri- and hypopericyst and the bulging central part of the cyst readily separates this form from all other species of Isabelidinium. In comparison, Isabelidinium belfastense (Cookson and Eisenack 1961) Lentin and Williams 1977 lacks contact between the endo- and pericyst in the central area of the cyst and has distinct apical and antapical granulation on the pericyst. The new species bears some ambital similarity to the specimen of 1. helfastense figured by Kjellstrom (1973, p. 19, fig. 11) which, from the photograph, has a rather high TAI (0.48) and also appears to have an endocystal capping (or mesophragm). In contrast to l. foucherii, however, the specimen figured by Kjellstrom has coarse pericystal granulation. If the presence or absence of coarse granulation on the periphragm is insignificant for the distinction between I. belfastense and I. foucherii, the specimen figured by Kjellstrom should be included in the synonymy of the latter species. Isabelidinium amphiatum (Mclntyre 1975) Lentin and Williams 1977 differs from I. foucherii in having a large hexagonal archaeopyle that extends nearly to the paracingulum, as well as in lacking a pronounced left antapical horn, and I. microarmum (Mclntyre 1975) Lentin and Williams 1977 differs in always having a distinguisable endo- and periphragm, as well as having echinae on the apical and antapical part of the periphragm. Isabelidinium glabrum (Cookson and Eisenack 1969) Lentin and Williams 1977 has an almost biconical ambitus, and no contact between the endo- and periphragm, as well as a somewhat lower TAI (0.46, estimated from the photograph of the holotype) than I. foucherii Isabelidinium hakeri (Deflandre and Cookson 1955) Lentin and Williams 1977 is widely fusiform in ambitus with a bell-shaped epipericyst and has no pronounced left antapical horn, and I. pellucidum (Deflandre and Cookson 1955) Lentin and Williams 1977 has an elongate oval ambitus and a distinct hexagonal archaeopyle. Isabelidinium greenense Marshall 1990 has abundant fine perforations surrounding the horns, and often has indications of a paracingulum. Extreme variants of the latter species (Marshall 1990, fig. 227 A, B) have some ambital resemblance to I. foucherii, but are much larger. Isabelidinium korojonense (Cookson and Eisenack 1958) Lentin and Williams 1977 and Manumiella? cretacea (Cookson 1956) Bujak and Davies 1983 are considerably smaller than l. foucherii. Furthermore, I. korojonense has a quadrangular ambitus and an iso-omegaform archaeopyle whereas M.? cretacea has an almost circular ambitus and a relatively low TAI.
Holotype: Schioler, 1992, pl.1, figs.6-7
Locus typicus: Arnager Limestone Formation, Arnager Harbour, Bornholm, Denmark
Stratum typicum: Coniacian
Original diagnosis: Schioler, 1992, p. 12, 16
A relatively large species of Isahelidinium that has slender epi- and hypopericysts which are sharply delimited from the rounded, almost bulging central area of the pericyst. The epipericyst is elongate, subtriangular in ambitus, without pronounced shoulders. Periarchaeopyle rounded steno-deltaform with TAI 0.5 or higher. Endocyst in contact with pericyst laterally and dorsoventrally. The periphragm is psilate to scabrate, but may possess discrete granules on the apical and antapical areas.
Original description: Schioler, 1992, p. 16, 20
Cavate peridinoid cyst. The central part of the periphragm, which is occupied by the
endocyst, is + 1.5 x wider than the cavate portlons of the cyst. The margins of the free part of the periphragm, close to their contact with the endocyst, are subparallel. The apical part of the epipericyst tapers gently to a blunt-pointed horn, giving the epipericyst the overall shape of an elongate triangle. The epipericyst has no (or only weakly developed) shoulders. The lateral margins of the hypopericyst are parallel from their contact with the endocyst, until the left side tapers into an acute antapical horn. The endocyst is subcircular and often slightly broader than long. and is capped posteriorly by a mucoid layer or mesophragm which extends 3-5 Ám into the hypopericoel, giving the impression of a thickened posterior part of the endophragm. Some of the specimens observed have lost their endocyst, due to damage of the periphragm (Plate 1, 8). Paracingulum not present, but indicated by a periphragmal fold on a few specimens. Parasulcal depression sometimes present on the hypopericyst. The periarchaeopyle is of type 1(2a) and gently rounded steno-deltaform with TAI often above 0.5. The TAI-value is relatively high for the genus, and results from the slenderness of the upper part of the pericyst. Perioperculum free. Endoarchaeopyle type not determined. Apart from the periarchaeopyle, no other indications of paratabulation were observed.
Dimensions: holotype range
length of pericyst: 120 µm 110-131 µm
width of pericyst: 49 µm 38-53 µm
length of endocyst: 39 µm 30-51 µm
width of endocyst: 46 µm 34-50 µm
Specimens measured: 35
Affinities:
Schioler, 1992, p. 20: The strong morphological contrast between the slender peri- and hypopericyst and the bulging central part of the cyst readily separates this form from all other species of Isabelidinium. In comparison, Isabelidinium belfastense (Cookson and Eisenack 1961) Lentin and Williams 1977 lacks contact between the endo- and pericyst in the central area of the cyst and has distinct apical and antapical granulation on the pericyst. The new species bears some ambital similarity to the specimen of 1. helfastense figured by Kjellstrom (1973, p. 19, fig. 11) which, from the photograph, has a rather high TAI (0.48) and also appears to have an endocystal capping (or mesophragm). In contrast to l. foucherii, however, the specimen figured by Kjellstrom has coarse pericystal granulation. If the presence or absence of coarse granulation on the periphragm is insignificant for the distinction between I. belfastense and I. foucherii, the specimen figured by Kjellstrom should be included in the synonymy of the latter species. Isabelidinium amphiatum (Mclntyre 1975) Lentin and Williams 1977 differs from I. foucherii in having a large hexagonal archaeopyle that extends nearly to the paracingulum, as well as in lacking a pronounced left antapical horn, and I. microarmum (Mclntyre 1975) Lentin and Williams 1977 differs in always having a distinguisable endo- and periphragm, as well as having echinae on the apical and antapical part of the periphragm. Isabelidinium glabrum (Cookson and Eisenack 1969) Lentin and Williams 1977 has an almost biconical ambitus, and no contact between the endo- and periphragm, as well as a somewhat lower TAI (0.46, estimated from the photograph of the holotype) than I. foucherii Isabelidinium hakeri (Deflandre and Cookson 1955) Lentin and Williams 1977 is widely fusiform in ambitus with a bell-shaped epipericyst and has no pronounced left antapical horn, and I. pellucidum (Deflandre and Cookson 1955) Lentin and Williams 1977 has an elongate oval ambitus and a distinct hexagonal archaeopyle. Isabelidinium greenense Marshall 1990 has abundant fine perforations surrounding the horns, and often has indications of a paracingulum. Extreme variants of the latter species (Marshall 1990, fig. 227 A, B) have some ambital resemblance to I. foucherii, but are much larger. Isabelidinium korojonense (Cookson and Eisenack 1958) Lentin and Williams 1977 and Manumiella? cretacea (Cookson 1956) Bujak and Davies 1983 are considerably smaller than l. foucherii. Furthermore, I. korojonense has a quadrangular ambitus and an iso-omegaform archaeopyle whereas M.? cretacea has an almost circular ambitus and a relatively low TAI.