Back
Microdinium gymnosuturum
MIcrodinium? gymnosuturum Smith, 1992
Smith, 1992, questionably included this species in Microdinium.
Holotype: Smith, 1992, figs.5a-b
Age: Late Campanian-Early Maastrichtian
Diagnosis: Smith, 1992, p. 342
A species, questionably placed in Microdiniun having paratabulation on the epicyst only, which is expressed by lack of ornament on the parasutures. The epicyst has 1 apical closing plate (acp), 3-4 apical paraplates, 5-6 anterior intercalary paraplates, and 7 precingular paraplates. There is intraspecific variability in the number and arrangement of apical and anterior intercalary paraplates.
Description: Smith, 1992, p. 342, 344
Cyst is ellipsoidal with a simple autophragm bearing no horns or projections. The surface ornament is composed of a dense covering of small (<1 Ám) grana over the entire cyst. The archaeopyle is apical and attached, of the type (tAtI)a. Accessory archaeopyle fissures (see Morphological term) are developed between the precingular paraplates, except between 6"" and 7"". A partial paratabulation is present, the parasutures are expressed as furrows devoid of the surface granulation. This paratabulation is normally developed on the epicyst and anterior margin of the paracingulum only. Rarely the posterior paracingular margin and some of the postcingular paraplates can be identified. The partial paratabulation formula for the cyst is acp, 3-4", 5-6a, 7"", Xc, as, Xs. The 7"" paraplate is always very small (Fig. 6 d). The number and configuration of the apical and intercalary paraplates is variable, at least three configurations can be identified (see Fig. 6). Variant 1 has the epicystal paraplate formula acp, 3", 5a, variant 2 has the epicystal formula acp, 3", 6a, and variant 3 has the epicystal formula acp, 4", 5a (Fig. 6).
Dimensions ( Ám):MinMeanMax
Overall length (not including adnate operculum)323640
Overall breadth283135
Specimens measured: 10
Affinities: Smith, 1992, p. 344
This species is questionably assigned to Microdinium as it possesses seven precingular paraplates, a (tAtI)a archaeopyle and a hypocyst which is larger than the epicyst.
Variant 1 with the epicystal paraplate formula acp, 3", 5a, 7"" is considered to be the basic paraplate configuration which is consistent with Phanerodinium carinatum Below (1987, p. 42, fig.10) a typical reresentative of fossil Cladopyxiaceae. This species was transferred to Microdinium by Lentin & Williams (1989, p. 290). Microdinium dentatum Vozzhenikova 1967 studied by Below (1987, p. 45) was also shown to have the epicystal paraplate formula 3", 5a, 7". It should be noted that Below (1987) stated the paraplate formulae of the two aforementioned species as 4", 4a, 7"", as he regarded the 5a paraplate as part of the apical series. This author feels that this paraplate is better regarded as an anterior intercalary paraplate as it does not contact the acp. From this it appears that typical representatives of the genus Microdinium have the same epicystal paratabulation as M.? gymosuturum sp. nov. In addition the epicystal paratabulation of the type species of Microdinium, M. ornatum figured by Evitt (1985, p.114, fig. 5.18 I) also has a comparable epicystal paraplate formula 3", 6a, 7"". However, using the Taylor-Evitt system of paraplate nomenclature, the small 7"" paraplate was designated by Evitt (1985) as 1i.
Variant 2 of M.? gymnosuturum sp. nov has an extra fourth apical paraplate which probably originates from the splitting of the 3" paraplate found in variant 1. Variant 3 is unusual as it has an exotic small triangular anterior intercalary paraplatelet, possibly originating from the splitting of paraplate 1a. Follo~ing the proposal of Evitt (1985 p. 92-93) the present author refers to the resulting paraplate fragments as "lav" and "lah", in order not to confuse the numbering of the remaining anterior intercalaries.
The expression of the parasutures in M.? gymnosuturum sp. nov. is different from other species in Microdinium. However, as the partial paratabulation and style of archaeopyle is consistent with the genus Microdinium, it is justified to questionably accommodate this new form in this genus. It is hoped that further studies will show whether this assignment can be maintained, eithernecessitating an eventual emendation of the generic diagnosis or the erection of a new genus.
Specimens of M.? gymnosuturum sp. nov. (variant 1) have been illustrated by Askin etal. 1991 (figs 13 m, n) in sediments from Cockburn Island as Microdinium/Druggidium sp., specimens of variants 1 and 2 are also present in material from False Island Point, Vega Island (personal observation).
Smith, 1992, questionably included this species in Microdinium.
Holotype: Smith, 1992, figs.5a-b
Age: Late Campanian-Early Maastrichtian
Diagnosis: Smith, 1992, p. 342
A species, questionably placed in Microdiniun having paratabulation on the epicyst only, which is expressed by lack of ornament on the parasutures. The epicyst has 1 apical closing plate (acp), 3-4 apical paraplates, 5-6 anterior intercalary paraplates, and 7 precingular paraplates. There is intraspecific variability in the number and arrangement of apical and anterior intercalary paraplates.
Description: Smith, 1992, p. 342, 344
Cyst is ellipsoidal with a simple autophragm bearing no horns or projections. The surface ornament is composed of a dense covering of small (<1 Ám) grana over the entire cyst. The archaeopyle is apical and attached, of the type (tAtI)a. Accessory archaeopyle fissures (see Morphological term) are developed between the precingular paraplates, except between 6"" and 7"". A partial paratabulation is present, the parasutures are expressed as furrows devoid of the surface granulation. This paratabulation is normally developed on the epicyst and anterior margin of the paracingulum only. Rarely the posterior paracingular margin and some of the postcingular paraplates can be identified. The partial paratabulation formula for the cyst is acp, 3-4", 5-6a, 7"", Xc, as, Xs. The 7"" paraplate is always very small (Fig. 6 d). The number and configuration of the apical and intercalary paraplates is variable, at least three configurations can be identified (see Fig. 6). Variant 1 has the epicystal paraplate formula acp, 3", 5a, variant 2 has the epicystal formula acp, 3", 6a, and variant 3 has the epicystal formula acp, 4", 5a (Fig. 6).
Dimensions ( Ám):MinMeanMax
Overall length (not including adnate operculum)323640
Overall breadth283135
Specimens measured: 10
Affinities: Smith, 1992, p. 344
This species is questionably assigned to Microdinium as it possesses seven precingular paraplates, a (tAtI)a archaeopyle and a hypocyst which is larger than the epicyst.
Variant 1 with the epicystal paraplate formula acp, 3", 5a, 7"" is considered to be the basic paraplate configuration which is consistent with Phanerodinium carinatum Below (1987, p. 42, fig.10) a typical reresentative of fossil Cladopyxiaceae. This species was transferred to Microdinium by Lentin & Williams (1989, p. 290). Microdinium dentatum Vozzhenikova 1967 studied by Below (1987, p. 45) was also shown to have the epicystal paraplate formula 3", 5a, 7". It should be noted that Below (1987) stated the paraplate formulae of the two aforementioned species as 4", 4a, 7"", as he regarded the 5a paraplate as part of the apical series. This author feels that this paraplate is better regarded as an anterior intercalary paraplate as it does not contact the acp. From this it appears that typical representatives of the genus Microdinium have the same epicystal paratabulation as M.? gymosuturum sp. nov. In addition the epicystal paratabulation of the type species of Microdinium, M. ornatum figured by Evitt (1985, p.114, fig. 5.18 I) also has a comparable epicystal paraplate formula 3", 6a, 7"". However, using the Taylor-Evitt system of paraplate nomenclature, the small 7"" paraplate was designated by Evitt (1985) as 1i.
Variant 2 of M.? gymnosuturum sp. nov has an extra fourth apical paraplate which probably originates from the splitting of the 3" paraplate found in variant 1. Variant 3 is unusual as it has an exotic small triangular anterior intercalary paraplatelet, possibly originating from the splitting of paraplate 1a. Follo~ing the proposal of Evitt (1985 p. 92-93) the present author refers to the resulting paraplate fragments as "lav" and "lah", in order not to confuse the numbering of the remaining anterior intercalaries.
The expression of the parasutures in M.? gymnosuturum sp. nov. is different from other species in Microdinium. However, as the partial paratabulation and style of archaeopyle is consistent with the genus Microdinium, it is justified to questionably accommodate this new form in this genus. It is hoped that further studies will show whether this assignment can be maintained, eithernecessitating an eventual emendation of the generic diagnosis or the erection of a new genus.
Specimens of M.? gymnosuturum sp. nov. (variant 1) have been illustrated by Askin etal. 1991 (figs 13 m, n) in sediments from Cockburn Island as Microdinium/Druggidium sp., specimens of variants 1 and 2 are also present in material from False Island Point, Vega Island (personal observation).