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Meiourogonyaulax distincta
Meiourogonyaulax distincta Smith and Harding, 2004, p.363,365,367, pl.4, figs.1-5; text-figs.3,4a-d.
Holotype: Smith and Harding, 2004, pl.4, figs.1-2.
Age: Early Valanginian.
Original description (Smith & Harding, 2004)
Holotype: Slide K17/0-10/3, England Finder coordinates N31/4. Sedgwick Museum number X.39371 ( Plate IV, 1,2). Paratype: Slide K17/0-5/2, England Finder coordinates N61. Sedgwick Museum number X.39372 ( Plate IV, 3). Type locality: Kashpir section, Volga Basin. Type horizon: Bed 24 (sample K17) sandstone with phosphatic concretions, Lower Valanginian. Range: only recorded in samples K17 and K18, both from the Early Valanginian (see Fig. 8). Etymology: named after its distinctive appearance in transmitted light.
Diagnosis: large, subspherical but slightly dorso-ventrally compressed proximate cysts, with a type (tA) archeopyle and a broad sulcal notch. The differentiated autophragm is sculpted into narrow muri which form a dense but evenly distributed reticulum with regularly sized and completely formed lumina, and which supports an entire (i.e. impunctate), microgranulate outer layer. Sulcus indicated by region of reduced height in the reticulum, sutural features indicate leptodinioid tabulation.
Description: large subspherical, but slightly dorso-ventrally compressed proximate cysts, often with a slightly flattened antapex. By strict definition ( Evitt, 1985, p. 65) the wall layer is a differentiated autophragm, and is sculpted into a dense and quite evenly distributed robust reticulation (Plate IV, 4) that supports a completely developed outer layer (Plate IV, 2). Mural height varies from 4.3 to 6.7 μm between specimens, but is generally markedly reduced at the sulcus (particularly at the junction of the sulcus and cingulum) to 1.5–5.7 μm. Mural width varies from 0.8 to 1.4 μm in all specimens measured. In cross section the muri are parallel-sided, but flare slightly at the contacts with the inner and outer layers of the differentiated autophragm. Luminal diameter may vary from 2 to 8 μm in a single specimen. Sutures are subtly indicated by muri of increased height and the sulcus and cingulum being underlain by muri of reduced height. Tabulation is most easily determined from the archeopyle margin (Plate IV, 1). The archeopyle is type (tA), with a free, simple polyplacoid operculum involving all four apical plate homologues (Plate IV, 5).
SEM examination reveals that the outermost layer of the autophragm is minutely granulate, and most specimens show masses of small circular intratabular depressions on the outer surface formed by ‘sagging’ of the outer layer into the lumina of the internal reticulation of the cyst wall (Plate V, 11). Moreover, the outer layer is clearly tabulate, although details of sulcal tabulation are suppressed (Plate V, 7). The central area of the sulcus bears small flagellar scars (Plate V, 7).
The tabulation formula is 4′, 6″, 6c, xs, 6‴, 1p, 1⁗, with a leptodinioid arrangement (Fig. 3). The presence/absence of an apical pore complex or an anterior intercalary (‘incidental’) plate cannot be confirmed.
The sutural areas are broad within the two adcingular series, between the post-cingular and antapical plates, within the cingular series, and at the junctions of the posterior intercalary plate with adjacent postcingular and antapical plates. Conversely, sutures are narrow between the adcingular and cingular series. Broad sutural regions appear to represent pandasutural bands, with fine striations on the overlapping plates perpendicular to the plate boundaries. Plates successively overlap from dorsal to ventral surfaces, and from the cingulum to the antapex (Fig. 4). The posterior intercalary is overlapped by post-cingular plate homologues *2‴ and *3‴, and itself overlaps *1‴, 1⁗ and the sulcal region. No complete specimens were studied, so plate overlap of the precingular series with the apical homologues has not been established.
Dimensions: width (across cingulum) (80) 87 (94) μm; height (antapex to archeopyle margin): (50) 59 (73) μm; depth: (80) 86 (90) μm. 12 specimens measured, all from sample K17.
Remarks: this species is allocated to the genus Meiourogonyaulax Sarjeant 1966 and not Lithodinia Eisenack emend. Gocht 1975, due to the development of a simple polyplacoid operculum (see Riding and Helby, 2001). The large cyst size and dense reticulum of narrow muri, and the pronounced reticulation of the differentiated autophragm make M. distincta quite different from other species of Meiourogonyaulax. This new species is not placed in Cassiculosphaeridia Davey 1969 or Valensiella Eisenack emend. Courtinat 1989 since most examples of these genera develop discontinuous but diaphanous ectophragms, and both are of uncertain gonyaulacalean affinity, whereas M. distincta reveals a clear leptodinioidean tabulation.
Holotype: Smith and Harding, 2004, pl.4, figs.1-2.
Age: Early Valanginian.
Original description (Smith & Harding, 2004)
Holotype: Slide K17/0-10/3, England Finder coordinates N31/4. Sedgwick Museum number X.39371 ( Plate IV, 1,2). Paratype: Slide K17/0-5/2, England Finder coordinates N61. Sedgwick Museum number X.39372 ( Plate IV, 3). Type locality: Kashpir section, Volga Basin. Type horizon: Bed 24 (sample K17) sandstone with phosphatic concretions, Lower Valanginian. Range: only recorded in samples K17 and K18, both from the Early Valanginian (see Fig. 8). Etymology: named after its distinctive appearance in transmitted light.
Diagnosis: large, subspherical but slightly dorso-ventrally compressed proximate cysts, with a type (tA) archeopyle and a broad sulcal notch. The differentiated autophragm is sculpted into narrow muri which form a dense but evenly distributed reticulum with regularly sized and completely formed lumina, and which supports an entire (i.e. impunctate), microgranulate outer layer. Sulcus indicated by region of reduced height in the reticulum, sutural features indicate leptodinioid tabulation.
Description: large subspherical, but slightly dorso-ventrally compressed proximate cysts, often with a slightly flattened antapex. By strict definition ( Evitt, 1985, p. 65) the wall layer is a differentiated autophragm, and is sculpted into a dense and quite evenly distributed robust reticulation (Plate IV, 4) that supports a completely developed outer layer (Plate IV, 2). Mural height varies from 4.3 to 6.7 μm between specimens, but is generally markedly reduced at the sulcus (particularly at the junction of the sulcus and cingulum) to 1.5–5.7 μm. Mural width varies from 0.8 to 1.4 μm in all specimens measured. In cross section the muri are parallel-sided, but flare slightly at the contacts with the inner and outer layers of the differentiated autophragm. Luminal diameter may vary from 2 to 8 μm in a single specimen. Sutures are subtly indicated by muri of increased height and the sulcus and cingulum being underlain by muri of reduced height. Tabulation is most easily determined from the archeopyle margin (Plate IV, 1). The archeopyle is type (tA), with a free, simple polyplacoid operculum involving all four apical plate homologues (Plate IV, 5).
SEM examination reveals that the outermost layer of the autophragm is minutely granulate, and most specimens show masses of small circular intratabular depressions on the outer surface formed by ‘sagging’ of the outer layer into the lumina of the internal reticulation of the cyst wall (Plate V, 11). Moreover, the outer layer is clearly tabulate, although details of sulcal tabulation are suppressed (Plate V, 7). The central area of the sulcus bears small flagellar scars (Plate V, 7).
The tabulation formula is 4′, 6″, 6c, xs, 6‴, 1p, 1⁗, with a leptodinioid arrangement (Fig. 3). The presence/absence of an apical pore complex or an anterior intercalary (‘incidental’) plate cannot be confirmed.
The sutural areas are broad within the two adcingular series, between the post-cingular and antapical plates, within the cingular series, and at the junctions of the posterior intercalary plate with adjacent postcingular and antapical plates. Conversely, sutures are narrow between the adcingular and cingular series. Broad sutural regions appear to represent pandasutural bands, with fine striations on the overlapping plates perpendicular to the plate boundaries. Plates successively overlap from dorsal to ventral surfaces, and from the cingulum to the antapex (Fig. 4). The posterior intercalary is overlapped by post-cingular plate homologues *2‴ and *3‴, and itself overlaps *1‴, 1⁗ and the sulcal region. No complete specimens were studied, so plate overlap of the precingular series with the apical homologues has not been established.
Dimensions: width (across cingulum) (80) 87 (94) μm; height (antapex to archeopyle margin): (50) 59 (73) μm; depth: (80) 86 (90) μm. 12 specimens measured, all from sample K17.
Remarks: this species is allocated to the genus Meiourogonyaulax Sarjeant 1966 and not Lithodinia Eisenack emend. Gocht 1975, due to the development of a simple polyplacoid operculum (see Riding and Helby, 2001). The large cyst size and dense reticulum of narrow muri, and the pronounced reticulation of the differentiated autophragm make M. distincta quite different from other species of Meiourogonyaulax. This new species is not placed in Cassiculosphaeridia Davey 1969 or Valensiella Eisenack emend. Courtinat 1989 since most examples of these genera develop discontinuous but diaphanous ectophragms, and both are of uncertain gonyaulacalean affinity, whereas M. distincta reveals a clear leptodinioidean tabulation.