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Meiourogonyaulax lacunosa
From Fensome et al., 2019:
Meiourogonyaulax lacunosa Duxbury, 2019, p.192–194, pl.17, figs.2–5,8,11,13. Holotype: Duxbury, 2019, pl.17, figs.8,11. Age: late Barremian.
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Paratype: Plate 17, Figures 2–4.
Type Locality: Heslerton No. 2 at 19.00 m (core chip), late Barremian age. Holotype E.F. K44.0, Paratype E.F. S51.4.
Derivation of Name: From the Latin lacunosus, full of hollows, cavities – in reference to the wall structure of this species.
Diagnosis: Alarge, thick-walled dinoflagellate cyst, subcircular in outline with finely and irregularly perforate “septa” forming a reticulum over much of the cyst surface, producing a pitted appearance. Tabulation of the formula ?', 6'', ?c, 6''', 1p, ?p.v., 1'''' is marked by low sutural ridges and by ectophragm folding along sutures. The height of the surface reticulum is particularly pronounced at the apex (producing a low, rounded prominence), around the cingulum and towards the antapex. There is a marked offset of cingular terminations and a clear sulcal furrow. The archeopyle is apical and the zig-zag archeopyle margin displays a distinct but shallow sulcal notch.
Dimensions: Holotype (complete): Length - 86 μm. Width - 81 μm.
Paratype (operculum lost): Length - 66 μm. Width - 86 μm.
Complete specimens (2 recorded): 86 (77) 68 μm × 81 (70) 58 μm.
Overall (operculum lost): Length - 73 (68) 63 μm; Width - 86 (74) 58 μm. Specimens Measured: 5.
Remarks: This species is recognisable even from fragmentary specimens, being covered in what appear to be subcircular pits resulting from the apparent thinning of the cyst wall at the centre of each mesh of the surface reticulum. The wall structure is similar to that in some species of Cassiculosphaeridia Davey 1969, particularly as described by Harding (1990, p. 49), for C. magna Davey 1974 emend. Harding 1990, although the presence of clearly-defined tabulation in M. lacunosa excludes this species from Cassiculosphaeridia. A similarly clear tabulation over a reticulate surface is observed in Ellipsoidictyum sagena (Duxbury 1980) Below 1982a, but that is a much smaller species with sutures usually, “delineated by single rows of pointed and occasionally ribbed denticles” (Harding 1990, p. 35). Meiourogonyaulax lacunosa is restricted to the 19.00 m sample (late Barremian) in the Heslerton No. 2 borehole and to the interval 11,190.00 ft to 11,154.00 ft in well 22/26a-2.
Meiourogonyaulax lacunosa Duxbury, 2019, p.192–194, pl.17, figs.2–5,8,11,13. Holotype: Duxbury, 2019, pl.17, figs.8,11. Age: late Barremian.
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Paratype: Plate 17, Figures 2–4.
Type Locality: Heslerton No. 2 at 19.00 m (core chip), late Barremian age. Holotype E.F. K44.0, Paratype E.F. S51.4.
Derivation of Name: From the Latin lacunosus, full of hollows, cavities – in reference to the wall structure of this species.
Diagnosis: Alarge, thick-walled dinoflagellate cyst, subcircular in outline with finely and irregularly perforate “septa” forming a reticulum over much of the cyst surface, producing a pitted appearance. Tabulation of the formula ?', 6'', ?c, 6''', 1p, ?p.v., 1'''' is marked by low sutural ridges and by ectophragm folding along sutures. The height of the surface reticulum is particularly pronounced at the apex (producing a low, rounded prominence), around the cingulum and towards the antapex. There is a marked offset of cingular terminations and a clear sulcal furrow. The archeopyle is apical and the zig-zag archeopyle margin displays a distinct but shallow sulcal notch.
Dimensions: Holotype (complete): Length - 86 μm. Width - 81 μm.
Paratype (operculum lost): Length - 66 μm. Width - 86 μm.
Complete specimens (2 recorded): 86 (77) 68 μm × 81 (70) 58 μm.
Overall (operculum lost): Length - 73 (68) 63 μm; Width - 86 (74) 58 μm. Specimens Measured: 5.
Remarks: This species is recognisable even from fragmentary specimens, being covered in what appear to be subcircular pits resulting from the apparent thinning of the cyst wall at the centre of each mesh of the surface reticulum. The wall structure is similar to that in some species of Cassiculosphaeridia Davey 1969, particularly as described by Harding (1990, p. 49), for C. magna Davey 1974 emend. Harding 1990, although the presence of clearly-defined tabulation in M. lacunosa excludes this species from Cassiculosphaeridia. A similarly clear tabulation over a reticulate surface is observed in Ellipsoidictyum sagena (Duxbury 1980) Below 1982a, but that is a much smaller species with sutures usually, “delineated by single rows of pointed and occasionally ribbed denticles” (Harding 1990, p. 35). Meiourogonyaulax lacunosa is restricted to the 19.00 m sample (late Barremian) in the Heslerton No. 2 borehole and to the interval 11,190.00 ft to 11,154.00 ft in well 22/26a-2.