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Perisseiasphaeridium cretaceum
From Fensome et al., 2019:
Perisseiasphaeridium cretaceum Duxbury, 2018, p.181, pl.7, figs.7,9–10. Holotype: Duxbury, 2018, pl.7, fig.7. Age: early Valanginian.
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Original description Duxbury, 2018:
Plate 7, figures 7, 9, 10
Holotype: Plate 7, Figure 7.
Paratype: Plate 7, Figure 10.
Type Locality: Well 20/01-8X at 8817.50 ft (conventional core chip), lower Valanginian. Holotype: E.F. D42.0. Paratype: E.F. G34.4
Derivation of Name: From Cretaceous – in reference to the recorded age of this species.
Diagnosis: Alarge, robust, chorate dinocyst. The smooth, spheroidal central body bears mesotabular, tubular processes, which expand distally into digitate terminations. Thickened annular areas are present where processes meet the main body. Holes are sometimes present within distal process terminations and, in rare cases, although they are proximally entire, the distal half to two-thirds of some process tubes are incomplete, being open on one side. Six cingular processes are also present, each comprising two very slender elements arising from a low, ellipsoidal proximal crest; these are of equal length to the tubular processes. The archeopyle is apical, tetratabular, and the operculum is invariably detached.
Dimensions: Holotype (operculum detached): Central body – 48 × 48 μm
Overall: 104 × 91 μm.
All measured specimens (operculum detached): Central body – 56 (49) 41 μm × 58 (51) 43 μm.
Overall: 124 (99) 81 μm × 114 (98) 89 μm.
Specimens Measured: 20.
Remarks: This species is very similar to Oligosphaeridium complex (White 1842) Davey and Williams 1966, the tubular processes differing only in small details, including the occasional presence of holes in the distal process terminations and the rare tendency to incomplete distal tube margins. The presence of slender cingular elements in addition to the tubular processes is, however, typical of Perisseiasphaeridium.
Another species similar to Perisseiasphaeridium cretaceum n. sp. is Perisseiasphaeridium insolitum Davey 1982, which is of a similar size and which has similar processes. However, P. insolitum is a more delicate species, tending to a thinner cyst wall and relatively slender tubular elements. In addition, the four sulcal processes described by Davey (op. cit.) and the occasional presence of longitudinal process striations were not seen in P. cretaceum n. sp. Also, the circular thickenings present at process bases in P. cretaceum n. sp. are not present in P. insolitum.
The age range given for Perisseiasphaeridium insolitum by Davey (op. cit.) in the Haldager No.1 borehole was early Portlandian to early Ryazanian, with its most frequent and youngest occurrence in core 2969'–2989', dated as late Portlandian to early Ryazanian. In the UKCS, Central North Sea, P. insolitum is common only within the uppermost middle Volgian, Zone UJP13 (Duxbury et al. 1999), being rare in the late Volgian, Subzone UJP14.1, well below the Ryazanian. The very short stratigraphic range of P. cretaceum n. sp. is within the lower Valanginian, Palyzone LKP6, immediately below the FAD of Oligosphaeridium complex. This and its very close morphological similarity to that species strongly suggeststhat P. cretaceum might be the immediate precursor of O. complex.
Perisseiasphaeridium cretaceum Duxbury, 2018, p.181, pl.7, figs.7,9–10. Holotype: Duxbury, 2018, pl.7, fig.7. Age: early Valanginian.
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Original description Duxbury, 2018:
Plate 7, figures 7, 9, 10
Holotype: Plate 7, Figure 7.
Paratype: Plate 7, Figure 10.
Type Locality: Well 20/01-8X at 8817.50 ft (conventional core chip), lower Valanginian. Holotype: E.F. D42.0. Paratype: E.F. G34.4
Derivation of Name: From Cretaceous – in reference to the recorded age of this species.
Diagnosis: Alarge, robust, chorate dinocyst. The smooth, spheroidal central body bears mesotabular, tubular processes, which expand distally into digitate terminations. Thickened annular areas are present where processes meet the main body. Holes are sometimes present within distal process terminations and, in rare cases, although they are proximally entire, the distal half to two-thirds of some process tubes are incomplete, being open on one side. Six cingular processes are also present, each comprising two very slender elements arising from a low, ellipsoidal proximal crest; these are of equal length to the tubular processes. The archeopyle is apical, tetratabular, and the operculum is invariably detached.
Dimensions: Holotype (operculum detached): Central body – 48 × 48 μm
Overall: 104 × 91 μm.
All measured specimens (operculum detached): Central body – 56 (49) 41 μm × 58 (51) 43 μm.
Overall: 124 (99) 81 μm × 114 (98) 89 μm.
Specimens Measured: 20.
Remarks: This species is very similar to Oligosphaeridium complex (White 1842) Davey and Williams 1966, the tubular processes differing only in small details, including the occasional presence of holes in the distal process terminations and the rare tendency to incomplete distal tube margins. The presence of slender cingular elements in addition to the tubular processes is, however, typical of Perisseiasphaeridium.
Another species similar to Perisseiasphaeridium cretaceum n. sp. is Perisseiasphaeridium insolitum Davey 1982, which is of a similar size and which has similar processes. However, P. insolitum is a more delicate species, tending to a thinner cyst wall and relatively slender tubular elements. In addition, the four sulcal processes described by Davey (op. cit.) and the occasional presence of longitudinal process striations were not seen in P. cretaceum n. sp. Also, the circular thickenings present at process bases in P. cretaceum n. sp. are not present in P. insolitum.
The age range given for Perisseiasphaeridium insolitum by Davey (op. cit.) in the Haldager No.1 borehole was early Portlandian to early Ryazanian, with its most frequent and youngest occurrence in core 2969'–2989', dated as late Portlandian to early Ryazanian. In the UKCS, Central North Sea, P. insolitum is common only within the uppermost middle Volgian, Zone UJP13 (Duxbury et al. 1999), being rare in the late Volgian, Subzone UJP14.1, well below the Ryazanian. The very short stratigraphic range of P. cretaceum n. sp. is within the lower Valanginian, Palyzone LKP6, immediately below the FAD of Oligosphaeridium complex. This and its very close morphological similarity to that species strongly suggeststhat P. cretaceum might be the immediate precursor of O. complex.