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Cassiculosphaeridia calvata
From Fensome et al., 2019:
Cassiculosphaeridia calvata Duxbury, 2019, p.182–183, pl.12, figs.3,6,9,12. Holotype: Duxbury, 2019, pl.12, figs.3,6,9. Age: late
Barremian.
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Original description Duxbury, 2019
Holotype: Plate 12, Figures 3, 6, 9.
Type Locality: Heslerton No. 2 at 26.00 m (core chip), late
Barremian age. Holotype: E.F. Y29.4.
Derivation of Name: From the Latin calvatus, made bare, bald - in reference to the reduced ornament.
Diagnosis: A large species of Cassiculosphaeridia characterised by significant reduction or loss of reticulation in the mid-dorsal and mid-ventral areas. Clear separation of autophragm and ectophragm is observed, particularly around the cingulum, producing a prominent paracingular bulge. Archeopyle tetratabular apical with the operculum usually lost.
Dimensions: Holotype: Length - 81 μm; Width - 101 μm.
Overall: Length - 81 (79) 76 μm; Width - 101 (96) 89 μm.
Specimens Measured: 4 (all with operculum detached).
Remarks: Harding (1990, p. 51) referred to the extinction of Cassiculosphaeridia magna Davey 1974 “during Hauptblatterton event times”, early Barremian, contrary to quoted ranges in Davey (1974) and Duxbury (1980) which included occurrences of a late Barremian age. He speculated that, “The anomalous ranges cited by Davey and Duxbury may well refer to records of a similar large reticulate dinocyst which occurs sporadically from the end of the Hauptblatterton event into late Barremian time. This form has an apical archeopyle (tA), but is also characterised by a prominent paracingular bulge and a noticeable sulcal groove”.
Although not illustrated by Harding, the “similar large reticulate dinocyst” referred to by him was noted in the current study and is described here as Cassiculosphaeridia calvata. The stratigraphic range of C. calvata is confined to the late Barremian in Heslerton No. 2 (26.00 m to 22.00 m samples), whereas C. magna ranges only as high as the 27.05 m sample. The transition between the two species appears therefore to be within the late Barremian, a younger level than suggested by Harding (op. cit., p. 51).
This species represents the youngest occurrence of large Cassiculosphaeridia spp. and its development is interpreted as a response to environmental pressures (currently unresolved) on the very long-ranging species Cassiculosphaeridia magna towards the top of its range. The transition from Cassiculosphaeridia magna to C. calvata appears to involve a “loosening” of the ectophragm layer and loss of the characteristic clear surface reticulation. Ultimately, only hints of the original reticulation are retained, confined to the cingulum and cyst margin, with the mid-dorsal and mid-ventral areas clear. The cingular bulge produced by wall layer separation is particularly notable.
Cassiculosphaeridia calvata Duxbury, 2019, p.182–183, pl.12, figs.3,6,9,12. Holotype: Duxbury, 2019, pl.12, figs.3,6,9. Age: late
Barremian.
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Original description Duxbury, 2019
Holotype: Plate 12, Figures 3, 6, 9.
Type Locality: Heslerton No. 2 at 26.00 m (core chip), late
Barremian age. Holotype: E.F. Y29.4.
Derivation of Name: From the Latin calvatus, made bare, bald - in reference to the reduced ornament.
Diagnosis: A large species of Cassiculosphaeridia characterised by significant reduction or loss of reticulation in the mid-dorsal and mid-ventral areas. Clear separation of autophragm and ectophragm is observed, particularly around the cingulum, producing a prominent paracingular bulge. Archeopyle tetratabular apical with the operculum usually lost.
Dimensions: Holotype: Length - 81 μm; Width - 101 μm.
Overall: Length - 81 (79) 76 μm; Width - 101 (96) 89 μm.
Specimens Measured: 4 (all with operculum detached).
Remarks: Harding (1990, p. 51) referred to the extinction of Cassiculosphaeridia magna Davey 1974 “during Hauptblatterton event times”, early Barremian, contrary to quoted ranges in Davey (1974) and Duxbury (1980) which included occurrences of a late Barremian age. He speculated that, “The anomalous ranges cited by Davey and Duxbury may well refer to records of a similar large reticulate dinocyst which occurs sporadically from the end of the Hauptblatterton event into late Barremian time. This form has an apical archeopyle (tA), but is also characterised by a prominent paracingular bulge and a noticeable sulcal groove”.
Although not illustrated by Harding, the “similar large reticulate dinocyst” referred to by him was noted in the current study and is described here as Cassiculosphaeridia calvata. The stratigraphic range of C. calvata is confined to the late Barremian in Heslerton No. 2 (26.00 m to 22.00 m samples), whereas C. magna ranges only as high as the 27.05 m sample. The transition between the two species appears therefore to be within the late Barremian, a younger level than suggested by Harding (op. cit., p. 51).
This species represents the youngest occurrence of large Cassiculosphaeridia spp. and its development is interpreted as a response to environmental pressures (currently unresolved) on the very long-ranging species Cassiculosphaeridia magna towards the top of its range. The transition from Cassiculosphaeridia magna to C. calvata appears to involve a “loosening” of the ectophragm layer and loss of the characteristic clear surface reticulation. Ultimately, only hints of the original reticulation are retained, confined to the cingulum and cyst margin, with the mid-dorsal and mid-ventral areas clear. The cingular bulge produced by wall layer separation is particularly notable.