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Phthanoperidinium obscurum
Phthanoperidinium obscurum Harland and Sharp, 1980, p.291,293–295, pl.1, figs.1–9, pl.2, figs.1–6; text-figs.3A–C.
Holotype: Harland and Sharp, 1980, pl.1, figs.1–2.
Age: late Eocene.
Derivation of name: Latin, obscurum; covered, dark, obscure, with reference to the obscure nature of this small and delicate cyst. Diagnosis: An orthoperidinioid cyst with small apical and antapical pericoels and parasutural ridges reflecting a tabulation of 4', 3a, 7", ?c, 5"', lps, 2"". Distinct apical structure often present. Ornament consists of penetabular and intratabular granules and bacules. Archeopyle, usually by loss of intercalary paraplate 2a, which is in a hexa configuration with its adjacent dorsal paraplates, and sometimes including paraplates 3" and 4". Dimensions: Holotype: length 52.0 µm, breadth 46.0 µm; Range: Length 31.0 [44.3] 54.0 µm, breadth 31.0 [43.3] 50.0 µm. Thirty-two specimens measured.
Description: The cyst is dorso-ventrally flattened and often appears fusiform in outline because of the poor development of the antapical horns, especially the left. The epicyst is cone-shaped whilst the hypocyst is hemispheroidal and they are separated by a planar or slightly laevo-rotary, non-indented paracingulum. The maximum displacement is 1/4 of the paracingulum's width. The cyst apex is surmounted by a low apical horn which in some specimens appears to be of a complex construction (Plate I, 3, 5, and 7) and may include a short, 1.0--2.0 gm, apical spine (Plate I, 7). Apart from small apical and antapical pericoels, separate wall layers cannot be distinguished elsewhere on the cyst. The ornamentation consists of pene- and intratabular granules and bacules, the latter often being bulbous or capitate distally. The penetabular ornament together with the faint, low parasutural ridges delineate a typical Phthanoperidiniurn tabulation (see Fig.3). The paracingulum and parasulcus are devoid of ornament and of any indication of tabulation; an exception to this is the recognition of a posterior parasulcal plate and a possible lateral or accessory parasulcal plate. Disposition of the ornament has also revealed, using the scanning electron microscope (see Plate II, 1 and 2) the presence of some growth bands, between certain paraplates. The standard hexa 2a intercalary paraplate is lost in archeopyle formation (Plate I, 1, 3 and 5; Plate II, 5 and 6) but may on occasions also include loss of paraplates 3" and 4" (Plate I, 7--9; Plate II, 3 and 4), a common phenomenon in Phthanoperidinium.
Holotype: Specimen MPK 2563; Plate I, 1 and 2. Type Horizon: Upper Headon Beds, late Eocene, 12.90--13.1 m depth in the I.G.S. Ramnor Inclosure Borehole, Hampshire, England. Variation: This species shows some variation in size and shape (Fig.2), with some specimens exhibiting an epicystal shoulder and variously developed horns.
Comparisons This species differs from all other described species of Phthanoperidinium in having low parasutural ridges, in being broader in shape and in the development of two, albeit small, antapical horns. It is also a small, pale and delicate cyst making it most inconspicuous in palynological preparations. The presence 295
of some penetabular ornament is only shared by two other species of the genus: P. echinaturn Eaton which is otherwise smooth and P. resistente (Morgenroth) Eisenack and Kjellström which has a prominent apical horn.
P. obscurum's association with the non-marine environment also makes it unique.
Holotype: Harland and Sharp, 1980, pl.1, figs.1–2.
Age: late Eocene.
Derivation of name: Latin, obscurum; covered, dark, obscure, with reference to the obscure nature of this small and delicate cyst. Diagnosis: An orthoperidinioid cyst with small apical and antapical pericoels and parasutural ridges reflecting a tabulation of 4', 3a, 7", ?c, 5"', lps, 2"". Distinct apical structure often present. Ornament consists of penetabular and intratabular granules and bacules. Archeopyle, usually by loss of intercalary paraplate 2a, which is in a hexa configuration with its adjacent dorsal paraplates, and sometimes including paraplates 3" and 4". Dimensions: Holotype: length 52.0 µm, breadth 46.0 µm; Range: Length 31.0 [44.3] 54.0 µm, breadth 31.0 [43.3] 50.0 µm. Thirty-two specimens measured.
Description: The cyst is dorso-ventrally flattened and often appears fusiform in outline because of the poor development of the antapical horns, especially the left. The epicyst is cone-shaped whilst the hypocyst is hemispheroidal and they are separated by a planar or slightly laevo-rotary, non-indented paracingulum. The maximum displacement is 1/4 of the paracingulum's width. The cyst apex is surmounted by a low apical horn which in some specimens appears to be of a complex construction (Plate I, 3, 5, and 7) and may include a short, 1.0--2.0 gm, apical spine (Plate I, 7). Apart from small apical and antapical pericoels, separate wall layers cannot be distinguished elsewhere on the cyst. The ornamentation consists of pene- and intratabular granules and bacules, the latter often being bulbous or capitate distally. The penetabular ornament together with the faint, low parasutural ridges delineate a typical Phthanoperidiniurn tabulation (see Fig.3). The paracingulum and parasulcus are devoid of ornament and of any indication of tabulation; an exception to this is the recognition of a posterior parasulcal plate and a possible lateral or accessory parasulcal plate. Disposition of the ornament has also revealed, using the scanning electron microscope (see Plate II, 1 and 2) the presence of some growth bands, between certain paraplates. The standard hexa 2a intercalary paraplate is lost in archeopyle formation (Plate I, 1, 3 and 5; Plate II, 5 and 6) but may on occasions also include loss of paraplates 3" and 4" (Plate I, 7--9; Plate II, 3 and 4), a common phenomenon in Phthanoperidinium.
Holotype: Specimen MPK 2563; Plate I, 1 and 2. Type Horizon: Upper Headon Beds, late Eocene, 12.90--13.1 m depth in the I.G.S. Ramnor Inclosure Borehole, Hampshire, England. Variation: This species shows some variation in size and shape (Fig.2), with some specimens exhibiting an epicystal shoulder and variously developed horns.
Comparisons This species differs from all other described species of Phthanoperidinium in having low parasutural ridges, in being broader in shape and in the development of two, albeit small, antapical horns. It is also a small, pale and delicate cyst making it most inconspicuous in palynological preparations. The presence 295
of some penetabular ornament is only shared by two other species of the genus: P. echinaturn Eaton which is otherwise smooth and P. resistente (Morgenroth) Eisenack and Kjellström which has a prominent apical horn.
P. obscurum's association with the non-marine environment also makes it unique.