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Oligosphaeridium abaculum
Oligosphaeridium abaculum Davey, 1979a, p.428,430,432–433, pl.48, figs.1–6; pl.49, figs.1–7; pl.50, figs.1,4,10–11; text-figs.1–2. Holotype: Davey, 1979a, pl.49, figs.1,3; Fauconnier and Masure, 2004, pl.56, figs.1–3. Age: Barremian–early Albian.
Original diagnosis: Davey, 1979, p. 428-430
Shape: the pericyst, excluding processes, was originally spheroidal with only minor dorso-ventral flattening.
Wall: The cysts wall is apparently two-layered, the two layers being closely appressed except where the periphragm alone forms the processes. The wall is lightly to densely intraperforate, the surface is smooth to scabrate and sparsely pitted. The processes are smooth.
Paratabulation: the parasutures may be defined by very low ridges or by an apparent change in the internal wall structure; the clarity of the parasutures varies from poor to good with those in the parasutural region being the most poorly defined. The paratabulation formula is pr, 4", 6"", 6c, 6""", 1p, 5s, 1"""".
Processes: the processes are of tubiform shape, and flare distally typically giving rise to six slight flexuous spines; neighbouring spines may be joined medially to each other. Process stem width varies according to position on the cyst, with the apical, posterior, intercalary, and parasulcal processes being the most narrow. These processes are also the shortest but variation in length is not pronounced; process length is approximately equal to half the endocyst diameter. The process formula is 4", 6"", 5""", 1p, 1s, 1""""; the first postcingular paraplate (1""") does not bear a process and the parasulcal process occurs on the posterior sulcal paraplate (ps). Archaeopyle: an apical archaeopyle always appears to be present, and is developed by the detachment of the apical paraplates as a unit (typa tA). It has a strong zigzag margin with a deep parasulcal notch.
Original description: Davey, 1979, p.430-433
Wall: the pericyst wall is approximately 0.5 Ám thick and is not obviously two-layered. The density of the intraperforation varies only slightly with the individual and the consequent spongy appearance is obvious under the light microscope (Pl.50, fig.10,11); under the SEM the wall surface is smooth to very ligthly pitted (Pl.48, figs.1-6). The parasutures are immediately obvious under the light microscope but under the SEM are difficult (pl.48, fig.1,2), and are sometimes impossible, todiscern. Rarely do they take the form of hollow ridges (Pl.48, fig.4). This lack or paucity of surface expression suggests that the parasutural markings may be mainly features fomed by a change in wall structure such as a marked decrease in wall intraperforation.
Processes: the processes are smooth, without internal wall structure and the wall thickness is less than 0.5 Ám. Proximally, the bases of the processes typically have small to relatively large perforations (Pl.48, fig.2,3). When these are well developed a definite weakness of the process wall is developed and the process easily becomes broken at this point (Pl.48, fig.5; Pl.49, fig.2,5).
Paratabulation: the epicyst and hypocyst are of approximate equal size and are separated by a narrow paracingulum which on the ventral surface, is displaced by its own width (text-fig.1a). The ends of the paracingulum do not overlap. The parasulcus is not sinusoidal but is rather displaced approximately half its width to the left at the paracingulum. The sulcal paraplates as, rs, ls and ps are well defined; as is large and elongate, rs and ls are small and rectangular, and ps is relatively large and rectangular. The latter has a semi-circular boundary with paraplate 1"""". Paraplate ra lies above paraplates rs and ls and to the right of 1"""; its boundary with cingular paraplate 6c is poorly defined. At the posterior end of paraplate as, abutting against paraplates 6c, ra and 1""", are two depressions which corresponds to the thecal flagellar pores. The right depression is the smaller and more deeply indented. A small ovoidal preapical paraplate (pr) occupies the centre of the apical series (text-fig.2b). The apical paraplates 1" and 4" are essentially four-sided with 1" having an additional small side that abutts against paraplate as. Paraplates 2" and 3" are basically five-sided. The precingular paraplates have arcuate long margins and are narrowest at the cingular paraplate boundaries and widest, particularly on the dorsal surface (text-fig.1b), where the precingular parasutures abutt against them. The postcingular paraplates are large and basically rectangular except for 1""" which is small, rectangular, and occurs beneath as and above 1p. The latter is five-sided. The antapical paraplate (1"""") is basically five-sided with paraplate ps indenting its ventral margin (text-fig.2a). Intraspecific variation: this is sligth and involves details such as process length, width, and clarity of paratabulation.
However, one rare distinctive variant exists in which the stems of the processes are very short or apparently absent (Pl.50, fig.1,4). In the latter case, the stellate distal process terminations lie directly on the endocyst.
Affinities:
Davey, 1979, p. 433: The more or less clear paratabulation of O.abaculum, which is defined by low ridges or, more usually, by internal wall structuring, differentiates this species from all previously described species. Davey (1979b) erected a granular species of Oligosphaeridium, O.verrucosum, from the Aptian of the northern Bay of Biscay which sometimes possesses smooth parasutural areas. However, the granular nature of the periphragm and the form of the paratabulation differentiates O.verrucosum from O.abaculum. Evitt et al. (1977, p.4) record that Wiggins and Engelhardt have found tabulate specimens of Oligosphaeridium in the Lower Cretaceous of Alaska; it may be that these are comparable to the present species.
Original diagnosis: Davey, 1979, p. 428-430
Shape: the pericyst, excluding processes, was originally spheroidal with only minor dorso-ventral flattening.
Wall: The cysts wall is apparently two-layered, the two layers being closely appressed except where the periphragm alone forms the processes. The wall is lightly to densely intraperforate, the surface is smooth to scabrate and sparsely pitted. The processes are smooth.
Paratabulation: the parasutures may be defined by very low ridges or by an apparent change in the internal wall structure; the clarity of the parasutures varies from poor to good with those in the parasutural region being the most poorly defined. The paratabulation formula is pr, 4", 6"", 6c, 6""", 1p, 5s, 1"""".
Processes: the processes are of tubiform shape, and flare distally typically giving rise to six slight flexuous spines; neighbouring spines may be joined medially to each other. Process stem width varies according to position on the cyst, with the apical, posterior, intercalary, and parasulcal processes being the most narrow. These processes are also the shortest but variation in length is not pronounced; process length is approximately equal to half the endocyst diameter. The process formula is 4", 6"", 5""", 1p, 1s, 1""""; the first postcingular paraplate (1""") does not bear a process and the parasulcal process occurs on the posterior sulcal paraplate (ps). Archaeopyle: an apical archaeopyle always appears to be present, and is developed by the detachment of the apical paraplates as a unit (typa tA). It has a strong zigzag margin with a deep parasulcal notch.
Original description: Davey, 1979, p.430-433
Wall: the pericyst wall is approximately 0.5 Ám thick and is not obviously two-layered. The density of the intraperforation varies only slightly with the individual and the consequent spongy appearance is obvious under the light microscope (Pl.50, fig.10,11); under the SEM the wall surface is smooth to very ligthly pitted (Pl.48, figs.1-6). The parasutures are immediately obvious under the light microscope but under the SEM are difficult (pl.48, fig.1,2), and are sometimes impossible, todiscern. Rarely do they take the form of hollow ridges (Pl.48, fig.4). This lack or paucity of surface expression suggests that the parasutural markings may be mainly features fomed by a change in wall structure such as a marked decrease in wall intraperforation.
Processes: the processes are smooth, without internal wall structure and the wall thickness is less than 0.5 Ám. Proximally, the bases of the processes typically have small to relatively large perforations (Pl.48, fig.2,3). When these are well developed a definite weakness of the process wall is developed and the process easily becomes broken at this point (Pl.48, fig.5; Pl.49, fig.2,5).
Paratabulation: the epicyst and hypocyst are of approximate equal size and are separated by a narrow paracingulum which on the ventral surface, is displaced by its own width (text-fig.1a). The ends of the paracingulum do not overlap. The parasulcus is not sinusoidal but is rather displaced approximately half its width to the left at the paracingulum. The sulcal paraplates as, rs, ls and ps are well defined; as is large and elongate, rs and ls are small and rectangular, and ps is relatively large and rectangular. The latter has a semi-circular boundary with paraplate 1"""". Paraplate ra lies above paraplates rs and ls and to the right of 1"""; its boundary with cingular paraplate 6c is poorly defined. At the posterior end of paraplate as, abutting against paraplates 6c, ra and 1""", are two depressions which corresponds to the thecal flagellar pores. The right depression is the smaller and more deeply indented. A small ovoidal preapical paraplate (pr) occupies the centre of the apical series (text-fig.2b). The apical paraplates 1" and 4" are essentially four-sided with 1" having an additional small side that abutts against paraplate as. Paraplates 2" and 3" are basically five-sided. The precingular paraplates have arcuate long margins and are narrowest at the cingular paraplate boundaries and widest, particularly on the dorsal surface (text-fig.1b), where the precingular parasutures abutt against them. The postcingular paraplates are large and basically rectangular except for 1""" which is small, rectangular, and occurs beneath as and above 1p. The latter is five-sided. The antapical paraplate (1"""") is basically five-sided with paraplate ps indenting its ventral margin (text-fig.2a). Intraspecific variation: this is sligth and involves details such as process length, width, and clarity of paratabulation.
However, one rare distinctive variant exists in which the stems of the processes are very short or apparently absent (Pl.50, fig.1,4). In the latter case, the stellate distal process terminations lie directly on the endocyst.
Affinities:
Davey, 1979, p. 433: The more or less clear paratabulation of O.abaculum, which is defined by low ridges or, more usually, by internal wall structuring, differentiates this species from all previously described species. Davey (1979b) erected a granular species of Oligosphaeridium, O.verrucosum, from the Aptian of the northern Bay of Biscay which sometimes possesses smooth parasutural areas. However, the granular nature of the periphragm and the form of the paratabulation differentiates O.verrucosum from O.abaculum. Evitt et al. (1977, p.4) record that Wiggins and Engelhardt have found tabulate specimens of Oligosphaeridium in the Lower Cretaceous of Alaska; it may be that these are comparable to the present species.