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Bourkidinium cylindricum
Bourkidinium? cylindicum, Dolding, 1992, p. 313
Name not validly published, since lodgment of holotype not specified.
Dolding, 1992, questionably included this species in Bourkidinium.
Holotype: Dolding, 1992, figs.3c,4a-c
Locus typicus: Humps Island, James Ross Island Area, Antarctica
Stratum typicum: Late Campanian
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Original description: [Dolding, 1992, p. 313-315]:
Diagnosis:
Elongate cyst possessing an apical archaeopyle and cylindrical, truncated processes located about the antapex and archaeopyle margin. A broad area devoid of processes, coincident with the paracingular region, is present. No evidence of paratabulation is seen.
Description:
Small, longitudinally elongate, ellipsoidal cyst having a length to breadth ratio of c. 1.5: 1.
Simple, hollow, distally open processes are confined to the archaeopyle margin and the antapex. The processes show very little variation in morphology, being cylindrical or tapering slightly towards the truncate terminations and are 7-11 µm long (roughly half the breadth of the central body). The processes immediately adjacent to the archaeopyle margin typically number 6 or 7 (less on damaged specimens, e.g. Figs 3d & 4d) and it is unclear whether they are tabulate or nontabulate. Nine to thirteen antapical processes are present and these are clearly non-tabulate. The holotype has seven precingular processes (Figs 3c, 4a-c) and a clearly visible parasulcal notch (Figs 3c, 4a, in addition to 13 antapical processes. However, it would seem that the number of antapical processes has been reduced due to a small tear in the hypocyst.
The cyst appears to be finely granulate, although this may be due to corrosion, and lacks any features of paratabulation.
Dimensions (in µm): Min Mean Max
Overall length (central body): 23 28 32
Overall breadth (central body): 16 19 21
Process length: 7 9 11
Process width: 2 3 3.5
Specimens measured: 5
Affinities:
The allocation of this species to Bourkidinium is tentative, since while having characteristic features of the genus, other factors prevent confident assignment.
The number and location of the antapical processes accords well with the type species. However, Morgan (1975, p.160) suggested that the processes were "probably associated with only the apical and antapical series of reflected plates.". Morgan (1975, p.161) also stated that "all, or all but one or two of the apical group of processes are on the free operculum". Specimens illustrated by Singh (1983, pl. 42, figs. 6,7) corroborate this point. Bourkidinium? cylindricum and B. psilatum clearly have processes associated with the precingular paraplates.
B. granulatum and B. psilatum both differ from B.? cylindricum, by being larger and bearing processes with flared or recurved rather than simple truncate terminations, (see Fig. 3). B. granulatum also possesses a densely granulate surface ornament.
Tanyosphaeridium differs from Bourkidinium in having more numerous, non-tabular processes which uniformly cover the entire central body. Bourkidinium ? cylindricum resembles Tanyosphaeridium isocalamus process morphology, but the processes of the later are uniformly distributed and more numerous. It has been noted that some specimens of Tanyosphaeridium exhibit a naked equatorial area (e.g. Drugg 1967, pl. 4, fig. 10). However, Drugg"s illustrated specimen has more numerous processes (especially in the precingular region) than seen on any species of Bourkidinium. Also, the exposed paracingular area is not as fully developed as that of Bourkidinium.
Kaiwaradinium also has processes concentrated at the poles, but these are plate-centred in the same manner as those of Oligosphaeridium Davey & Williams 1966; emend. Davey 1982.
Clearly Bourkidinium and Tanyosphaeridium are very closely related and Bourkidinium? cylindricum appears to possess features characteristic of both genera. However, it is questionably assigned to Bourkidinium since all the recorded specimens have processes restricted towards the poles and the apical processes are contiguous with the archaeopyle margin (and are not present in a more adcingular position).
Name not validly published, since lodgment of holotype not specified.
Dolding, 1992, questionably included this species in Bourkidinium.
Holotype: Dolding, 1992, figs.3c,4a-c
Locus typicus: Humps Island, James Ross Island Area, Antarctica
Stratum typicum: Late Campanian
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Original description: [Dolding, 1992, p. 313-315]:
Diagnosis:
Elongate cyst possessing an apical archaeopyle and cylindrical, truncated processes located about the antapex and archaeopyle margin. A broad area devoid of processes, coincident with the paracingular region, is present. No evidence of paratabulation is seen.
Description:
Small, longitudinally elongate, ellipsoidal cyst having a length to breadth ratio of c. 1.5: 1.
Simple, hollow, distally open processes are confined to the archaeopyle margin and the antapex. The processes show very little variation in morphology, being cylindrical or tapering slightly towards the truncate terminations and are 7-11 µm long (roughly half the breadth of the central body). The processes immediately adjacent to the archaeopyle margin typically number 6 or 7 (less on damaged specimens, e.g. Figs 3d & 4d) and it is unclear whether they are tabulate or nontabulate. Nine to thirteen antapical processes are present and these are clearly non-tabulate. The holotype has seven precingular processes (Figs 3c, 4a-c) and a clearly visible parasulcal notch (Figs 3c, 4a, in addition to 13 antapical processes. However, it would seem that the number of antapical processes has been reduced due to a small tear in the hypocyst.
The cyst appears to be finely granulate, although this may be due to corrosion, and lacks any features of paratabulation.
Dimensions (in µm): Min Mean Max
Overall length (central body): 23 28 32
Overall breadth (central body): 16 19 21
Process length: 7 9 11
Process width: 2 3 3.5
Specimens measured: 5
Affinities:
The allocation of this species to Bourkidinium is tentative, since while having characteristic features of the genus, other factors prevent confident assignment.
The number and location of the antapical processes accords well with the type species. However, Morgan (1975, p.160) suggested that the processes were "probably associated with only the apical and antapical series of reflected plates.". Morgan (1975, p.161) also stated that "all, or all but one or two of the apical group of processes are on the free operculum". Specimens illustrated by Singh (1983, pl. 42, figs. 6,7) corroborate this point. Bourkidinium? cylindricum and B. psilatum clearly have processes associated with the precingular paraplates.
B. granulatum and B. psilatum both differ from B.? cylindricum, by being larger and bearing processes with flared or recurved rather than simple truncate terminations, (see Fig. 3). B. granulatum also possesses a densely granulate surface ornament.
Tanyosphaeridium differs from Bourkidinium in having more numerous, non-tabular processes which uniformly cover the entire central body. Bourkidinium ? cylindricum resembles Tanyosphaeridium isocalamus process morphology, but the processes of the later are uniformly distributed and more numerous. It has been noted that some specimens of Tanyosphaeridium exhibit a naked equatorial area (e.g. Drugg 1967, pl. 4, fig. 10). However, Drugg"s illustrated specimen has more numerous processes (especially in the precingular region) than seen on any species of Bourkidinium. Also, the exposed paracingular area is not as fully developed as that of Bourkidinium.
Kaiwaradinium also has processes concentrated at the poles, but these are plate-centred in the same manner as those of Oligosphaeridium Davey & Williams 1966; emend. Davey 1982.
Clearly Bourkidinium and Tanyosphaeridium are very closely related and Bourkidinium? cylindricum appears to possess features characteristic of both genera. However, it is questionably assigned to Bourkidinium since all the recorded specimens have processes restricted towards the poles and the apical processes are contiguous with the archaeopyle margin (and are not present in a more adcingular position).