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Perisseiasphaeridium inusitatum
Perisseiasphaeridium inusitatum Stevens and Helby, 1987
Holotype: Stevens and Helby, 1987, Fig.14A-B,E-F
Locus typicus: Exmouth Plateau, Australia
Stratum typicum: Early Berriasian
Original description: Stevens and Helby, 1987, p. 179-180
Cysts skolochorate and subspherical, commonly with a pronounced square to rectangular outline.
Cyst wall: 2-layered with endophragm and periphragm closely appressed, except around process bases. Both layers less than 1 µm thick. Endophragm smooth, but with short radial striations evident at the bases of the larger processes on some specimens. Periphragm smooth, scabrate or more commonly perforate (perforations are fine, closely packed, and less than 0.5µm in diameter); perforation not normally extending onto the shaft of the processes.
Processes: Periphragm bearing both solid and hollow intratabular processes, and on some specimens low parasutural ridges. Single, intratabular hollow, distally open processes originating from each of the precingular, postcingular, posterior intercalary and antapical paraplates. Process length ranging up to 40 µm and width varying from 2-16 µm at mid shaft. Basal diameter of these processes varying from 3-10 µm; end of each shaft distinguished by an annular thickening of the endophragm (Fig. 14F). Periphragm may be detached at these locations. Processes may display scattered, large, irregular perforations, up to 5 µm in maximum dimension, extremities generally completely branched (Fig.13Ca-c, 14F). Processes on the cingular paraplate series occurring as paired, intratabular spines (Figs 13Cd-f, 14G). Processes solid, slender (up to 28 µm long and about 1µm wide along most of their length), commonly pointed, but may be branched and generally with slightly expanded bases (up to 1.5 µm diameter). Paired cingular processes, up to 1 µm wide and 14 µm long, may be connected proximally by a low ridge (Figs 13Ce, 14G), and may have thin trabecular cross-bars connecting the pair, although these do not occur above mid shaft (Fig. 13C). Sulcal processes not observed. Free or attached opercula not observed so processes on apical series not confirmed. By comparison with other chorate cysts 4 apical processes assumed to be present.
Paratabulation: expressed by low parasutural ridges, intratabular processes and the archeopyle. Parasutural ridges generally apparent at the paracingulum but may be well-developed over the remainder of the cyst. Paratabulation gonyaulacacean; ?4 ", 6", 6c, 5""", 1p, 1"""", 0s. Smaller paraplates such as 4", 6", 2""", 1p marked by shorter, narrower, intratabular processes with much smaller basal diameters. Cingular paraplates 2c to 5c appearing elongate and 6-sided, while 1c and 6c possibly 4-sided. Variable distance between paired processes on the cingular paraplates, proportional to the long dimension of the paraplate.
Archeopyle: apical, type uncertain; principal archeopyle suture zigzag, accessory sutures often extending almost to the paracingulum; sulcal notch relatively deep. Operculum: free (not observed in sample residues). Archeopyle outline indicating that the operculum would comprise 4 apical paraplates.
Paracingulum: generally marked by low parasutural ridges (0.5 µm high and 0.5-1 µm wide) and characterized by paired intratabular processes on each paraplate.
Ends of the paracingulum offset by 1 to 1.5 paracingulum widths. Anterior margin of the parasulcus marked by a sulcal notch, parasulcus itself commonly poorly delineated.
Dimensions:
Cyst length (dehisced and excluding processes) 50(71)86 µm, cyst width (excluding processes) 44(65)80 µm; 25 specimens
Variability:
The cyst size and shape and the ratio of process length to overall body width (approximately 1:2) are fairly constant. In contrast, process size and shape is extremely variable, particularly in the precingular and postcingular series, where, within one series, process size is apparently relative to the size of the paraplates, e.g. the 4"" and 6"" and the 2""" and 6""" have smaller processes than those on the other paraplates in these two series (Fig.13A,B). The process shape varies with basal diameter and the degree of branching (Fig.13C). The paired cingular processes are slender and fairly constant in size; the processes in each pair may be connected by cross bars or a low proximate ridge (Figs 13Cd-e, 14E,G) within each cingular paraplate. The paratabulation is indicated by the intratabular processes, but also on many specimens by low parasutural ridges (Fig.14A,B).
Affinities:
Stevens and Helby, 1987, p. 181: Perisseiasphaeridium insitatum closely resembles P. insolitum Davey 1982. The latter species is distinguished by the
occurrence of up to 4 sulcal processes and the predominance of bifurcate tips on the paired cingulate processes. Perisseiasphaeridium insolitum is nontabulate and has simple tubiform processes which are less complexly divided distally than P. inusitatum. It also lacks the proximal cross bars, which often connect the paired cingulate processes, and the annular thickening of the endophragm at the base of the major processes both of which typify P. inusitatum. Davey (1982, p.19) commented that Perisseiasphaeridium represented a morphology transitional between Oligosphaeridium Davey & Williams in Davey et al. 1966 and Systematophora Klement 1960. The diagnosis of Perisseiasphaeridium given by Stover & Evitt (1978, p.73) does not specify the number of processess on each cingular paraplate. However, the disposition of cingular processes on the type species, P. pannosus (Davey & Williams in Davey et al. 1966, fig.15; pl.ll, fig.8) suggests that at least some of the cingular paraplates bear paired processes.
Holotype: Stevens and Helby, 1987, Fig.14A-B,E-F
Locus typicus: Exmouth Plateau, Australia
Stratum typicum: Early Berriasian
Original description: Stevens and Helby, 1987, p. 179-180
Cysts skolochorate and subspherical, commonly with a pronounced square to rectangular outline.
Cyst wall: 2-layered with endophragm and periphragm closely appressed, except around process bases. Both layers less than 1 µm thick. Endophragm smooth, but with short radial striations evident at the bases of the larger processes on some specimens. Periphragm smooth, scabrate or more commonly perforate (perforations are fine, closely packed, and less than 0.5µm in diameter); perforation not normally extending onto the shaft of the processes.
Processes: Periphragm bearing both solid and hollow intratabular processes, and on some specimens low parasutural ridges. Single, intratabular hollow, distally open processes originating from each of the precingular, postcingular, posterior intercalary and antapical paraplates. Process length ranging up to 40 µm and width varying from 2-16 µm at mid shaft. Basal diameter of these processes varying from 3-10 µm; end of each shaft distinguished by an annular thickening of the endophragm (Fig. 14F). Periphragm may be detached at these locations. Processes may display scattered, large, irregular perforations, up to 5 µm in maximum dimension, extremities generally completely branched (Fig.13Ca-c, 14F). Processes on the cingular paraplate series occurring as paired, intratabular spines (Figs 13Cd-f, 14G). Processes solid, slender (up to 28 µm long and about 1µm wide along most of their length), commonly pointed, but may be branched and generally with slightly expanded bases (up to 1.5 µm diameter). Paired cingular processes, up to 1 µm wide and 14 µm long, may be connected proximally by a low ridge (Figs 13Ce, 14G), and may have thin trabecular cross-bars connecting the pair, although these do not occur above mid shaft (Fig. 13C). Sulcal processes not observed. Free or attached opercula not observed so processes on apical series not confirmed. By comparison with other chorate cysts 4 apical processes assumed to be present.
Paratabulation: expressed by low parasutural ridges, intratabular processes and the archeopyle. Parasutural ridges generally apparent at the paracingulum but may be well-developed over the remainder of the cyst. Paratabulation gonyaulacacean; ?4 ", 6", 6c, 5""", 1p, 1"""", 0s. Smaller paraplates such as 4", 6", 2""", 1p marked by shorter, narrower, intratabular processes with much smaller basal diameters. Cingular paraplates 2c to 5c appearing elongate and 6-sided, while 1c and 6c possibly 4-sided. Variable distance between paired processes on the cingular paraplates, proportional to the long dimension of the paraplate.
Archeopyle: apical, type uncertain; principal archeopyle suture zigzag, accessory sutures often extending almost to the paracingulum; sulcal notch relatively deep. Operculum: free (not observed in sample residues). Archeopyle outline indicating that the operculum would comprise 4 apical paraplates.
Paracingulum: generally marked by low parasutural ridges (0.5 µm high and 0.5-1 µm wide) and characterized by paired intratabular processes on each paraplate.
Ends of the paracingulum offset by 1 to 1.5 paracingulum widths. Anterior margin of the parasulcus marked by a sulcal notch, parasulcus itself commonly poorly delineated.
Dimensions:
Cyst length (dehisced and excluding processes) 50(71)86 µm, cyst width (excluding processes) 44(65)80 µm; 25 specimens
Variability:
The cyst size and shape and the ratio of process length to overall body width (approximately 1:2) are fairly constant. In contrast, process size and shape is extremely variable, particularly in the precingular and postcingular series, where, within one series, process size is apparently relative to the size of the paraplates, e.g. the 4"" and 6"" and the 2""" and 6""" have smaller processes than those on the other paraplates in these two series (Fig.13A,B). The process shape varies with basal diameter and the degree of branching (Fig.13C). The paired cingular processes are slender and fairly constant in size; the processes in each pair may be connected by cross bars or a low proximate ridge (Figs 13Cd-e, 14E,G) within each cingular paraplate. The paratabulation is indicated by the intratabular processes, but also on many specimens by low parasutural ridges (Fig.14A,B).
Affinities:
Stevens and Helby, 1987, p. 181: Perisseiasphaeridium insitatum closely resembles P. insolitum Davey 1982. The latter species is distinguished by the
occurrence of up to 4 sulcal processes and the predominance of bifurcate tips on the paired cingulate processes. Perisseiasphaeridium insolitum is nontabulate and has simple tubiform processes which are less complexly divided distally than P. inusitatum. It also lacks the proximal cross bars, which often connect the paired cingulate processes, and the annular thickening of the endophragm at the base of the major processes both of which typify P. inusitatum. Davey (1982, p.19) commented that Perisseiasphaeridium represented a morphology transitional between Oligosphaeridium Davey & Williams in Davey et al. 1966 and Systematophora Klement 1960. The diagnosis of Perisseiasphaeridium given by Stover & Evitt (1978, p.73) does not specify the number of processess on each cingular paraplate. However, the disposition of cingular processes on the type species, P. pannosus (Davey & Williams in Davey et al. 1966, fig.15; pl.ll, fig.8) suggests that at least some of the cingular paraplates bear paired processes.