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Callaiosphaeridium
From Fensome et al., 2019:
Callaiosphaeridium, Davey and Williams, 1966b, p.103.
Emendations: Duxbury, 1980, p.113; Below, 1981a, p.27.
Nomenclatural junior synonym: Hexasphaera Clarke and Verdier, 1967, which has the same type.
Taxonomic junior synonym: Avellodinium, according to Below (1981a, p.27) -- however, Lentin and Williams (1981, p.23) retained Avellodinium. Type: Deflandre and Courteville, 1939, pl.4, fig.1, as Hystrichosphaeridium asymmetricum.
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Original description: [Davey and Williams, 1966]:
Diagnosis:
Chorate cysts with sub-spherical central body composed of two layers. Processes intratabular and of two types: (i) cingular processes large and tubular, open distally, and (ii) apical, precingular, postcingular and sulcal processes solid. Antapical processes absent. Reflected tabulation inferred from arrangement of processes is 1`(-2`), 6``, 6c, 5```, 1p, 0````, and 0-1s. Thickenings of periphragm join all except cingular processes. Archeopyle epitractal, suture just above cingular processes.
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Modified description:
Stover and Evitt 1978, p. 202:
Synopsis: Cysts skolochorate, acavate; body subspherical; paratabulation gonyaulacacean, indicated by parasutural ridges between gonal processes on epicyst and hypocyst and by intratabular paracingular processes; archeopyle, Type tAtP.
Description:
Shape: Body subspherical.
Wall relationships: Endophragm and periphragm appressed between processes, the periphragm forming processes and parasutural ridges; acavate.
Wall features: Parasutural ridges extend between bases of furcate gonal processes on epicyst and hypocyst. Paracingular intratabular processes tubular to trumpet shaped; gonal processes slender, generally solid with furcate tips.
Paratabulation: Indicated by intratabular processes and by parasutural ridges between gonal processes; gonyaulacacean, formula: 1-2`, 5-6``, 6c, 5```, 1````, and 1-3s.
Archeopyle: Combination epicystal, Type tAtP; operculum probably attached, but commonly separated; outline circular.
Paracingulum: Indicated by six intratabular processes.
Parasulcus: Location may be indicated by one or more small processes near the paracingulum.
Size: Intermediate to large.
Affinities:
Callaiosphaeridium differs from Spiniferites in having paracingular intratabular processes and an epicystal archeopyle, whereas Spiniferites has gonal or gonal and intergonal processes and a precingular archeopyle. Actinotheca, like Callaiosphaeridium, has an epicystal archeopyle, but its paracingulum is expressed by high thin septa rather than by intratabular processes.
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Emended descriptions:
Duxbury, 1980:
Diagnosis:
Spheroidal to ellipsoidal dinoflagellate cysts. The paracingulum has six tubular, distally open processes which are roughly equidistant from one another and which bear secondary, flexuous spines near their distal terminations. The epi- and hypocystal areas bear long gonal and parasutural
processes connected by parasutural crests of varying height. Such parasutural crests may be merely low surface ridges or may attain a height equal to that of the gonal processes. In the latter case, individual gonal and parasutural spines may be indistinguishable and the epi- and hypocystal areas may be considered to bear only high parasutural crests. The paratabulation is typically gonyaulacacean and the archeoplye is epicystal.
Affinities:
The main characteristic feature of Callaiosphaeridium is the presence of six tubular, paracingular processes. In other respects, it may be compared with Avellodinium Duxbury, 1977 and Heslertonia Sarjeant, 1966 emend. Duxbury, 1980. Avellodinium bears only gonal processes and has only very low parasutural crests. Heslertonia has only high, regular parasutural crests without tubular or Avellodinium-type processes. In the epi- and hypocystal areas, therefore, Callaiosphaeridium may combine the gonal process type of Avellodinium with the high parasutural crests typical of Heslertonia.
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Below, 1981: (Translation: LPP)
Diagnosis:
Chorate, bilayered cyst. Outline subspheroidal. Paratabulation gonyaulacoid 4`, 6``, 6c, 6```, 1pc, 1````, Xs. Low ridges or membranes follow the parasutures and connect gonal, solid or hollow, distally polyfurcate processes. Parasutural processes, taeniate in cross-section and distally furcate, may be developed. Paracingulum level, hexatabular with per paraplate one intratabular process, markedly tubular or similar to the gonal processes. Parasulcus subdivided or as an inarticulate broad zone. Combination archaeopyle (AP).
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Emended description Niechwedowicz, 2021:
Synonymy.
1966 Callaiosphaeridium Davey & Williams: p. 103.
1967 Hexasphaera Clarke & Verdier: p. 42 (illegitimate name).
1978 Callaiosphaeridium Davey & Williams 1966 – Stover &
Evitt: p. 202.
1980 Callaiosphaeridium Davey & Williams 1966 – Duxbury: p. 113.
1983 Callaiosphaeridium Davey & Williams 1966 – Duxbury: p. 40.
2009 Callaiosphaeridium Davey & Williams 1966 – Fensome, Williams, & MacRae: p. 17, 18.
2016 Callaiosphaeridium Davey & Williams 1966 – Fensome, Williams, & Nøhr-Hansen: p. 30.
Type. Callaiosphaeridium asymmetricum (Deflandre & Courteville 1939) Davey & Williams 1966.
Emended diagnosis. Gonyaulacacean (leptodinioid) chorate cysts with subspheroidal to ovoidal central body, bearing gonal (processes) and sutural (ridges, septa, processes) features, clearly reflecting plate arrangement. The processes are of two types: (i) six prominent equatorial processes (tubular, broad, distally open) in gonal positions; and (ii) simple processes (solid, slender, or taeniate) in gonal or intergonal positions, located elsewhere on epi- and hypocyst excluding cingulum. Processes of both types commonly are connected by sutural septa of variable height (not higher than the processes), with distal margin entire or irregular; septa sometimes are distally supported by thickening (transverse ridge parallel to plate suture). The archaeopyle is epicystal.
Emended description. Chorate cysts with subspheroidal to ovoidal central body, bearing gonal (rarely intergonal) processes, sutural ridges and septa; suturocavate. Epi- and hypocyst are of similar size, or epicyst is slightly smaller. The central body surface is internally smooth, and externally it is smooth, scabrate, or rugulate, with plate margins smooth and often perforate in regular manner (lines of perforations parallel to plate boundaries); septa and processes are generally smooth. The processes are of two types: (i) six equatorial processes, tubular, broad, located in gonal positions between cingular and postcingular plate series; and (ii) slender processes, predominantly gonal, rarely intergonal, positioned elsewhere on epi- and hypocyst, except for the anterior and posterior margin of the cingulum. The equatorial processes are distinct, tubiform, distally flared, and open, terminating with few spines, or with low crests with incorporated numerous short spines. The equatorial processes bear more or less expressed longitudinal striae (three or four) extending frompostcingular and cingular sutures. The slender gonal processes are generally solid (sometimes partially hollow), resulting from merging of sutural septa at gonal points; distally they are commonly furcate, occasionally with additional bifurcation of second order. The intergonal processes, if present, are taeniate. Equatorial and slender processes are connected by a more or less complete network of sutural ridges and septa of variable height (not higher than the processes); equatorial processes are sometimes interconnected basally by low septa or septum-like hollow membranous structures. Septa have entire or irregular distal margin, in the latter case forming taeniate intergonal processes; distally septa may be supported by thickening (transverse ridge parallel to plate suture), crowned with short spines incorporated into low crests. The tabulation formula is 40, 60’, 6c, 60’’, 1p, 5 s, 10’’’, expressed by the archaeopyle and arrangement of gonal and sutural features, indicating leptodinioid pattern (sexiform hypocystal configuration, L-type ventral organisation, and neutral torsion of the hypocyst; see Figure 4); sutural ridges between apical and sulcal plates may be reduced or absent. The archaeopyle is epicystal (formula A1–40 þ P1–60 ’), operculum is free or attached ventrally.
Remarks. The present emendation emphasises the leptodinioid tabulation pattern, the gonal nature of the tubular processes, and variability in the development of septa and processes in Callaiosphaeridium. The sutural features indicating tabulation in Callaiosphaeridium may be present at almost all plate boundaries, but may be only partially developed or absent in the sulcal and apical regions (see also Duxbury 1983, pl. 5, fig. 12, text-fig. 18; Khowaja- Ateequzzaman and Garg 2004, pl. 1, fig. 7). The sutural ridge is also absent from the anterior margin of the cingulum (the boundary between the precingular and cingular series is defined by the archaeopyle suture), but contrary to Davey and Williams (1966, p. 103) and Evitt (1985, p. 244), the posterior margin of the cingulum and the boundaries between particular cingular plates are clearly defined by low sutural ridges (Plate 2, figures 1–5; Plate 3, figures 5–7; Plate 4, figure 17; Plate 5, figures 3, 4, 7–10). These plate boundaries may, however, be poorly visible on specimens in apical or antapical views, which is quite common in the case of Callaiosphaeridium specimens lacking the operculum. The distribution of sutural and gonal features in Callaiosphaeridium asymmetricum (Deflandre & Courteville 1939) Davey & Williams 1966 clearly indicates the leptodinioid affinity of this genus. This is suggested by a sexiform hypocystal configuration (plate 10’’’ is in contact with six plates, i.e. ps, 1p, 30’’–60’’; see Plate 2, figures 13, 16), L-type ventral organisation (longitudinal, more or less straight sulcus; Plate 2, figures 13, 14), and neutral torsion (boundaries between plates 30’/40’ and 40’’/50’’ are in line; Plate 2, figures 16, 17; Plate 3, figures 1, 2). A similar pattern was also observed in C. bicoronatum Niechwedowicz in Niechwedowicz & Walaszczyk 2021 (see Plate 5, figures 5–12, 16, 18). The processes in Callaiosphaeridium are mainly gonal (including the large tubular ones), and rarely intergonal (these are present only in C. trycherium). The characteristic six equatorial processes in Callaiosphaeridium are not intratabular (as suggested before, e.g. Davey and Williams 1966, p. 103; Stover and Evitt 1978, p. 202; Below 1981, p. 27; Evitt 1985, p. 181, 188, 243, 245), but occupy gonal positions (compare also Duxbury 1980, p. 114; Fensome et al. 2009; p. 17, 2016, p. 30). This is indicated by the distribution of faint sutural ridges on the cyst surface: tubular processes arise at locations where three or four sutural ridges meet (see Figures 4 and 5; Plate 3, figures 5–7; Plate 5, figures 8–10). These ridges separate postcingular and cingular plates, and extend onto the tubular processes in the form of longitudinal striae (Plate 2, figures 7, 10; Plate 3, figure 11; Plate 4, figure 17). Since the tubular processes in Callaiosphaeridium are gonal and positioned between the cingular and postcingular series, they should rather be referred to as equatorial, not cingular. The gonal position of such processes is not an unusual feature; e.g. gonal, hollow, and distally open processes are known in Spiniferites pseudofurcatus (see Evitt 1985, p. 228; Riding and Lucas-Clark 2016, p. 45, pl. 8, figs 1, 2), and S. procerus (see Marheinecke 1992, p. 29). What should also be emphasised is the variability in the development of septa and processes occupying the epi- and hypocyst and the presence of suturocavation. Suturocavation is commonly manifested in the presence of lines of perforations running along the sutures near plate margins, and it is visible under both SEM (Plate 2, figures 5–8; Plate 3, figures 5–7, 9; Plate 4, figures 8, 9, 17) and TLM (Plate 2, figure 2; Plate 5, figure 10). Suturocavation is recognised in C. asymmetricum, C. bicoronatum (see Niechwedowicz and Walaszczyk 2021), and most probably in C. trycherium (see Duxbury 1980, p. 114). In fact, the manifestation of suturocavation has been noticed in C. asymmetricum by prior workers, but identified previously as ‘globular [structures], [present] especially along the ribs … [that refer] to small spherical spaces between the endophragm and the periphragm’ (Davey and Williams 1966, p. 104) or ‘strings of pearl-like cavities … at the base of the septa’ (author’s translation from Below 1981, p. 29) and illustrated by Harding (1990, pl. 21, fig. 4).
Comparison. The possession of an epicystal archaeopyle, and more broadly an overall similarity in appearance, make Actinotheca Cookson & Eisenack 1960a, Avellodinium Duxbury 1977, and Heslertonia Sarjeant 1966b very similar to Callaiosphaeridium. Actinotheca differs in having the equatorial processes, or rather ‘intergrown membranes’ or ‘equatorial row of somewhat boxlike chambers’ (Evitt 1985, p. 243, 244), extended more in the equatorial plane. Additionally, it is characterised by the absence or weak development of epiand hypocystal sutural and gonal features (ridges, septa, processes), unlike Callaiosphaeridium. Avellodinium, similar to Callaiosphaeridium, has gonal and intergonal processes. However, its equatorial processes are not tubular, but are comparable in shape with other processes. Heslertonia has high and regularly developed sutural septa covering the entire cyst sutures. Morphologically, Callaiosphaeridium is a transitional form between Avellodinium and Heslertonia. In ventral, dorsal, or lateral views, Callaiosphaeridium specimens with attached operculum resemble Spiniferites Mantell 1850. The gonal, distally furcate processes occupying the polar regions in Callaiosphaeridium (e.g. in C. asymmetricum) are very similar to those in Spiniferites (see Evitt 1985, p. 244). The latter genus, however, differs from Callaiospharedium in having S-type ventral organisation and a precingular archaeopyle.
Stratigraphical comments. Callaiosphaeridium is biostratigraphically important for the Campanian–Maastrichtian boundary; C. asymmetricum disappears in the uppermost Campanian (Habib and Miller 1989; Kurita and Mclntyre 1994; Nøhr-Hansen 1996; Slimani 2001; Skupien and Mohamed 2008; Niechwedowicz and Walaszczyk 2021), and the genus finally disappears in the lowermost Maastrichtian in Poland with its youngest representative, C. bicoronatum (Niechwedowicz and Walaszczyk 2021).
Callaiosphaeridium, Davey and Williams, 1966b, p.103.
Emendations: Duxbury, 1980, p.113; Below, 1981a, p.27.
Nomenclatural junior synonym: Hexasphaera Clarke and Verdier, 1967, which has the same type.
Taxonomic junior synonym: Avellodinium, according to Below (1981a, p.27) -- however, Lentin and Williams (1981, p.23) retained Avellodinium. Type: Deflandre and Courteville, 1939, pl.4, fig.1, as Hystrichosphaeridium asymmetricum.
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Original description: [Davey and Williams, 1966]:
Diagnosis:
Chorate cysts with sub-spherical central body composed of two layers. Processes intratabular and of two types: (i) cingular processes large and tubular, open distally, and (ii) apical, precingular, postcingular and sulcal processes solid. Antapical processes absent. Reflected tabulation inferred from arrangement of processes is 1`(-2`), 6``, 6c, 5```, 1p, 0````, and 0-1s. Thickenings of periphragm join all except cingular processes. Archeopyle epitractal, suture just above cingular processes.
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Modified description:
Stover and Evitt 1978, p. 202:
Synopsis: Cysts skolochorate, acavate; body subspherical; paratabulation gonyaulacacean, indicated by parasutural ridges between gonal processes on epicyst and hypocyst and by intratabular paracingular processes; archeopyle, Type tAtP.
Description:
Shape: Body subspherical.
Wall relationships: Endophragm and periphragm appressed between processes, the periphragm forming processes and parasutural ridges; acavate.
Wall features: Parasutural ridges extend between bases of furcate gonal processes on epicyst and hypocyst. Paracingular intratabular processes tubular to trumpet shaped; gonal processes slender, generally solid with furcate tips.
Paratabulation: Indicated by intratabular processes and by parasutural ridges between gonal processes; gonyaulacacean, formula: 1-2`, 5-6``, 6c, 5```, 1````, and 1-3s.
Archeopyle: Combination epicystal, Type tAtP; operculum probably attached, but commonly separated; outline circular.
Paracingulum: Indicated by six intratabular processes.
Parasulcus: Location may be indicated by one or more small processes near the paracingulum.
Size: Intermediate to large.
Affinities:
Callaiosphaeridium differs from Spiniferites in having paracingular intratabular processes and an epicystal archeopyle, whereas Spiniferites has gonal or gonal and intergonal processes and a precingular archeopyle. Actinotheca, like Callaiosphaeridium, has an epicystal archeopyle, but its paracingulum is expressed by high thin septa rather than by intratabular processes.
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Emended descriptions:
Duxbury, 1980:
Diagnosis:
Spheroidal to ellipsoidal dinoflagellate cysts. The paracingulum has six tubular, distally open processes which are roughly equidistant from one another and which bear secondary, flexuous spines near their distal terminations. The epi- and hypocystal areas bear long gonal and parasutural
processes connected by parasutural crests of varying height. Such parasutural crests may be merely low surface ridges or may attain a height equal to that of the gonal processes. In the latter case, individual gonal and parasutural spines may be indistinguishable and the epi- and hypocystal areas may be considered to bear only high parasutural crests. The paratabulation is typically gonyaulacacean and the archeoplye is epicystal.
Affinities:
The main characteristic feature of Callaiosphaeridium is the presence of six tubular, paracingular processes. In other respects, it may be compared with Avellodinium Duxbury, 1977 and Heslertonia Sarjeant, 1966 emend. Duxbury, 1980. Avellodinium bears only gonal processes and has only very low parasutural crests. Heslertonia has only high, regular parasutural crests without tubular or Avellodinium-type processes. In the epi- and hypocystal areas, therefore, Callaiosphaeridium may combine the gonal process type of Avellodinium with the high parasutural crests typical of Heslertonia.
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Below, 1981: (Translation: LPP)
Diagnosis:
Chorate, bilayered cyst. Outline subspheroidal. Paratabulation gonyaulacoid 4`, 6``, 6c, 6```, 1pc, 1````, Xs. Low ridges or membranes follow the parasutures and connect gonal, solid or hollow, distally polyfurcate processes. Parasutural processes, taeniate in cross-section and distally furcate, may be developed. Paracingulum level, hexatabular with per paraplate one intratabular process, markedly tubular or similar to the gonal processes. Parasulcus subdivided or as an inarticulate broad zone. Combination archaeopyle (AP).
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Emended description Niechwedowicz, 2021:
Synonymy.
1966 Callaiosphaeridium Davey & Williams: p. 103.
1967 Hexasphaera Clarke & Verdier: p. 42 (illegitimate name).
1978 Callaiosphaeridium Davey & Williams 1966 – Stover &
Evitt: p. 202.
1980 Callaiosphaeridium Davey & Williams 1966 – Duxbury: p. 113.
1983 Callaiosphaeridium Davey & Williams 1966 – Duxbury: p. 40.
2009 Callaiosphaeridium Davey & Williams 1966 – Fensome, Williams, & MacRae: p. 17, 18.
2016 Callaiosphaeridium Davey & Williams 1966 – Fensome, Williams, & Nøhr-Hansen: p. 30.
Type. Callaiosphaeridium asymmetricum (Deflandre & Courteville 1939) Davey & Williams 1966.
Emended diagnosis. Gonyaulacacean (leptodinioid) chorate cysts with subspheroidal to ovoidal central body, bearing gonal (processes) and sutural (ridges, septa, processes) features, clearly reflecting plate arrangement. The processes are of two types: (i) six prominent equatorial processes (tubular, broad, distally open) in gonal positions; and (ii) simple processes (solid, slender, or taeniate) in gonal or intergonal positions, located elsewhere on epi- and hypocyst excluding cingulum. Processes of both types commonly are connected by sutural septa of variable height (not higher than the processes), with distal margin entire or irregular; septa sometimes are distally supported by thickening (transverse ridge parallel to plate suture). The archaeopyle is epicystal.
Emended description. Chorate cysts with subspheroidal to ovoidal central body, bearing gonal (rarely intergonal) processes, sutural ridges and septa; suturocavate. Epi- and hypocyst are of similar size, or epicyst is slightly smaller. The central body surface is internally smooth, and externally it is smooth, scabrate, or rugulate, with plate margins smooth and often perforate in regular manner (lines of perforations parallel to plate boundaries); septa and processes are generally smooth. The processes are of two types: (i) six equatorial processes, tubular, broad, located in gonal positions between cingular and postcingular plate series; and (ii) slender processes, predominantly gonal, rarely intergonal, positioned elsewhere on epi- and hypocyst, except for the anterior and posterior margin of the cingulum. The equatorial processes are distinct, tubiform, distally flared, and open, terminating with few spines, or with low crests with incorporated numerous short spines. The equatorial processes bear more or less expressed longitudinal striae (three or four) extending frompostcingular and cingular sutures. The slender gonal processes are generally solid (sometimes partially hollow), resulting from merging of sutural septa at gonal points; distally they are commonly furcate, occasionally with additional bifurcation of second order. The intergonal processes, if present, are taeniate. Equatorial and slender processes are connected by a more or less complete network of sutural ridges and septa of variable height (not higher than the processes); equatorial processes are sometimes interconnected basally by low septa or septum-like hollow membranous structures. Septa have entire or irregular distal margin, in the latter case forming taeniate intergonal processes; distally septa may be supported by thickening (transverse ridge parallel to plate suture), crowned with short spines incorporated into low crests. The tabulation formula is 40, 60’, 6c, 60’’, 1p, 5 s, 10’’’, expressed by the archaeopyle and arrangement of gonal and sutural features, indicating leptodinioid pattern (sexiform hypocystal configuration, L-type ventral organisation, and neutral torsion of the hypocyst; see Figure 4); sutural ridges between apical and sulcal plates may be reduced or absent. The archaeopyle is epicystal (formula A1–40 þ P1–60 ’), operculum is free or attached ventrally.
Remarks. The present emendation emphasises the leptodinioid tabulation pattern, the gonal nature of the tubular processes, and variability in the development of septa and processes in Callaiosphaeridium. The sutural features indicating tabulation in Callaiosphaeridium may be present at almost all plate boundaries, but may be only partially developed or absent in the sulcal and apical regions (see also Duxbury 1983, pl. 5, fig. 12, text-fig. 18; Khowaja- Ateequzzaman and Garg 2004, pl. 1, fig. 7). The sutural ridge is also absent from the anterior margin of the cingulum (the boundary between the precingular and cingular series is defined by the archaeopyle suture), but contrary to Davey and Williams (1966, p. 103) and Evitt (1985, p. 244), the posterior margin of the cingulum and the boundaries between particular cingular plates are clearly defined by low sutural ridges (Plate 2, figures 1–5; Plate 3, figures 5–7; Plate 4, figure 17; Plate 5, figures 3, 4, 7–10). These plate boundaries may, however, be poorly visible on specimens in apical or antapical views, which is quite common in the case of Callaiosphaeridium specimens lacking the operculum. The distribution of sutural and gonal features in Callaiosphaeridium asymmetricum (Deflandre & Courteville 1939) Davey & Williams 1966 clearly indicates the leptodinioid affinity of this genus. This is suggested by a sexiform hypocystal configuration (plate 10’’’ is in contact with six plates, i.e. ps, 1p, 30’’–60’’; see Plate 2, figures 13, 16), L-type ventral organisation (longitudinal, more or less straight sulcus; Plate 2, figures 13, 14), and neutral torsion (boundaries between plates 30’/40’ and 40’’/50’’ are in line; Plate 2, figures 16, 17; Plate 3, figures 1, 2). A similar pattern was also observed in C. bicoronatum Niechwedowicz in Niechwedowicz & Walaszczyk 2021 (see Plate 5, figures 5–12, 16, 18). The processes in Callaiosphaeridium are mainly gonal (including the large tubular ones), and rarely intergonal (these are present only in C. trycherium). The characteristic six equatorial processes in Callaiosphaeridium are not intratabular (as suggested before, e.g. Davey and Williams 1966, p. 103; Stover and Evitt 1978, p. 202; Below 1981, p. 27; Evitt 1985, p. 181, 188, 243, 245), but occupy gonal positions (compare also Duxbury 1980, p. 114; Fensome et al. 2009; p. 17, 2016, p. 30). This is indicated by the distribution of faint sutural ridges on the cyst surface: tubular processes arise at locations where three or four sutural ridges meet (see Figures 4 and 5; Plate 3, figures 5–7; Plate 5, figures 8–10). These ridges separate postcingular and cingular plates, and extend onto the tubular processes in the form of longitudinal striae (Plate 2, figures 7, 10; Plate 3, figure 11; Plate 4, figure 17). Since the tubular processes in Callaiosphaeridium are gonal and positioned between the cingular and postcingular series, they should rather be referred to as equatorial, not cingular. The gonal position of such processes is not an unusual feature; e.g. gonal, hollow, and distally open processes are known in Spiniferites pseudofurcatus (see Evitt 1985, p. 228; Riding and Lucas-Clark 2016, p. 45, pl. 8, figs 1, 2), and S. procerus (see Marheinecke 1992, p. 29). What should also be emphasised is the variability in the development of septa and processes occupying the epi- and hypocyst and the presence of suturocavation. Suturocavation is commonly manifested in the presence of lines of perforations running along the sutures near plate margins, and it is visible under both SEM (Plate 2, figures 5–8; Plate 3, figures 5–7, 9; Plate 4, figures 8, 9, 17) and TLM (Plate 2, figure 2; Plate 5, figure 10). Suturocavation is recognised in C. asymmetricum, C. bicoronatum (see Niechwedowicz and Walaszczyk 2021), and most probably in C. trycherium (see Duxbury 1980, p. 114). In fact, the manifestation of suturocavation has been noticed in C. asymmetricum by prior workers, but identified previously as ‘globular [structures], [present] especially along the ribs … [that refer] to small spherical spaces between the endophragm and the periphragm’ (Davey and Williams 1966, p. 104) or ‘strings of pearl-like cavities … at the base of the septa’ (author’s translation from Below 1981, p. 29) and illustrated by Harding (1990, pl. 21, fig. 4).
Comparison. The possession of an epicystal archaeopyle, and more broadly an overall similarity in appearance, make Actinotheca Cookson & Eisenack 1960a, Avellodinium Duxbury 1977, and Heslertonia Sarjeant 1966b very similar to Callaiosphaeridium. Actinotheca differs in having the equatorial processes, or rather ‘intergrown membranes’ or ‘equatorial row of somewhat boxlike chambers’ (Evitt 1985, p. 243, 244), extended more in the equatorial plane. Additionally, it is characterised by the absence or weak development of epiand hypocystal sutural and gonal features (ridges, septa, processes), unlike Callaiosphaeridium. Avellodinium, similar to Callaiosphaeridium, has gonal and intergonal processes. However, its equatorial processes are not tubular, but are comparable in shape with other processes. Heslertonia has high and regularly developed sutural septa covering the entire cyst sutures. Morphologically, Callaiosphaeridium is a transitional form between Avellodinium and Heslertonia. In ventral, dorsal, or lateral views, Callaiosphaeridium specimens with attached operculum resemble Spiniferites Mantell 1850. The gonal, distally furcate processes occupying the polar regions in Callaiosphaeridium (e.g. in C. asymmetricum) are very similar to those in Spiniferites (see Evitt 1985, p. 244). The latter genus, however, differs from Callaiospharedium in having S-type ventral organisation and a precingular archaeopyle.
Stratigraphical comments. Callaiosphaeridium is biostratigraphically important for the Campanian–Maastrichtian boundary; C. asymmetricum disappears in the uppermost Campanian (Habib and Miller 1989; Kurita and Mclntyre 1994; Nøhr-Hansen 1996; Slimani 2001; Skupien and Mohamed 2008; Niechwedowicz and Walaszczyk 2021), and the genus finally disappears in the lowermost Maastrichtian in Poland with its youngest representative, C. bicoronatum (Niechwedowicz and Walaszczyk 2021).