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Seriliodinium explicatum
Seriliodinium explicatum Eaton 1996
Holotype: Eaton 1996: Plate I, l, 2; Figs. 2A,B, 5A,B.
Type locality: Black Sea floor, latitude 42o05'N, longitude 36o51'E.
Stratigraphic horizon: Plio-Pleistocene.
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G.L. Williams short notes on species, Mesozoic-Cenozoic dinocyst course, Urbino, Italy, May 17-22, 1999
Seriliodinium explicatum Eaton, 1976. Aaccording to Eaton (1996), this is characterised by the variable development of parasutural membranes at one or more specific sites. On the ventral hypocyst there are two longitudinal sites, to the left and right of the mid-ventral area. On the dorsal epicyst there are four sites, around the lateral and anterior margins of the precingular archeopyle. With minimal development at a particular site, shallow membranes may be proximal to the cyst body, lateral to the limiting processes, or subjacent to the limiting trabeculae. When fully developed, these membranes form curtain-like structures which display a variable range of fenestrations. Size: overall length 61-106 µm, overall width 58-108 µm, cyst body length 38-63 µm, width 29-50 µm, maximum length of processes 17-37 µm.
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Original Diagnosis: Eaton 1996, p. 153
A species of Seriliodinium which is characterised by the variable development of parasutural membranes at one or more of several specific sites. On the ventral hypocyst there are two longitudinal sites, to the left and right of the mid-ventral area. On the dorsal epicyst there are four sites, around the lateral and anterior margins of the precingular archaeopyle. With minimal development at a particular site, shallow membranes may be proximal to the cyst body, lateral to the limiting processes, or subjacent to the limiting trabeculae. When fully developed, these membranes form curtain-like structures which display a variable range of fenestration.
Original Description: Eaton 1996, p. 153-167
In dorso-ventral view the cyst body usually has a rounded cruciform outline, although this may be modified to rounded polygonal or oval. The cyst wall is thin (usually #1.0Ám), and composed of two layers which can be distinguished where there is separation, i.e. in the vicinity of the processes. The inner endophragm represents most of the wall thickness, and is apparently smooth. The outer periphragm is very thin, and may be smooth, scabrate, faintly grano-punctate, or distinctly granular with adjacent granular elements sometimes fused together. This range of surface texture may be displayed on the processes and associated membranes. Processes are characteristically confined to the peripheral region of the cyst body, leaving the mid-ventral and mid-dorsal areas essentially process-free. This distribution results in an overall marginate appearance and a preferred dorso-ventral orientation. The majority of the processes are gonal in position, and often display a rod-like core structure; they appear to be mainly solid, but may show some proximal pericoel development and expansion. These gonal processes are distally expanded and trifurcate, with the trifurcations either free or united by trabeculae. There are also processes which are more complex, and occupy several gonal sites. These processes are hollow, at least proximally, and may also display rod-like structures which represent individual gonal elements. Distally these processes are expanded and multifurcate ("multigonal" of Marheinecke, 1992), but the furcations can be related to the trifurcate form of individual gonal elements; this individuality may be emphasised by varying degrees of branching, and/or the development of large fenestrations. The distal furcations may be free or united by trabeculae.
Parasutural septa may occur, as low, ridge-like thickenings which can extend on to the processes, or as membranes of variable height which link adjacent processes. Membrane development may obscure the individuality of linked, adjacent processes, but their separate core structures are usually evident (Plate II, 1, 4). Extreme membrane development results in curtain-like structures which occupy specific parasutural sites. The membranous septa are variably and irregularly fenestrate; the curtain-like membranes range from virtually entire with only a few small openings, to complex, open net-like structures. When the potential sites of these curtain-like structures show minimal membranes, they can be considered to be occupied by a large fenestration which restricts the surrounding membrane to shallow features proximal to the cyst body, lateral to the limiting processes, and subjacent to the limiting trabeculae (PlateI, 3). In intermediate forms, membrane development may show posterior ventral bias (Plate I, 5, 6) and anterior dorsal bias (Plate II, 1), as well as left or right bias. In specimens displaying highly fenestrate membranes, the processes may be similarly fenestrate, and the cyst body usually displays a coarse, granular surface texture (Plate II, 6). Such examples are not common, but again intermediate forms also occur (Plate II, 5).
The trabeculae are single, rope-like, and parasutural, and form a simple network which distally unites the processes in the peripheral region of the cyst, characteristically leaving the mid-ventral and mid-dorsal areas open. Each trabecula is developed as a single element from a particular process furcation, but the restricted areal development of the trabeculae leaves some furcations free. The trabeculae are variable in thickness (usually c. 1.0µm), overall and individually, and typically show undulating margins which in extreme examples can result in a beaded appearance (Plate I, 5). They also have a dense texture which is usually discernable even where the trabeculae merge with, and distally limit, the curtain-like membranes, and where they merge with the distal furcations of the processes. The distribution of the trabeculae is directly related to paratabulation, with each trabecula reflecting a particular parasuture.
The distribution of the processes, parasutural septa and membranes, and trabeculae, defines a partial gonyaulacoid paratabulation with the Kofoidian formula: 3', 6", 6c, 6"', lp, 1"", Xs (Fig. 5). The simple precingular archaeopyle is formed by the loss of paraplate 3". On the epicyst, the majority of the processes occupy single gonal sites on the boundary between the apical and precingular zones, with a complex process usually developed at the boundary between the ventral apical and anterior parasulcal zones (Plate I, 1, 5; Fig. 2A,C). In some specimens this process is divided (Plate I, 4, Plate II, 3; Fig. 3A). These epicystal processes are united distally by a ring of trabeculae which reflects the apical/precingular zonal boundary. The slender polar process ("x" in Figs. 2B, 3A,B,D) which marks the meeting point of the apical paraplates is linked to the trabecular ring by a single trabecula which reflects the 2'/3' boundary. In the precingular zone, paraplates 2"-5" are well defined with trabeculae reflecting all parasutures except the posterior margin of 3". Paraplates 1" and 6" are partially defined by parasutures, but their boundaries with the parasulcal zone are poorly marked and not reflected by trabeculae. Paraplate 6" is triangular, and does contact the ventral apical zone (Fig. 5A,C,E). Variably fenestrate curtain-like membranes may be developed on the 2"/3" and 3"/4" boundaries, and along the anterior margin of 3" i.e. the 2'/3" and 3'/3" boundaries (Plate I, 2; Plate II, 1, 2; Figs. 2B, 3B).
The paracingulum is characterised by complex processes which separate lc: 2c: 3c-4c: 5c: 6c: parasulcus. The boundaries of the paracingular well defined by low septa in the mid-dorsal and dorso-lateral areas (Plate I, 6; Fig. 2D). Trabeculae reflect the boundaries of the paracingular zone along the posterior margins of 2", 5" and 6", and the anterior margins of 2"' 5"' and 6"'. On the hypocyst, processes occupy gonal sites on the boundaries between the postcingular, posterior intercalary, posterior parasulcal and antapical zones, and septal membranes often unite the most closely adjacent ones (Figs. 2, 3A,B). In the postcingular zone, paraplates 2"'-6"' are readily distinguishable, with 4"' sometimes fully defined by septa (Plate I, 6; Fig. 2D). Paraplates 2"' 6"' are fully reflected by trabeculae, but those that reflect the lateral boundaries of 4"' are not consistently developed (compare Figs. 2B,D and 3B,D). Paraplate 1"' is often well defined parasuturally, except for its boundary with the parasulcal zone, and it may be fully reflected by trabeculae. Variably fenestrate, curtain-like membranes are often developed along the 1"'/2"' and 6"'/parasulcal zone boundaries (Plate I, 1, 4, 5; Figs. 2A,C, 3A). Posterior intercalary lp and antapical 1"" are well defined, and fully reflected by trabeculae. The mid-ventral area is usually devoid of processes and septa, and the only consistent feature defined within the parasulcal zone is the posterior sulcal paraplate (p.s.). The anterior margin of p.s. may be marked by a transverse septal membrane which varies from low (Plate III, 6; Figs. 4A, 6A) to curtain-like and fenestrate (Plate III, 4; Fig. 6B), and links the two ventral curtain-like structures which mark the 1"'/2"' and 6"'/parasulcal zone boundaries. This transverse parasuture is often reflected by a trabecula, with or without the corresponding septal feature (compare Fig. 2C with Figs. 4A and 6A,B).
Dimensions: Holotype, overall: length 87µm, width 85µm; cyst body: length 52µm, width 40µm; maximum length of processes: 32µm. Observed range, overall: length 61-(79)-106µm, width 58-(79)-108µm; cyst body: length 38-(48)-63µm, width 29-(38.5)-50µm; maximum length of processes: 17-(25)-37µm. Number of specimens measured: 169.
Discussion: The marginate appearance of Seriliodinium explicatum is enhanced by the fact that its processes tend to "splay out" towards the periphery, suggesting a certain degree of dorsolateral compression. When there is minimal compression, the arrangement of the trabeculae is more readily appreciated (Fig. 4A,B). The general absence of trabeculae across the mid-ventral and mid-dorsal areas leaves several processes with some distally free furcations, including the complex anterior ventral process, and the complex processes at the lc/2c, 2c/3c and 4c/5c boundaries. The variable development of trabeculae reflecting the 3"'/4"' and 4"'/5"' boundaries leaves the paired process at the posterior margin of 4"' with one or two free furcations (Plate II, 2, 5; compare Fig. 2B,D with Fig. 3B,D). Although the gonyaulacoid paratabulation is not fully defined, the hypocystal pattern is sexiform and the visible features indicate an S-type ventral arrangement (Evitt, 1985). The ventral limits of the paracingulum are also not well defined, but this key structure is certainly laevorotatory, and its alignment to the left and right of the mid-ventral area suggests a ventral displacement of at least its own width. Other paratabulatory features include the relative position of precingular 6" which is anterior to the right posterior parasulcal area, rather than 6c (and 6"'), and the relative size of 1"' which appears to be quite broad. The relationship between 1"' and lp and p.s. is variable. In many specimens 1"' is in contact posteriorly with lp and p.s. (Fig. 5A,C,G), but in others there only appears to be contact with lp (Figs. 4B, 5E). Also, only three apical paraplates have been distinguished, but it is possible that the area designated 1' in Figs. 2, 3 and 5 represents two ventral apicals (1' and 4'). A hint of this division is suggested by the arrangement of the free furcations associated with the complex anterior ventral process, but the evidence is not conclusive.
The foregoing description and remarks are based on the examination of a large number of specimens of S. explicatum, but some anomalous features were also noted. Occasionally, reduced processes are developed in the mid-ventral area (Plate III, 4; "y" in Fig. 6B). In the illustrated example, this simple, short, isolated process apparently lies on the boundary between lc and the parasulcus, and is probably gonal in position. Its dense texture is comparable with the usual style of the trabeculae. The specimen in Plate III, 2 and Fig. 7A shows a mid-dorsal process associated with the anterior margin of the paracingulum. This process is bifurcate, and distally united by trabeculae with the two adjacent, typically complex, paracingular processes at the 2c/3c and 4c/5c boundaries. The furcations of the latter two processes, which are united by the two anomalous trabecular elements, are usually distally free. A similar mid-dorsal process associated with the posterior margin of the paracingulum is shown in Fig. 7B. This is also bifurcate, and has the dense trabecular texture, but it is distally free with no evidence of accompanying trabeculae. These anomalous features are still parasutural in position; however, the specimen in Plate III, 1 and Fig. 7C shows a non-parasutural feature. A process associated with the anterior margin of the mid-dorsal paracingulum is again featured, with a distal transverse trabecular link to the right adjacent paracingular process (4c/Sc boundary), and a free furcation which could be potentially united with the left adjacent paracingular process (2c/3c boundary). However, there is also a longitudinal trabecular link to the transverse trabecula which reflects the 3'/3" boundary. The intersection between the two is not gonal in position, and the anomalous trabecula appears to cross 3". This is the only non-parasutural feature recorded in this study of S. explicatum.
In discussing the generic concept of Seriliodinium, comparisons have necessarily been made with other trabeculate cysts of the Spiniferites complex (Evitt, 1985), but in terms of specific morphology Seriliodinium explicatum is most similar to the non-trabeculate chorate cyst Spiniferites cruciformis Wall and Dale ln wan et al., 1973. Both species are characterised by a rounded cruciform cyst body shape, and both are capable of developing curtain-like membranes at specific parasutural sites. The examination of rare examples of Spiniferites cruciformis recorded in the present study shows that it may also display a posterior ventral transverse septal membrane (Plate IV, 4, 5; Fig. 6C,D). This feature is also shown in a specimen illustrated in Wall et al. (1973, pl. 2, fig. 3). As in Seriliodinium explicatum, this membrane defines the anterior margin of the posterior sulcal paraplate (p.s.), and links the ventral longitudinal curtain-like membranes. Furthermore, the parasutural position of the latter membranes in Spiniferites cruciformis appears to be comparable with that of Seriliodinium explicatum ( 1"'/2"' and 6"'/parasulcal zone). In their original description of Spiniferites cruciformis, Wall and Dale made the same interpretation of the right membrane, but they considered that the left membrane marked the 2"'/3"' boundary. The 1"'/2"' interpretation preferred here, can be applied to the specimen illustrated in Wall et al. (1973, pl. 2, fig. 3) which shows this membrane impinging on the left anterior margin of lp, rather than running into the left posterior margin. These details as well as the other morphological similarities, suggest that a very close relationship existed between Seriliodinium explicatum and Spiniferites cruciformis, and presumably between their corresponding motile stages.
The morphological style of Seriliodinium explicatum and its possible significance
Although Seriliodinium explicatum can be referred to the Spiniferites complex of Evitt (1985) on the basis of the general form and distribution of its processes and its overall paratabulation pattern, it also displays features which appear to be atypical of this complex. These are, a cruciform cyst body shape, the presence of single parasutural trabeculae, and the selective development of processes and trabeculae which results in a marginate overall appearance. As noted earlier, a cruciform cyst body shape is also a characteristic of Spiniferites cruciformis. In their original description of this species, Wall and Dale (in Wall et al., 1973) interpreted this feature and the considerable range of morphological variation displayed by S. cruciformis, as adaptive mor- phological responses to abnormal salinities. They considered S. cruciformis to be a cool water, stenohaline, low salinity species which thrived in the Black Sea during the Late Pleistocene to earliest Mesoholocene period, but was unable to survive the subsequent influx of saline water when it was replaced by euryhaline marine species (see also Wall and Dale, 1973). Wall and Dale's interpretation of the palaeoenvironmental significance of Spiniferites cruciformis was supported by independent evidence which is not available with respect to Seriliodinium explicatum. However, it seems reasonable to suggest that the cruciform cyst body shape and range of variation in Seriliodinium explicatum also represent adaptive morphological responses to a stressed environment, possibly associated with abnormal (?low) salinity. The furcate processes which characterise the Spiniferites complex usually display bifid terminations (Evitt, 1985, fig. 10.7), and distal linkage of these terminations would normally be achieved through the development of paired penitabular trabeculae as in Nematosphaeropsis. These paired penitabular elements may be separate as in Nematosphaeropsis rigida Wrenn, 1988 (see also Evitt, 1985, fig. 10.7M), or variably united by membrane as in Nematosphaeropsis lativittatus Wrenn, 1988 (see also Evitt, 1985, fig. 10.7N). The extensive development of such membrane would ultimately result in a distal network of relatively broad trabeculae which are essentially parasutural in position, although they actually consist of laterally united, paired penitabular elements. This is the situation in Nematosphaeropsis downiei Brown, 1986, and apparently also in Unipontidinium aquaeductum (Piasecki) Wrenn, 1988 (see Piasecki, 1980, pl. 5, figs. 1, 2). The same overall effect is achieved in Seriliodinium explicatum, but here the trabeculae appear to be genuinely single and parasutural, with no evidence of penitabular pairing. It is possible that the potential uniting of paired trabecular elements in Nematosphaeropsis through membrane development is an adaptive morphological response to environmental influence. If so, then the development of single trabeculae in Seriliodinium explicatum could be an extreme response to a stressed environment, resulting in the suppression of the bifid terminations of the processes and their influence on trabecular development. Cysts referable to the Spiniferites complex may appear to have a fairly regular distribution of processes and parasutural ornament, but in fact these features are often suppressed on the ventral surface and especially in the parasulcal area (Evitt, 1985). In Seriliodinium explicatum this suppression is considerable, and results in both the mid-ventral and mid-dorsal areas usually being devoid of processes and trabeculae. Again it is possible that this selective development of processes and trabeculae which produces a marginate overall form, is also an adaptive morphological response to a stressed environment. This interpretation implies that under appropriate environmental conditions the parent dinoflagellate of S. explicatum might have produced a more conventional cyst morphotype with a full complement of processes and trabeculae. If so, then the occasional and apparently anomalous development of these morphological elements on the mid-ventral and mid-dorsal areas in Seriliodinium explicatum may represent an abortive attempt to produce that more conventional cyst morphotype.
Holotype: Eaton 1996: Plate I, l, 2; Figs. 2A,B, 5A,B.
Type locality: Black Sea floor, latitude 42o05'N, longitude 36o51'E.
Stratigraphic horizon: Plio-Pleistocene.
--------------------------------------------------
G.L. Williams short notes on species, Mesozoic-Cenozoic dinocyst course, Urbino, Italy, May 17-22, 1999
Seriliodinium explicatum Eaton, 1976. Aaccording to Eaton (1996), this is characterised by the variable development of parasutural membranes at one or more specific sites. On the ventral hypocyst there are two longitudinal sites, to the left and right of the mid-ventral area. On the dorsal epicyst there are four sites, around the lateral and anterior margins of the precingular archeopyle. With minimal development at a particular site, shallow membranes may be proximal to the cyst body, lateral to the limiting processes, or subjacent to the limiting trabeculae. When fully developed, these membranes form curtain-like structures which display a variable range of fenestrations. Size: overall length 61-106 µm, overall width 58-108 µm, cyst body length 38-63 µm, width 29-50 µm, maximum length of processes 17-37 µm.
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Original Diagnosis: Eaton 1996, p. 153
A species of Seriliodinium which is characterised by the variable development of parasutural membranes at one or more of several specific sites. On the ventral hypocyst there are two longitudinal sites, to the left and right of the mid-ventral area. On the dorsal epicyst there are four sites, around the lateral and anterior margins of the precingular archaeopyle. With minimal development at a particular site, shallow membranes may be proximal to the cyst body, lateral to the limiting processes, or subjacent to the limiting trabeculae. When fully developed, these membranes form curtain-like structures which display a variable range of fenestration.
Original Description: Eaton 1996, p. 153-167
In dorso-ventral view the cyst body usually has a rounded cruciform outline, although this may be modified to rounded polygonal or oval. The cyst wall is thin (usually #1.0Ám), and composed of two layers which can be distinguished where there is separation, i.e. in the vicinity of the processes. The inner endophragm represents most of the wall thickness, and is apparently smooth. The outer periphragm is very thin, and may be smooth, scabrate, faintly grano-punctate, or distinctly granular with adjacent granular elements sometimes fused together. This range of surface texture may be displayed on the processes and associated membranes. Processes are characteristically confined to the peripheral region of the cyst body, leaving the mid-ventral and mid-dorsal areas essentially process-free. This distribution results in an overall marginate appearance and a preferred dorso-ventral orientation. The majority of the processes are gonal in position, and often display a rod-like core structure; they appear to be mainly solid, but may show some proximal pericoel development and expansion. These gonal processes are distally expanded and trifurcate, with the trifurcations either free or united by trabeculae. There are also processes which are more complex, and occupy several gonal sites. These processes are hollow, at least proximally, and may also display rod-like structures which represent individual gonal elements. Distally these processes are expanded and multifurcate ("multigonal" of Marheinecke, 1992), but the furcations can be related to the trifurcate form of individual gonal elements; this individuality may be emphasised by varying degrees of branching, and/or the development of large fenestrations. The distal furcations may be free or united by trabeculae.
Parasutural septa may occur, as low, ridge-like thickenings which can extend on to the processes, or as membranes of variable height which link adjacent processes. Membrane development may obscure the individuality of linked, adjacent processes, but their separate core structures are usually evident (Plate II, 1, 4). Extreme membrane development results in curtain-like structures which occupy specific parasutural sites. The membranous septa are variably and irregularly fenestrate; the curtain-like membranes range from virtually entire with only a few small openings, to complex, open net-like structures. When the potential sites of these curtain-like structures show minimal membranes, they can be considered to be occupied by a large fenestration which restricts the surrounding membrane to shallow features proximal to the cyst body, lateral to the limiting processes, and subjacent to the limiting trabeculae (PlateI, 3). In intermediate forms, membrane development may show posterior ventral bias (Plate I, 5, 6) and anterior dorsal bias (Plate II, 1), as well as left or right bias. In specimens displaying highly fenestrate membranes, the processes may be similarly fenestrate, and the cyst body usually displays a coarse, granular surface texture (Plate II, 6). Such examples are not common, but again intermediate forms also occur (Plate II, 5).
The trabeculae are single, rope-like, and parasutural, and form a simple network which distally unites the processes in the peripheral region of the cyst, characteristically leaving the mid-ventral and mid-dorsal areas open. Each trabecula is developed as a single element from a particular process furcation, but the restricted areal development of the trabeculae leaves some furcations free. The trabeculae are variable in thickness (usually c. 1.0µm), overall and individually, and typically show undulating margins which in extreme examples can result in a beaded appearance (Plate I, 5). They also have a dense texture which is usually discernable even where the trabeculae merge with, and distally limit, the curtain-like membranes, and where they merge with the distal furcations of the processes. The distribution of the trabeculae is directly related to paratabulation, with each trabecula reflecting a particular parasuture.
The distribution of the processes, parasutural septa and membranes, and trabeculae, defines a partial gonyaulacoid paratabulation with the Kofoidian formula: 3', 6", 6c, 6"', lp, 1"", Xs (Fig. 5). The simple precingular archaeopyle is formed by the loss of paraplate 3". On the epicyst, the majority of the processes occupy single gonal sites on the boundary between the apical and precingular zones, with a complex process usually developed at the boundary between the ventral apical and anterior parasulcal zones (Plate I, 1, 5; Fig. 2A,C). In some specimens this process is divided (Plate I, 4, Plate II, 3; Fig. 3A). These epicystal processes are united distally by a ring of trabeculae which reflects the apical/precingular zonal boundary. The slender polar process ("x" in Figs. 2B, 3A,B,D) which marks the meeting point of the apical paraplates is linked to the trabecular ring by a single trabecula which reflects the 2'/3' boundary. In the precingular zone, paraplates 2"-5" are well defined with trabeculae reflecting all parasutures except the posterior margin of 3". Paraplates 1" and 6" are partially defined by parasutures, but their boundaries with the parasulcal zone are poorly marked and not reflected by trabeculae. Paraplate 6" is triangular, and does contact the ventral apical zone (Fig. 5A,C,E). Variably fenestrate curtain-like membranes may be developed on the 2"/3" and 3"/4" boundaries, and along the anterior margin of 3" i.e. the 2'/3" and 3'/3" boundaries (Plate I, 2; Plate II, 1, 2; Figs. 2B, 3B).
The paracingulum is characterised by complex processes which separate lc: 2c: 3c-4c: 5c: 6c: parasulcus. The boundaries of the paracingular well defined by low septa in the mid-dorsal and dorso-lateral areas (Plate I, 6; Fig. 2D). Trabeculae reflect the boundaries of the paracingular zone along the posterior margins of 2", 5" and 6", and the anterior margins of 2"' 5"' and 6"'. On the hypocyst, processes occupy gonal sites on the boundaries between the postcingular, posterior intercalary, posterior parasulcal and antapical zones, and septal membranes often unite the most closely adjacent ones (Figs. 2, 3A,B). In the postcingular zone, paraplates 2"'-6"' are readily distinguishable, with 4"' sometimes fully defined by septa (Plate I, 6; Fig. 2D). Paraplates 2"' 6"' are fully reflected by trabeculae, but those that reflect the lateral boundaries of 4"' are not consistently developed (compare Figs. 2B,D and 3B,D). Paraplate 1"' is often well defined parasuturally, except for its boundary with the parasulcal zone, and it may be fully reflected by trabeculae. Variably fenestrate, curtain-like membranes are often developed along the 1"'/2"' and 6"'/parasulcal zone boundaries (Plate I, 1, 4, 5; Figs. 2A,C, 3A). Posterior intercalary lp and antapical 1"" are well defined, and fully reflected by trabeculae. The mid-ventral area is usually devoid of processes and septa, and the only consistent feature defined within the parasulcal zone is the posterior sulcal paraplate (p.s.). The anterior margin of p.s. may be marked by a transverse septal membrane which varies from low (Plate III, 6; Figs. 4A, 6A) to curtain-like and fenestrate (Plate III, 4; Fig. 6B), and links the two ventral curtain-like structures which mark the 1"'/2"' and 6"'/parasulcal zone boundaries. This transverse parasuture is often reflected by a trabecula, with or without the corresponding septal feature (compare Fig. 2C with Figs. 4A and 6A,B).
Dimensions: Holotype, overall: length 87µm, width 85µm; cyst body: length 52µm, width 40µm; maximum length of processes: 32µm. Observed range, overall: length 61-(79)-106µm, width 58-(79)-108µm; cyst body: length 38-(48)-63µm, width 29-(38.5)-50µm; maximum length of processes: 17-(25)-37µm. Number of specimens measured: 169.
Discussion: The marginate appearance of Seriliodinium explicatum is enhanced by the fact that its processes tend to "splay out" towards the periphery, suggesting a certain degree of dorsolateral compression. When there is minimal compression, the arrangement of the trabeculae is more readily appreciated (Fig. 4A,B). The general absence of trabeculae across the mid-ventral and mid-dorsal areas leaves several processes with some distally free furcations, including the complex anterior ventral process, and the complex processes at the lc/2c, 2c/3c and 4c/5c boundaries. The variable development of trabeculae reflecting the 3"'/4"' and 4"'/5"' boundaries leaves the paired process at the posterior margin of 4"' with one or two free furcations (Plate II, 2, 5; compare Fig. 2B,D with Fig. 3B,D). Although the gonyaulacoid paratabulation is not fully defined, the hypocystal pattern is sexiform and the visible features indicate an S-type ventral arrangement (Evitt, 1985). The ventral limits of the paracingulum are also not well defined, but this key structure is certainly laevorotatory, and its alignment to the left and right of the mid-ventral area suggests a ventral displacement of at least its own width. Other paratabulatory features include the relative position of precingular 6" which is anterior to the right posterior parasulcal area, rather than 6c (and 6"'), and the relative size of 1"' which appears to be quite broad. The relationship between 1"' and lp and p.s. is variable. In many specimens 1"' is in contact posteriorly with lp and p.s. (Fig. 5A,C,G), but in others there only appears to be contact with lp (Figs. 4B, 5E). Also, only three apical paraplates have been distinguished, but it is possible that the area designated 1' in Figs. 2, 3 and 5 represents two ventral apicals (1' and 4'). A hint of this division is suggested by the arrangement of the free furcations associated with the complex anterior ventral process, but the evidence is not conclusive.
The foregoing description and remarks are based on the examination of a large number of specimens of S. explicatum, but some anomalous features were also noted. Occasionally, reduced processes are developed in the mid-ventral area (Plate III, 4; "y" in Fig. 6B). In the illustrated example, this simple, short, isolated process apparently lies on the boundary between lc and the parasulcus, and is probably gonal in position. Its dense texture is comparable with the usual style of the trabeculae. The specimen in Plate III, 2 and Fig. 7A shows a mid-dorsal process associated with the anterior margin of the paracingulum. This process is bifurcate, and distally united by trabeculae with the two adjacent, typically complex, paracingular processes at the 2c/3c and 4c/5c boundaries. The furcations of the latter two processes, which are united by the two anomalous trabecular elements, are usually distally free. A similar mid-dorsal process associated with the posterior margin of the paracingulum is shown in Fig. 7B. This is also bifurcate, and has the dense trabecular texture, but it is distally free with no evidence of accompanying trabeculae. These anomalous features are still parasutural in position; however, the specimen in Plate III, 1 and Fig. 7C shows a non-parasutural feature. A process associated with the anterior margin of the mid-dorsal paracingulum is again featured, with a distal transverse trabecular link to the right adjacent paracingular process (4c/Sc boundary), and a free furcation which could be potentially united with the left adjacent paracingular process (2c/3c boundary). However, there is also a longitudinal trabecular link to the transverse trabecula which reflects the 3'/3" boundary. The intersection between the two is not gonal in position, and the anomalous trabecula appears to cross 3". This is the only non-parasutural feature recorded in this study of S. explicatum.
In discussing the generic concept of Seriliodinium, comparisons have necessarily been made with other trabeculate cysts of the Spiniferites complex (Evitt, 1985), but in terms of specific morphology Seriliodinium explicatum is most similar to the non-trabeculate chorate cyst Spiniferites cruciformis Wall and Dale ln wan et al., 1973. Both species are characterised by a rounded cruciform cyst body shape, and both are capable of developing curtain-like membranes at specific parasutural sites. The examination of rare examples of Spiniferites cruciformis recorded in the present study shows that it may also display a posterior ventral transverse septal membrane (Plate IV, 4, 5; Fig. 6C,D). This feature is also shown in a specimen illustrated in Wall et al. (1973, pl. 2, fig. 3). As in Seriliodinium explicatum, this membrane defines the anterior margin of the posterior sulcal paraplate (p.s.), and links the ventral longitudinal curtain-like membranes. Furthermore, the parasutural position of the latter membranes in Spiniferites cruciformis appears to be comparable with that of Seriliodinium explicatum ( 1"'/2"' and 6"'/parasulcal zone). In their original description of Spiniferites cruciformis, Wall and Dale made the same interpretation of the right membrane, but they considered that the left membrane marked the 2"'/3"' boundary. The 1"'/2"' interpretation preferred here, can be applied to the specimen illustrated in Wall et al. (1973, pl. 2, fig. 3) which shows this membrane impinging on the left anterior margin of lp, rather than running into the left posterior margin. These details as well as the other morphological similarities, suggest that a very close relationship existed between Seriliodinium explicatum and Spiniferites cruciformis, and presumably between their corresponding motile stages.
The morphological style of Seriliodinium explicatum and its possible significance
Although Seriliodinium explicatum can be referred to the Spiniferites complex of Evitt (1985) on the basis of the general form and distribution of its processes and its overall paratabulation pattern, it also displays features which appear to be atypical of this complex. These are, a cruciform cyst body shape, the presence of single parasutural trabeculae, and the selective development of processes and trabeculae which results in a marginate overall appearance. As noted earlier, a cruciform cyst body shape is also a characteristic of Spiniferites cruciformis. In their original description of this species, Wall and Dale (in Wall et al., 1973) interpreted this feature and the considerable range of morphological variation displayed by S. cruciformis, as adaptive mor- phological responses to abnormal salinities. They considered S. cruciformis to be a cool water, stenohaline, low salinity species which thrived in the Black Sea during the Late Pleistocene to earliest Mesoholocene period, but was unable to survive the subsequent influx of saline water when it was replaced by euryhaline marine species (see also Wall and Dale, 1973). Wall and Dale's interpretation of the palaeoenvironmental significance of Spiniferites cruciformis was supported by independent evidence which is not available with respect to Seriliodinium explicatum. However, it seems reasonable to suggest that the cruciform cyst body shape and range of variation in Seriliodinium explicatum also represent adaptive morphological responses to a stressed environment, possibly associated with abnormal (?low) salinity. The furcate processes which characterise the Spiniferites complex usually display bifid terminations (Evitt, 1985, fig. 10.7), and distal linkage of these terminations would normally be achieved through the development of paired penitabular trabeculae as in Nematosphaeropsis. These paired penitabular elements may be separate as in Nematosphaeropsis rigida Wrenn, 1988 (see also Evitt, 1985, fig. 10.7M), or variably united by membrane as in Nematosphaeropsis lativittatus Wrenn, 1988 (see also Evitt, 1985, fig. 10.7N). The extensive development of such membrane would ultimately result in a distal network of relatively broad trabeculae which are essentially parasutural in position, although they actually consist of laterally united, paired penitabular elements. This is the situation in Nematosphaeropsis downiei Brown, 1986, and apparently also in Unipontidinium aquaeductum (Piasecki) Wrenn, 1988 (see Piasecki, 1980, pl. 5, figs. 1, 2). The same overall effect is achieved in Seriliodinium explicatum, but here the trabeculae appear to be genuinely single and parasutural, with no evidence of penitabular pairing. It is possible that the potential uniting of paired trabecular elements in Nematosphaeropsis through membrane development is an adaptive morphological response to environmental influence. If so, then the development of single trabeculae in Seriliodinium explicatum could be an extreme response to a stressed environment, resulting in the suppression of the bifid terminations of the processes and their influence on trabecular development. Cysts referable to the Spiniferites complex may appear to have a fairly regular distribution of processes and parasutural ornament, but in fact these features are often suppressed on the ventral surface and especially in the parasulcal area (Evitt, 1985). In Seriliodinium explicatum this suppression is considerable, and results in both the mid-ventral and mid-dorsal areas usually being devoid of processes and trabeculae. Again it is possible that this selective development of processes and trabeculae which produces a marginate overall form, is also an adaptive morphological response to a stressed environment. This interpretation implies that under appropriate environmental conditions the parent dinoflagellate of S. explicatum might have produced a more conventional cyst morphotype with a full complement of processes and trabeculae. If so, then the occasional and apparently anomalous development of these morphological elements on the mid-ventral and mid-dorsal areas in Seriliodinium explicatum may represent an abortive attempt to produce that more conventional cyst morphotype.