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Gonyaulacysta adecta
Original description (Sarjeant, 1982b):
Gonyaulacysta jurassica (DEFLANDRE) subsp. adecta nov.
Pls. 1-3; pl. 4, figs. 1-4, 9; pl 6, figs. 4-5, 9.
DERIVATION OF NAME: Greek adektos, unbitten, unmolested.
DIAGNOSIS: A su bspecies of Gonyaulacysta jurassica in which no hypotractal cavation is developed. nor is an opisthopyle present.
HOLOTYPE: Specimen SWS 2459/1/6, slide GMUS Pd 249; figured herein, pl. 1, fig. 2. (Lodged in the collections of the Department of Geological Sciences, University of Saskatchewan, Canada). Middle Jurassic (Callovian: Jason Zone), Isle of Skye, western Scotland.
PARATYPES: Specimens SWS 2459/1/134 and SwS 2459/1/138, slide GMUS Pd 249; figured herein, pl. 1, figs. 5, 6. Same locality and lodgement.
DIMENSIONS: Holotype: overall length 77 μm, length of epiperico el 20 μm, length of endoblast 50 um, overall breadth 62 μm, breadth of endoblast 45 um. Paratype A: overall length 57 μm, length of epipericoel 12 μm, length of endoblast 45 μm, overall breadth 37 μm, breadth of endoblast 31 μm.
Paratype B: overall length 79 μm, length of epipericoel 22 μm, length of endoblast 56 μm, overall breadth 51 μm, breadth of endoblast 43 μm. Range of dimensions: overall length 56-105 μm. 30 specimens measured.
DISCUSSION: Though both subspecies here distinguished are present together in the type horizon for the species, each has an independent history, G. jurassica subsp. adecta appearing in the Callovian and fading out in the Early Jurassic, G. jurassica subsp. jurassica developing from it in the early Oxfordian and persisting into the Middle or Late Kimmeridgian. Since each thus has not only an evolutionary but also a stratigraphical significance, nomenclatural separation appears justified despite their overlapping range.
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Emended description Riding et al., 2022:
Emended diagnosis: A subpentagonal, epicavate species of Gonyaulacysta, intermediate in size, with a short to moderately long apical horn. The L-type sulcus may be slightly sigmoidal. Most of the sutural features are denticulate/echinate sutural crests and ridges. The periphragm and endophragm are normally smooth. An opisthopyle is not observable/present.
Emended description: A subpentagonal species of Gonyaulacysta, intermediate in size, with epicavate or rarely cornucavate cyst organisation. The boundaries/sutures between the precingular and postcingular plates may be suturocavate. The pericyst has a short to relatively prominent horn, which is distally truncate. The endocyst may exhibit a small rounded apical protuberance, or it may be ovoidal. The tabulation is indicated by denticulate/echinate sutural ridges or crests which are variable in height. The denticles/echinae are longest at the top of the precingular plates, around the cingulum and the antapical plate. The antapical plate may have small gonal spines up to 4 μm in length. The periphragm and endophragm are both smooth, but low relief nontabular ornamentation may be present. The periphragmis significantly thinner than the endophragm. Because the two wall layers are not separated in the antapical region the opisthopyle, if developed, is not visible. The sulcuses of some specimens are slightly sigmoidal in overall shape.
Holotype: Specimen SWS 2459/1/6, slide GMUS Pd 249 (Sarjeant, 1982, pl. 1, 2). Sample from the Dunans Shale Member of the Staffin Shale Formation (Kosmoceras jason ammonite zone), Trotternish, Isle of Skye, northwestern Scotland (NGR NG 472708). Curated in the Department of Geological Sciences, University of Saskatoon, Saskatchewan, Canada.
Comments: Sarjeant (1982) noted that Gonyaulacysta jurassica, as originally conceived, can either be bicavate or epicavate. Furthermore, Sarjeant (1982) perceptively reported that the earliest representatives of Gonyaulacysta jurassica, largely from the Callovian, are exclusively epicavate and have relatively short horns, whereas the bicavate morphotypes are confined to the Oxfordian and Kimmeridgian. The holotype of Gonyaulacysta jurassica is earliest Oxfordian in age, and is bicavate. Therefore, the largely Callovian epicavate forms can be consistently and easily separated from Gonyaulacysta jurassica. They were assigned to Gonyaulacysta jurassica subsp. adecta by Sarjeant (1982, p. 30–31). The subspecies name was derived from the Greek adektos, meaning unbitten or unmolested. Given that our principal aimis to simplify the taxonomy of the genus Gonyaulacysta, we here elevate Gonyaulacysta jurassica subsp. adecta to species status. Gonyaulacysta adecta is a distinctly subpentangular, relatively squat species with an epicavate or cornucavate structure that is virtually always distinguishable from the bicavate Gonyaulacysta jurassica. However, Gonyaulacysta adecta and Gonyaulacysta jurassica overlap in size and so, if the antapical area is damaged or obscured, the differentiation of these species may be problematical. All specimens of Gonyaulacysta adecta have an apical horn of moderate size. Because the cyst-wall layers are in contact in the antapical region, an opisthopyle on the ps plate may well be present but is never visible. Many specimens are apparently not suturocavate, but some individuals demonstrably exhibit this feature in the precingular and postcingular plate series (Plate III, 8). Riding (1983, pl. 2, 9) illustrated a specimen which clearly exhibits suturocavation within the 1′′ plate. In Gonyaulacysta adecta, both cyst wall layers are normally smooth, however low-relief, nontabular ornamentation such as granules may occasionally be present. The endophragm is markedly thick and robust. The sutural crests posterior to the apical series are often relatively high (up to 9 μm) and are distally echinate or denticulate. Gonyaulacysta adecta is emended here to expand the somewhat brief diagnosis of Sarjeant (1982, p. 30), who did not document the morphology in detail. This species is appreciably variable in size, as discussed below. As mentioned above, in some specimens the sulcus is slightly sigmoidal. This phenomenon is simply a relatively minor intraspecific variation of the L-type sulcus in Gonyaulacysta adecta where the sutures of the left-hand sides of plates 1′′, 1c and 2′′′, and the right-hand sides of plates 6′′, 6c and 6′′′ are curved and undulating (Fig. 5B, Plate III, 4 and Plate IV, 1, 2, 7), as opposed to being more or less straight (Fig. 5A, Plate III, 5, 6). This tendency towards a sigmoidal shape is relatively minor, and does not imply that these morphotypes have a S-type sulcus of Evitt (1985). This phenomenon represents intraspecific variability. There is a morphological continuum between these two morphotypes, and the two extremes are illustrated in Fig. 5.
Comparison: Gonyaulacysta jurassica is the most similar species to. Gonyaulacysta adecta, but the former is consistently bicavate and has an opisthopyle (Fig. 2).
Dimensions: Sarjeant (1982, p. 30) designated two paratypes of Gonyaulacysta jurassica subsp. adecta,which he labelled A and B; the two are markedly different in size. The overall lengths of paratypes A and B are 57 μm and 79 μm respectively according to Sarjeant (1982). In order to investigate the size range of Gonyaulacysta adecta, the length and width of 303 specimensweremeasured and plotted (Fig. 6, Supplementary material Appendix 3, table 2). These data show that this species is strikingly variable in size, ranging from44 μm to 93 μmin length. However, Gonyaulacysta adecta displays a continuum in size variability, and it is not possible to consistently identify the two paratypes documented by Sarjeant (1982) (Fig. 6). Further investigations were carried out to study the size variability of this species throughout the Callovian. One continuous and wellpreserved Callovian succession fromthe Denver Sluice Borehole in Norfolk, eastern England, and five sections together forming a composite from England and Scotland were studied. The latter section comprises material from the Nettleton Bottom, Warboys and Warlingham boreholes in central and southern England, as well as outcrop material from Bletchley Brick Pit, Buckinghamshire, southern England, and Staffin Bay, Isle of Skye, northwest Scotland (Woollam and Riding, 1983, appendix 1; Riding, 1987; Riding and Thomas, 1997). The dimensions of Gonyaulacysta adecta from each ammonite zone in these successions were measured, and we compared the two sections (Fig. 7). Note that the Proplanulites koenigi/Sigaloceras calloviense ammonite zones and Kosmoceras jason/Erymnoceras coronatum ammonite zones could not be differentiated in the English succession. The trends recorded in each zone in the Denver Sluice Borehole and the composite section are based on data in Supplementarymaterial, Appendix 3, tables 3, 4. The lower Callovian strata are characterised by relatively small specimens that only occasionally approach 70 μm in length. Large specimens, exceeding 70 μmin length, first appeared in the Kosmoceras jason ammonite zone, and became more common throughout themiddle and upper Callovian, resulting in constant co-existence of small and large specimens. In the uppermost Callovian (Quenstedtoceras lamberti ammonite zone), the minimum length apparently increased (Fig. 7). The size patterns recorded in both succssions are extremely similar, suggesting that this trend of increasing size with time is consistent. Size data based on four specimens of Gonyaulacysta adecta from the uppermost Bathonian (Clydoniceras discus ammonite zone) of southwest England are as follows: length of pericyst, 49 (57) 62; length of apical horn, 9 (9.5) 11; length of epipericyst, 29 (38) 44; length of hypopericyst, 7 (15) 20; length of endocyst, 40 (45) 51; and width at cingulum, 38 (44) 51 (Supplementary material, Appendix 3, table 5). The dimensions of 60 specimens examined herein from the lower Callovian are: length of pericyst, 49 (58) 67; length of apical horn, 6 (9) 16; length of epipericyst, 29 (37) 49; length of hypopericyst, 9 (16) 27; length of endocyst, 33 (42) 58; and width at cingulum, 31 (43) 62 (Supplementary material, Appendix 3, table 6). These results compare well with the dimensions of paratype A of Sarjeant (1982), i.e. length of pericyst, 57; length of epipericoel, 12; length of endocyst, 45;width of endocyst, 31;width at cingulum, 37 (Sarjeant, 1982, p. 30). The dimensions of 30 specimens from the upper Callovian are: length of pericyst, 53 (67) 84; length of apical horn, 7 (13) 20; length of epipericyst, 33 (45) 55; length of hypopericyst, 9 (19) 27; length of endocyst, 38 (50) 64; and width at cingulum, 40 (46) 64 (Supplementary material, Appendix 3, table 7). The average size increased during the late Callovian due to the first occurrences of larger forms in the middle Callovian, as discussed above.
Geographical and stratigraphical distribution: Gonyaulacysta adecta is the earliest species of the genus. It ranges from the earliest Bathonian (Zigzagiceras zigzag ammonite zone) to the middle Oxfordian (Cardioceras tenuiserratum ammonite zone) of the Northern Hemisphere (Fig. 3; Sarjeant, 1972; Fenton et al., 1980; Feist Burkhardt and Wille, 1992; Poulsen, 1996; Riding and Thomas, 1997; Riding et al., 1999; Riding, 2005b; Wiggan et al., 2017; Wiggan et al., 2018). Occasional reports of Gonyaulacysta adecta from the late Oxfordian are known; these may represent reworking. For example, Riding (1987, fig. 5) noted an isolated occurrence from the lowermost upper Oxfordian (Amoeboceras glosense ammonite zone) of Eastern England. Also, the specieswas encountered in the upper Oxfordian (Amoeboceras serratum and Amoeboceras regulare ammonite zones) by Thomas and Cox (1988, fig. 3). The latter report appears to be substantially inconsistentwith all other investigations. Gonyaulacysta adecta is extremely rare in the Bathonian and its consistent range is early Callovian (Macrocephalites herveyiammonite zone) tomiddle Oxfordian (Cardioceras tenuiserratum ammonite zone). It is normally common throughout the Callovian (Fig. 3; Riding and Thomas, 1997; Riding, 2005b).
Gonyaulacysta adecta has never been reported from Australasia. However, it occurs in the upper Callovian Lotena Formation of the Neuquén Basin in west-central Argentina (Martinez and Quattrocchio, 2004; Quattrocchio et al., 2007; Riding et al., 2011). The Argentinian late Callovian dinoflagellate cyst assemblages are more similar to their counterparts in Europe than in Australia. This may be due to a marine connection between South America and Europe in the Hispanic Corridor, and the circulation of the circum-Tropical Marine Current (Riding et al., 2011). Gonyaulacysta adecta was also reported from the Callovian–Oxfordian Maguazo Unit of Ecuador (Litherland et al., 1994).
Gonyaulacysta jurassica (DEFLANDRE) subsp. adecta nov.
Pls. 1-3; pl. 4, figs. 1-4, 9; pl 6, figs. 4-5, 9.
DERIVATION OF NAME: Greek adektos, unbitten, unmolested.
DIAGNOSIS: A su bspecies of Gonyaulacysta jurassica in which no hypotractal cavation is developed. nor is an opisthopyle present.
HOLOTYPE: Specimen SWS 2459/1/6, slide GMUS Pd 249; figured herein, pl. 1, fig. 2. (Lodged in the collections of the Department of Geological Sciences, University of Saskatchewan, Canada). Middle Jurassic (Callovian: Jason Zone), Isle of Skye, western Scotland.
PARATYPES: Specimens SWS 2459/1/134 and SwS 2459/1/138, slide GMUS Pd 249; figured herein, pl. 1, figs. 5, 6. Same locality and lodgement.
DIMENSIONS: Holotype: overall length 77 μm, length of epiperico el 20 μm, length of endoblast 50 um, overall breadth 62 μm, breadth of endoblast 45 um. Paratype A: overall length 57 μm, length of epipericoel 12 μm, length of endoblast 45 μm, overall breadth 37 μm, breadth of endoblast 31 μm.
Paratype B: overall length 79 μm, length of epipericoel 22 μm, length of endoblast 56 μm, overall breadth 51 μm, breadth of endoblast 43 μm. Range of dimensions: overall length 56-105 μm. 30 specimens measured.
DISCUSSION: Though both subspecies here distinguished are present together in the type horizon for the species, each has an independent history, G. jurassica subsp. adecta appearing in the Callovian and fading out in the Early Jurassic, G. jurassica subsp. jurassica developing from it in the early Oxfordian and persisting into the Middle or Late Kimmeridgian. Since each thus has not only an evolutionary but also a stratigraphical significance, nomenclatural separation appears justified despite their overlapping range.
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Emended description Riding et al., 2022:
Emended diagnosis: A subpentagonal, epicavate species of Gonyaulacysta, intermediate in size, with a short to moderately long apical horn. The L-type sulcus may be slightly sigmoidal. Most of the sutural features are denticulate/echinate sutural crests and ridges. The periphragm and endophragm are normally smooth. An opisthopyle is not observable/present.
Emended description: A subpentagonal species of Gonyaulacysta, intermediate in size, with epicavate or rarely cornucavate cyst organisation. The boundaries/sutures between the precingular and postcingular plates may be suturocavate. The pericyst has a short to relatively prominent horn, which is distally truncate. The endocyst may exhibit a small rounded apical protuberance, or it may be ovoidal. The tabulation is indicated by denticulate/echinate sutural ridges or crests which are variable in height. The denticles/echinae are longest at the top of the precingular plates, around the cingulum and the antapical plate. The antapical plate may have small gonal spines up to 4 μm in length. The periphragm and endophragm are both smooth, but low relief nontabular ornamentation may be present. The periphragmis significantly thinner than the endophragm. Because the two wall layers are not separated in the antapical region the opisthopyle, if developed, is not visible. The sulcuses of some specimens are slightly sigmoidal in overall shape.
Holotype: Specimen SWS 2459/1/6, slide GMUS Pd 249 (Sarjeant, 1982, pl. 1, 2). Sample from the Dunans Shale Member of the Staffin Shale Formation (Kosmoceras jason ammonite zone), Trotternish, Isle of Skye, northwestern Scotland (NGR NG 472708). Curated in the Department of Geological Sciences, University of Saskatoon, Saskatchewan, Canada.
Comments: Sarjeant (1982) noted that Gonyaulacysta jurassica, as originally conceived, can either be bicavate or epicavate. Furthermore, Sarjeant (1982) perceptively reported that the earliest representatives of Gonyaulacysta jurassica, largely from the Callovian, are exclusively epicavate and have relatively short horns, whereas the bicavate morphotypes are confined to the Oxfordian and Kimmeridgian. The holotype of Gonyaulacysta jurassica is earliest Oxfordian in age, and is bicavate. Therefore, the largely Callovian epicavate forms can be consistently and easily separated from Gonyaulacysta jurassica. They were assigned to Gonyaulacysta jurassica subsp. adecta by Sarjeant (1982, p. 30–31). The subspecies name was derived from the Greek adektos, meaning unbitten or unmolested. Given that our principal aimis to simplify the taxonomy of the genus Gonyaulacysta, we here elevate Gonyaulacysta jurassica subsp. adecta to species status. Gonyaulacysta adecta is a distinctly subpentangular, relatively squat species with an epicavate or cornucavate structure that is virtually always distinguishable from the bicavate Gonyaulacysta jurassica. However, Gonyaulacysta adecta and Gonyaulacysta jurassica overlap in size and so, if the antapical area is damaged or obscured, the differentiation of these species may be problematical. All specimens of Gonyaulacysta adecta have an apical horn of moderate size. Because the cyst-wall layers are in contact in the antapical region, an opisthopyle on the ps plate may well be present but is never visible. Many specimens are apparently not suturocavate, but some individuals demonstrably exhibit this feature in the precingular and postcingular plate series (Plate III, 8). Riding (1983, pl. 2, 9) illustrated a specimen which clearly exhibits suturocavation within the 1′′ plate. In Gonyaulacysta adecta, both cyst wall layers are normally smooth, however low-relief, nontabular ornamentation such as granules may occasionally be present. The endophragm is markedly thick and robust. The sutural crests posterior to the apical series are often relatively high (up to 9 μm) and are distally echinate or denticulate. Gonyaulacysta adecta is emended here to expand the somewhat brief diagnosis of Sarjeant (1982, p. 30), who did not document the morphology in detail. This species is appreciably variable in size, as discussed below. As mentioned above, in some specimens the sulcus is slightly sigmoidal. This phenomenon is simply a relatively minor intraspecific variation of the L-type sulcus in Gonyaulacysta adecta where the sutures of the left-hand sides of plates 1′′, 1c and 2′′′, and the right-hand sides of plates 6′′, 6c and 6′′′ are curved and undulating (Fig. 5B, Plate III, 4 and Plate IV, 1, 2, 7), as opposed to being more or less straight (Fig. 5A, Plate III, 5, 6). This tendency towards a sigmoidal shape is relatively minor, and does not imply that these morphotypes have a S-type sulcus of Evitt (1985). This phenomenon represents intraspecific variability. There is a morphological continuum between these two morphotypes, and the two extremes are illustrated in Fig. 5.
Comparison: Gonyaulacysta jurassica is the most similar species to. Gonyaulacysta adecta, but the former is consistently bicavate and has an opisthopyle (Fig. 2).
Dimensions: Sarjeant (1982, p. 30) designated two paratypes of Gonyaulacysta jurassica subsp. adecta,which he labelled A and B; the two are markedly different in size. The overall lengths of paratypes A and B are 57 μm and 79 μm respectively according to Sarjeant (1982). In order to investigate the size range of Gonyaulacysta adecta, the length and width of 303 specimensweremeasured and plotted (Fig. 6, Supplementary material Appendix 3, table 2). These data show that this species is strikingly variable in size, ranging from44 μm to 93 μmin length. However, Gonyaulacysta adecta displays a continuum in size variability, and it is not possible to consistently identify the two paratypes documented by Sarjeant (1982) (Fig. 6). Further investigations were carried out to study the size variability of this species throughout the Callovian. One continuous and wellpreserved Callovian succession fromthe Denver Sluice Borehole in Norfolk, eastern England, and five sections together forming a composite from England and Scotland were studied. The latter section comprises material from the Nettleton Bottom, Warboys and Warlingham boreholes in central and southern England, as well as outcrop material from Bletchley Brick Pit, Buckinghamshire, southern England, and Staffin Bay, Isle of Skye, northwest Scotland (Woollam and Riding, 1983, appendix 1; Riding, 1987; Riding and Thomas, 1997). The dimensions of Gonyaulacysta adecta from each ammonite zone in these successions were measured, and we compared the two sections (Fig. 7). Note that the Proplanulites koenigi/Sigaloceras calloviense ammonite zones and Kosmoceras jason/Erymnoceras coronatum ammonite zones could not be differentiated in the English succession. The trends recorded in each zone in the Denver Sluice Borehole and the composite section are based on data in Supplementarymaterial, Appendix 3, tables 3, 4. The lower Callovian strata are characterised by relatively small specimens that only occasionally approach 70 μm in length. Large specimens, exceeding 70 μmin length, first appeared in the Kosmoceras jason ammonite zone, and became more common throughout themiddle and upper Callovian, resulting in constant co-existence of small and large specimens. In the uppermost Callovian (Quenstedtoceras lamberti ammonite zone), the minimum length apparently increased (Fig. 7). The size patterns recorded in both succssions are extremely similar, suggesting that this trend of increasing size with time is consistent. Size data based on four specimens of Gonyaulacysta adecta from the uppermost Bathonian (Clydoniceras discus ammonite zone) of southwest England are as follows: length of pericyst, 49 (57) 62; length of apical horn, 9 (9.5) 11; length of epipericyst, 29 (38) 44; length of hypopericyst, 7 (15) 20; length of endocyst, 40 (45) 51; and width at cingulum, 38 (44) 51 (Supplementary material, Appendix 3, table 5). The dimensions of 60 specimens examined herein from the lower Callovian are: length of pericyst, 49 (58) 67; length of apical horn, 6 (9) 16; length of epipericyst, 29 (37) 49; length of hypopericyst, 9 (16) 27; length of endocyst, 33 (42) 58; and width at cingulum, 31 (43) 62 (Supplementary material, Appendix 3, table 6). These results compare well with the dimensions of paratype A of Sarjeant (1982), i.e. length of pericyst, 57; length of epipericoel, 12; length of endocyst, 45;width of endocyst, 31;width at cingulum, 37 (Sarjeant, 1982, p. 30). The dimensions of 30 specimens from the upper Callovian are: length of pericyst, 53 (67) 84; length of apical horn, 7 (13) 20; length of epipericyst, 33 (45) 55; length of hypopericyst, 9 (19) 27; length of endocyst, 38 (50) 64; and width at cingulum, 40 (46) 64 (Supplementary material, Appendix 3, table 7). The average size increased during the late Callovian due to the first occurrences of larger forms in the middle Callovian, as discussed above.
Geographical and stratigraphical distribution: Gonyaulacysta adecta is the earliest species of the genus. It ranges from the earliest Bathonian (Zigzagiceras zigzag ammonite zone) to the middle Oxfordian (Cardioceras tenuiserratum ammonite zone) of the Northern Hemisphere (Fig. 3; Sarjeant, 1972; Fenton et al., 1980; Feist Burkhardt and Wille, 1992; Poulsen, 1996; Riding and Thomas, 1997; Riding et al., 1999; Riding, 2005b; Wiggan et al., 2017; Wiggan et al., 2018). Occasional reports of Gonyaulacysta adecta from the late Oxfordian are known; these may represent reworking. For example, Riding (1987, fig. 5) noted an isolated occurrence from the lowermost upper Oxfordian (Amoeboceras glosense ammonite zone) of Eastern England. Also, the specieswas encountered in the upper Oxfordian (Amoeboceras serratum and Amoeboceras regulare ammonite zones) by Thomas and Cox (1988, fig. 3). The latter report appears to be substantially inconsistentwith all other investigations. Gonyaulacysta adecta is extremely rare in the Bathonian and its consistent range is early Callovian (Macrocephalites herveyiammonite zone) tomiddle Oxfordian (Cardioceras tenuiserratum ammonite zone). It is normally common throughout the Callovian (Fig. 3; Riding and Thomas, 1997; Riding, 2005b).
Gonyaulacysta adecta has never been reported from Australasia. However, it occurs in the upper Callovian Lotena Formation of the Neuquén Basin in west-central Argentina (Martinez and Quattrocchio, 2004; Quattrocchio et al., 2007; Riding et al., 2011). The Argentinian late Callovian dinoflagellate cyst assemblages are more similar to their counterparts in Europe than in Australia. This may be due to a marine connection between South America and Europe in the Hispanic Corridor, and the circulation of the circum-Tropical Marine Current (Riding et al., 2011). Gonyaulacysta adecta was also reported from the Callovian–Oxfordian Maguazo Unit of Ecuador (Litherland et al., 1994).