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Scriniodinium multistratum
Scriniodinium multistratum sp. nov., Pestchevitskaya and Lebedeva, 2017
Holotype: TsSGM, sp. 1, slide 35.1, specimen 35.1-1, N28, Pl. 14, figs. 1‒3, Pestchevitskaya and Lebedeva, 2017
Location: Nordvik section, northern Siberia (Paksa Peninsula, outcrop. 32, Bed 16; Boreal Berriasian)
Strat. range: Late Jurassic ‒ early Cretaceous (Upper Volgian and Berriasian)
Diagnosis:
Cyst proximate, circumcavate, commonly large, rarely medium-sized. Pericyst and mesocyst rounded or rounded rhombic, with short, widely conical apical horn, occasionally turning into shoulders. Endocyst rounded or rounded rhombic. Periphragm transparent, shagreen. Mesophragm and endophragm thicker, fine-grained. Tabulation faintly manifested by sutural features; formula 1pr (Q), 4', 6'', Xc, 5s, 6''', 1''''. Cingulum expressed in broad fold limited by narrow ridges. Archeopyle precingular, type 1P3 .
Description:
The cyst is proximate, cavate. The pericyst is rounded or rounded rhombic, with a somewhat projecting cingular region. The cyst is compressed mostly laterally. The apical region has a short widely conical horn, occasionally passing into prominent “shoulders”. The endocyst is rounded or rounded rhombic, usually with a projection along the cingulum in the left lateral region.
Wall: The periphragm is transparent, with a shagreen surface. The endophragm is denser, with a fine-grained sculpture. There are often one or two additional layers forming one or more mesocysts between the endo- and pericyst. It is usually difficult to discern multilayers, because the mesocysts have a sufficiently dense and slightly transparent wall and are sculptured by small granules, like the endocyst. The mesocysts have the same shape as the pericyst or smoother.
The pericoel between peri- and mesocyst is very narrow, often hardly discernible, sometimes almost absent. The pericoel between meso- and endocyst is wider, clearly visible, especially in the apical and antapical regions.
Archeopyle: precingular, of 1P 3 type; the operculum is usually attached.
Tabulation: weakly expressed by thin sutural lines on endo-, meso-, and pericyst. Due to the transparency of peri- and mesophragm, the sutural lines overlap each other;
therefore, ralations of the plates are very difficult to determine. Tabulation is gonyaulacoid; the formula 4', 6'', ?c, 5s, 6''', 1''''. Plate 4' in the apical series projects beyond the row and does not contact with the pore complex, which is bordered by a small extra plate Q (or Cv). Plates 1'', 3'', and 4'' are the largest in the precingular series. The first plate 1'' has a wide trapezoidal shape typical for gonyaulacoid dinocysts. The last plate 6'' is not reduced near the sulcus and comes in contact with the first 1' plate of the apical series. Plates 3''', 4''', and 5''' are the largest in the postcingular series. The first plate 1''' is strongly reduced. Suture 4'''/5''' is positioned more distally relative to suture 3''/4''; thus, the cyst has a sinistral torsion. The cingulum is marked by a broad fold bounded by narrow ribs. The sulcus is wide on the pericyst, narrower on the endocyst, straight or slightly oblique to the left in the precingular region; the right additional plate (ras) is in contact with the first precingular 1''. The postsulcal plate (ps) is large, wide, sometimes divided into two parts in the lower region. In addition to the pore complex, the sulcus has additional pores inthe middle of the postsulcal plate and at its junction with the left and right sulcal plates.
Variability:
The shape of the pericyst and endocyst is the most variable trait, ranging from round to oval rhombic. Rounded cysts have well-pronounced “shoulders”; in oval rhombic cysts, they are smoothed. The pericoel width also varies, which may be connected with the different positions of a cyst.
Measurements: in µm:
Holotype: pericyst length, 122; width, 110; endocyst length, 101; endocyst width, 96; apical horn length, 9; cingulum width, 8; apical pericoel length, 19; antapical pericoel length, 2.
Other specimens: pericyst, length 109–144; width, 73–135; endocyst length, 89–112; endocyst width, 73–118; apical horn length, 7–15; cingulum width, 7–13; apical pericoel length, 9–30; antapical pericoel length, 2–10.
The number of measured specimens: 20.
Affinities/Comparison:
The presence of several internal cysts distinguishes S. multistratum sp. nov. from morphologically similar species. The rhombic cyst and granular sculpture on the endocyst makes the new species similar to S. galeritum Klement (Klement, 1960). Distinctive features of the latter are the wider pericoel, especially in the antapical region, and a typical pitted‒meshed sculpture of the pericyst. The species S. intribile Riley, S. campanula, S. anceps (Raynaud) Jan du Chêne et al., and S. playfordii have rounded or rounded rhomboid cysts with an apical horn. The new species differs from S. intribile (Fisher, Riley, 1980) in the more pronounced sculpture, well-developed pericoel in the apical and antapical regions, sufficiently clear tabulation, and the presence of a cingular fold. The species S. campanula (Gocht, 1959; Below, 1981) has a larger apical horn, thinner transparent cyst and well-pronounced expansion in the cingular region, forming typical protruding cingular fold. The species S. anceps (Jan du Chêne et al., 1986) has a longer and wider apical horn, rather wide pericoel, finer sculpture on the peri- and endophragm; the cingulum is marked by a notch forming the cingular fold. In S. playfordii Cookson et Eisenack (Cookson and Eisenack, 1960), the pericoel is also wider, the periphragm is smooth, and the endophragm is finely pitted; the tabulation is expressed only by the archeopyle and cingulum. The species S. granulatum (Raynaud) Jan du Chêne et al. and S. glabrum (Duxbury) Jan du Chêne et al., having
oval or rounded oval cysts, a small apical horn, and tabulation, are also morphologically similar. However, these species are distinguished by the well-pronounced broad pericoel. Moreover, the endocyst of S. glabrum (Duxbury, 1977) has a shagreen sculpture, while in S. granulatum (Jan du Chêne et al., 1986), it is
granulated only on the hypocyst.
Remarks:
In S. multistratum, tabulation of the ventral area is L type: the sulcus is almost straight, the sixth precingular plate 6'' is nonreduced, pentagonal, and comes in contact with the first apical plate 1'. It is noteworthy that the sulcus is sometimes oblique to the left, which is not typical for gonyaulacoid dinocysts. The sulcus usually curves to the right and, in the case of strong curvature, it becomes sigmoid (S-type). Suture 4'''/5''' of the new species is positioned more distally than suture 3''/4'', i.e., it shows sinistral torsion of the Leptodinium type. This type of structure is also characterized by a strongly reduced additional postcingular 1p plate, which is sometimes included in the sulcal region. Perhaps, a homologue of this plate is present in S. multistratum and included in the postsulcal ps plate, dividing it into two parts in the lower part.
In addition to the usual pore complex, there are additional pores on the sulcus. In living dinoflagellates, they usually serve as an exit for the feeding pallum or pseudopodia (Konovalova, 1998; Pugachev, 2011). Thus, it can be assumed that dinoflagellates producing the cysts S. multistratum were heterotrophic or mixotrophic. Based on available material, it is generally accepted that the heterotrophic feeding mode is more typical for peridinioid dinoflagellates rather than for gonyaulacoid (Taylor, 1987; Okolodkov, 1999; Pugachev, 2011). However, it should be noted that peridinioid dinoflagellates became a flourishing group in the Late Cretaceous; therefore, ecological niches that were later occupied by peridinioids could have belonged to representatives of the order Gonyaulacales in the Late Jurassic–Early Cretaceous.
Some specimens have pores (or structures resembling craters) of uncertain function at the junction of pre- and postcingular plates with sulcal and apical series (Pl. 14, figs. 4, 5). These formations are observed in both peri- and endocysts, but each pore is only discernible at a certain focus due to the multiple layers, making them difficult to display. They are only visible in some specimens, possibly because of varying preservation. Extant dinoflagellates sometimes also have pores in the plates of the apical, pre-, and postcingular series, which provide exit for protective stinging filaments (Konovalova, 1998). The most numerous occurrences of the species S. multistratum are confined to the Upper Volgian deposits of the Craspedites okensis Ammonite Zone, which is often characterized by anaerobic conditions developing in a deepwater part of the lower sublittoral in the Nordvik section (Nikitenko et al., 2008). Perhaps, the new species emerged in connection with the development of adaptations to conditions unfavorable for dinoflagellates.
Holotype: TsSGM, sp. 1, slide 35.1, specimen 35.1-1, N28, Pl. 14, figs. 1‒3, Pestchevitskaya and Lebedeva, 2017
Location: Nordvik section, northern Siberia (Paksa Peninsula, outcrop. 32, Bed 16; Boreal Berriasian)
Strat. range: Late Jurassic ‒ early Cretaceous (Upper Volgian and Berriasian)
Diagnosis:
Cyst proximate, circumcavate, commonly large, rarely medium-sized. Pericyst and mesocyst rounded or rounded rhombic, with short, widely conical apical horn, occasionally turning into shoulders. Endocyst rounded or rounded rhombic. Periphragm transparent, shagreen. Mesophragm and endophragm thicker, fine-grained. Tabulation faintly manifested by sutural features; formula 1pr (Q), 4', 6'', Xc, 5s, 6''', 1''''. Cingulum expressed in broad fold limited by narrow ridges. Archeopyle precingular, type 1P3 .
Description:
The cyst is proximate, cavate. The pericyst is rounded or rounded rhombic, with a somewhat projecting cingular region. The cyst is compressed mostly laterally. The apical region has a short widely conical horn, occasionally passing into prominent “shoulders”. The endocyst is rounded or rounded rhombic, usually with a projection along the cingulum in the left lateral region.
Wall: The periphragm is transparent, with a shagreen surface. The endophragm is denser, with a fine-grained sculpture. There are often one or two additional layers forming one or more mesocysts between the endo- and pericyst. It is usually difficult to discern multilayers, because the mesocysts have a sufficiently dense and slightly transparent wall and are sculptured by small granules, like the endocyst. The mesocysts have the same shape as the pericyst or smoother.
The pericoel between peri- and mesocyst is very narrow, often hardly discernible, sometimes almost absent. The pericoel between meso- and endocyst is wider, clearly visible, especially in the apical and antapical regions.
Archeopyle: precingular, of 1P 3 type; the operculum is usually attached.
Tabulation: weakly expressed by thin sutural lines on endo-, meso-, and pericyst. Due to the transparency of peri- and mesophragm, the sutural lines overlap each other;
therefore, ralations of the plates are very difficult to determine. Tabulation is gonyaulacoid; the formula 4', 6'', ?c, 5s, 6''', 1''''. Plate 4' in the apical series projects beyond the row and does not contact with the pore complex, which is bordered by a small extra plate Q (or Cv). Plates 1'', 3'', and 4'' are the largest in the precingular series. The first plate 1'' has a wide trapezoidal shape typical for gonyaulacoid dinocysts. The last plate 6'' is not reduced near the sulcus and comes in contact with the first 1' plate of the apical series. Plates 3''', 4''', and 5''' are the largest in the postcingular series. The first plate 1''' is strongly reduced. Suture 4'''/5''' is positioned more distally relative to suture 3''/4''; thus, the cyst has a sinistral torsion. The cingulum is marked by a broad fold bounded by narrow ribs. The sulcus is wide on the pericyst, narrower on the endocyst, straight or slightly oblique to the left in the precingular region; the right additional plate (ras) is in contact with the first precingular 1''. The postsulcal plate (ps) is large, wide, sometimes divided into two parts in the lower region. In addition to the pore complex, the sulcus has additional pores inthe middle of the postsulcal plate and at its junction with the left and right sulcal plates.
Variability:
The shape of the pericyst and endocyst is the most variable trait, ranging from round to oval rhombic. Rounded cysts have well-pronounced “shoulders”; in oval rhombic cysts, they are smoothed. The pericoel width also varies, which may be connected with the different positions of a cyst.
Measurements: in µm:
Holotype: pericyst length, 122; width, 110; endocyst length, 101; endocyst width, 96; apical horn length, 9; cingulum width, 8; apical pericoel length, 19; antapical pericoel length, 2.
Other specimens: pericyst, length 109–144; width, 73–135; endocyst length, 89–112; endocyst width, 73–118; apical horn length, 7–15; cingulum width, 7–13; apical pericoel length, 9–30; antapical pericoel length, 2–10.
The number of measured specimens: 20.
Affinities/Comparison:
The presence of several internal cysts distinguishes S. multistratum sp. nov. from morphologically similar species. The rhombic cyst and granular sculpture on the endocyst makes the new species similar to S. galeritum Klement (Klement, 1960). Distinctive features of the latter are the wider pericoel, especially in the antapical region, and a typical pitted‒meshed sculpture of the pericyst. The species S. intribile Riley, S. campanula, S. anceps (Raynaud) Jan du Chêne et al., and S. playfordii have rounded or rounded rhomboid cysts with an apical horn. The new species differs from S. intribile (Fisher, Riley, 1980) in the more pronounced sculpture, well-developed pericoel in the apical and antapical regions, sufficiently clear tabulation, and the presence of a cingular fold. The species S. campanula (Gocht, 1959; Below, 1981) has a larger apical horn, thinner transparent cyst and well-pronounced expansion in the cingular region, forming typical protruding cingular fold. The species S. anceps (Jan du Chêne et al., 1986) has a longer and wider apical horn, rather wide pericoel, finer sculpture on the peri- and endophragm; the cingulum is marked by a notch forming the cingular fold. In S. playfordii Cookson et Eisenack (Cookson and Eisenack, 1960), the pericoel is also wider, the periphragm is smooth, and the endophragm is finely pitted; the tabulation is expressed only by the archeopyle and cingulum. The species S. granulatum (Raynaud) Jan du Chêne et al. and S. glabrum (Duxbury) Jan du Chêne et al., having
oval or rounded oval cysts, a small apical horn, and tabulation, are also morphologically similar. However, these species are distinguished by the well-pronounced broad pericoel. Moreover, the endocyst of S. glabrum (Duxbury, 1977) has a shagreen sculpture, while in S. granulatum (Jan du Chêne et al., 1986), it is
granulated only on the hypocyst.
Remarks:
In S. multistratum, tabulation of the ventral area is L type: the sulcus is almost straight, the sixth precingular plate 6'' is nonreduced, pentagonal, and comes in contact with the first apical plate 1'. It is noteworthy that the sulcus is sometimes oblique to the left, which is not typical for gonyaulacoid dinocysts. The sulcus usually curves to the right and, in the case of strong curvature, it becomes sigmoid (S-type). Suture 4'''/5''' of the new species is positioned more distally than suture 3''/4'', i.e., it shows sinistral torsion of the Leptodinium type. This type of structure is also characterized by a strongly reduced additional postcingular 1p plate, which is sometimes included in the sulcal region. Perhaps, a homologue of this plate is present in S. multistratum and included in the postsulcal ps plate, dividing it into two parts in the lower part.
In addition to the usual pore complex, there are additional pores on the sulcus. In living dinoflagellates, they usually serve as an exit for the feeding pallum or pseudopodia (Konovalova, 1998; Pugachev, 2011). Thus, it can be assumed that dinoflagellates producing the cysts S. multistratum were heterotrophic or mixotrophic. Based on available material, it is generally accepted that the heterotrophic feeding mode is more typical for peridinioid dinoflagellates rather than for gonyaulacoid (Taylor, 1987; Okolodkov, 1999; Pugachev, 2011). However, it should be noted that peridinioid dinoflagellates became a flourishing group in the Late Cretaceous; therefore, ecological niches that were later occupied by peridinioids could have belonged to representatives of the order Gonyaulacales in the Late Jurassic–Early Cretaceous.
Some specimens have pores (or structures resembling craters) of uncertain function at the junction of pre- and postcingular plates with sulcal and apical series (Pl. 14, figs. 4, 5). These formations are observed in both peri- and endocysts, but each pore is only discernible at a certain focus due to the multiple layers, making them difficult to display. They are only visible in some specimens, possibly because of varying preservation. Extant dinoflagellates sometimes also have pores in the plates of the apical, pre-, and postcingular series, which provide exit for protective stinging filaments (Konovalova, 1998). The most numerous occurrences of the species S. multistratum are confined to the Upper Volgian deposits of the Craspedites okensis Ammonite Zone, which is often characterized by anaerobic conditions developing in a deepwater part of the lower sublittoral in the Nordvik section (Nikitenko et al., 2008). Perhaps, the new species emerged in connection with the development of adaptations to conditions unfavorable for dinoflagellates.