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Fallaciodinium minutum
Fallaciodinium minutum Harding, 1998, p.91,93,95-97, pl.1, figs.1-3,5-8,10-11; pl.2, figs.1-13; text-figs.1a-c.
Holotype: Harding, 1998, pl.1, figs.1-2.
Age: late Barremian.
Fallaciodinium minutum Harding, sp. nov. (Plate I, l 3,5 6, 7-8, 10-11; Plate II, 1-13)
Holotype: Plate I, 1 2.
Repository: Sedgwick Museum, Cambridge, England. Catalogue number: X.27547.
Type locality/horizon: Otto Gott claypit, nr. Sarstedt, Germany, Bed 197 (Mutterlose, 1984). Etymology: Latin, minutus meaning minute. Diagnosis: A proximochorate dinocyst of diminutive size (<20 µm diameter), appearing superficially like an acanthomorph acritarch, but displaying atypical cladopyxiinean tabulation and a Type Ea (epicystal) archaeopyle.
Description:
Shape: Cyst ambitus is subcircular to ovoidal and the cyst shows slight primary dorsoventral compression. Where elongation is developed this occurs along the apico-antapical axis (although dorso-ventral compaction of excysted specimens causes artificial elongation across the cingulum). Greatest width along the posterior cingular/postcingular suture. Hyposome is large and comprises over half of the length of the cyst. The cingulum is also very broad and accounts for a further quarter of the cyst length. The episome is, by comparison, relatively small, making up the remaining cyst length.
Phragma: The autophragm is thin (~0.2- 0.4 µm), but despite this the shape is retained well due to the small size of the cyst. The cyst surface ranges from laevigate to a much more elaborate covering of a variety of sculptural elements in which the tabulation is obscured by the development of very coarse disorganised rugulae 0.1 0.2 µm wide (Plate II, 10, 12). However, most specimens lie between these two end members, and although possessing some delicate rugulae which radiate out from the spine bases and isolated grana, the tabulation is clearly displayed (Plate II ). The spines exhibited by most specimens are short (generally ~1.5 µm, exceptionally up to 3.5 gm long: Plate II, 11 ), acuminate and distributed in an intratabular fashion. In some specimens the spines are poorly developed or perhaps slightly corroded and appear as small hemispherical domes (~0.75 µm in diameter: Plate II, 6, 9 and 10).
Tabulation: Tabulation is occasionally discernible on specimens observed by light microscopy (Plate I, 1, 3, 5 and 6), but is more readily visible in the electron microscope. Tabulation is generally expressed by sutural ridges of semicircular cross section (~0.25 gm in width). More rarely the sutures are asymmetrical in section and indicate the direction of plate overlap. In a few specimens the plate margins give the impression of penitabular features (Plate II, 6 and 10). Tabulation formula: 1pr, 4', 3a, 7", Xc, 6'", 2"", 4s.
Archaeopyle: Type Ea, epitractal, involving all apical, anterior intercalary and precingular plates as a simple polyplacoid operculum which appears to be adnate along the sulcus (Plate I, 2, 8, 11; Plate II, 4) and is only removed from some damaged specimens.
Cingulum: Characteristically broad, measuring 2.5-3.0 µm from anterior to posterior margin, but showing no indentation into the cyst body (Plate II, 1-2, 13). Slight laevorotatory displacement, but appears virtually planar, uncertain total number of plates.
Sulcus: Pronounced inverted omegaform shape, dominated by a broad spatulate posterior sulcal (ps or Z) plate (Plate II, 7 and 9). Anterior sulcal greatly elongated and proportionally much larger than usually seen on typically partiform episomes (Plate II, 4).
Dimensions: Maximum cyst length, less processes (19) 16.7 (14) µm; width of cyst, less processes (19) 15.9 (12) µm; 38 specimens measured from sample 197/4/83, residue CH116.
Remarks: The tabulation of this new species shows features common to two suborders, the Cladopyxiineae and the Glenodiniineae (Fig. 1). However, the hyposome shows a typical partiform, asymmetrical arrangement of plates on the antapex as a single antapical and a posterior intercalary (Plate II, 9), rather than the two symmetrical antapical plates of the glenodiniineans. This argues for a placement amongst the Suborder Cladopyxiineae (R.A. Fensome, pets. commun., 1997). Familial determination is less straightforward. Referral to the Family Cladopyxiaceae is suggested due to the fact that it displays cladopyxiinean tabulation and possesses climactal plates of unequal sizes, but such an assignation must remain uncertain as cysts with epitractal archaeopyles are specifically excluded from the Cladopyxiaceae (Fensome et al., 1993, p. 73). This would therefore argue for inclusion within the Family Mancodiniaceae, Subfamily Luehndeoideae. The use of such a variable character such as archaeopyle type to justify exclusion from a family might, however, be deemed inappropriate when weighed against the apparently more invariant parameter of plate size. The area occupied by the climactal plates measures only ~4 µm across, and many plates bear small spines, thus rendering determination of the tabulation scheme a complex task. However, it is clear that the arrangement of the climactals is somewhat atypical in comparison with published cladopyxiinean schemes. The fourth precingular is planate and the second anterior intercalary plate is linteloid, a situation perhaps more suggestive of a peridiniinean tabulation (Plate II, 3). Migration of the second apical plate along the suture between the third apical and the first anterior intercalary has resulted in contact with the second anterior intercalary; thus the third apical and the first anterior intercalary no longer have a common suture and the second intercalary is therefore seven-sided (Plate II, 3; Fig. IB). The other modification is the reduction in length of the first apical and the concomitant increase in the length of the anterior sulcal (as). Surface sculpture is markedly different from specimen to specimen as stated, although it has not been discounted that the variations in spine length on such small specimens may be caused by selective corrosion by oxidative processes. Despite the small size of the specimens, it has been possible in most instances to deduce the direction of plate overlap or the tegulation pattern (Figs. I B and C), although the information is too incomplete to construct a tegulation formula. The episomal keystone plate can be identified as the third precingular (3"), whereas that on the hyposome is the fourth postcingular plate (4'").
Comparisons: The diminutive size and processes of P. minutum serve immediately to distinguish it from all other described dinocyst taxa with Type E archaeopyles. In a similar manner, the tabulation and archaeopyle style set this species apart from all other described cladopyxiineans. Care must be taken to recognise the cyst as a dinocyst and not as an acanthomorph acritarch with a median-split excystment opening (Strother, 1996), as at high magnifications the tabulation is difficult to discern (indeed few specimens could be located to illustrate this feature). The new taxon is accompanied in the residues by abundant Resticulasphaera medusae Harding, 1990, another small dinocyst with a Type Ea archaeopyle (Plate I, 12). However, the tabulation displayed by this species consists only of a broad cingular area immediately antapically of the archaeopyle suture, it is much thinner-walled, and the surface ornament consists of more densely packed, longer fibrils. The Tethyan Biorbifera johnewingii Habib, 1972 is another species displaying a Type Ea achaeopyle, although being larger and more distinctively elongate about the apico-antapical axis and displaying a slightly depressed cingular area (Plate I, 4 and 9). In the SEM it can be seen that similarities do exist in the wall sculpture of both B. johnewingii and Fallaciodinium minutum (Plate I, 9; Plate II, 12), but the former possesses more densely packed acuminate spines on a cyst surface covered with short fibrils ("very fine luxuriate setules"--translation of Below, 1987, p. 63). Below (1987, p. 63, fig. 21) described an unusual tabulation for B. johnewingii on the basis of his electron microscopic examination (pr, 4', 1a, 5", 6c, 5’’’, 1’’’’,ns), although he specifically stated that "the areation (sic) is clearly discernible only on the hypocyst of certain specimens. The subdivision of the epicyst and cingulum is deduced by comparison with Valvaeodinium." Fensome et al. (1993, p. 145) placed both Valvaeodinium and the monospecific genus Biorbifera within the Family Comparodiniaceae Vozzhennikova, 1979, which they deemed as being of uncertain ordinal status due to the fact that the tabulation "is so unlike any known peridinialean episomal tabulation". On the basis of published electron micrographs (Below, 1987, pl. 27) and new observations (e.g. Plate I, 9), the present author considers Below's tabulation scheme for Biorbifera as uncertain and requiring of further corroboration, in both epi- and hyposomal regions.
Holotype: Harding, 1998, pl.1, figs.1-2.
Age: late Barremian.
Fallaciodinium minutum Harding, sp. nov. (Plate I, l 3,5 6, 7-8, 10-11; Plate II, 1-13)
Holotype: Plate I, 1 2.
Repository: Sedgwick Museum, Cambridge, England. Catalogue number: X.27547.
Type locality/horizon: Otto Gott claypit, nr. Sarstedt, Germany, Bed 197 (Mutterlose, 1984). Etymology: Latin, minutus meaning minute. Diagnosis: A proximochorate dinocyst of diminutive size (<20 µm diameter), appearing superficially like an acanthomorph acritarch, but displaying atypical cladopyxiinean tabulation and a Type Ea (epicystal) archaeopyle.
Description:
Shape: Cyst ambitus is subcircular to ovoidal and the cyst shows slight primary dorsoventral compression. Where elongation is developed this occurs along the apico-antapical axis (although dorso-ventral compaction of excysted specimens causes artificial elongation across the cingulum). Greatest width along the posterior cingular/postcingular suture. Hyposome is large and comprises over half of the length of the cyst. The cingulum is also very broad and accounts for a further quarter of the cyst length. The episome is, by comparison, relatively small, making up the remaining cyst length.
Phragma: The autophragm is thin (~0.2- 0.4 µm), but despite this the shape is retained well due to the small size of the cyst. The cyst surface ranges from laevigate to a much more elaborate covering of a variety of sculptural elements in which the tabulation is obscured by the development of very coarse disorganised rugulae 0.1 0.2 µm wide (Plate II, 10, 12). However, most specimens lie between these two end members, and although possessing some delicate rugulae which radiate out from the spine bases and isolated grana, the tabulation is clearly displayed (Plate II ). The spines exhibited by most specimens are short (generally ~1.5 µm, exceptionally up to 3.5 gm long: Plate II, 11 ), acuminate and distributed in an intratabular fashion. In some specimens the spines are poorly developed or perhaps slightly corroded and appear as small hemispherical domes (~0.75 µm in diameter: Plate II, 6, 9 and 10).
Tabulation: Tabulation is occasionally discernible on specimens observed by light microscopy (Plate I, 1, 3, 5 and 6), but is more readily visible in the electron microscope. Tabulation is generally expressed by sutural ridges of semicircular cross section (~0.25 gm in width). More rarely the sutures are asymmetrical in section and indicate the direction of plate overlap. In a few specimens the plate margins give the impression of penitabular features (Plate II, 6 and 10). Tabulation formula: 1pr, 4', 3a, 7", Xc, 6'", 2"", 4s.
Archaeopyle: Type Ea, epitractal, involving all apical, anterior intercalary and precingular plates as a simple polyplacoid operculum which appears to be adnate along the sulcus (Plate I, 2, 8, 11; Plate II, 4) and is only removed from some damaged specimens.
Cingulum: Characteristically broad, measuring 2.5-3.0 µm from anterior to posterior margin, but showing no indentation into the cyst body (Plate II, 1-2, 13). Slight laevorotatory displacement, but appears virtually planar, uncertain total number of plates.
Sulcus: Pronounced inverted omegaform shape, dominated by a broad spatulate posterior sulcal (ps or Z) plate (Plate II, 7 and 9). Anterior sulcal greatly elongated and proportionally much larger than usually seen on typically partiform episomes (Plate II, 4).
Dimensions: Maximum cyst length, less processes (19) 16.7 (14) µm; width of cyst, less processes (19) 15.9 (12) µm; 38 specimens measured from sample 197/4/83, residue CH116.
Remarks: The tabulation of this new species shows features common to two suborders, the Cladopyxiineae and the Glenodiniineae (Fig. 1). However, the hyposome shows a typical partiform, asymmetrical arrangement of plates on the antapex as a single antapical and a posterior intercalary (Plate II, 9), rather than the two symmetrical antapical plates of the glenodiniineans. This argues for a placement amongst the Suborder Cladopyxiineae (R.A. Fensome, pets. commun., 1997). Familial determination is less straightforward. Referral to the Family Cladopyxiaceae is suggested due to the fact that it displays cladopyxiinean tabulation and possesses climactal plates of unequal sizes, but such an assignation must remain uncertain as cysts with epitractal archaeopyles are specifically excluded from the Cladopyxiaceae (Fensome et al., 1993, p. 73). This would therefore argue for inclusion within the Family Mancodiniaceae, Subfamily Luehndeoideae. The use of such a variable character such as archaeopyle type to justify exclusion from a family might, however, be deemed inappropriate when weighed against the apparently more invariant parameter of plate size. The area occupied by the climactal plates measures only ~4 µm across, and many plates bear small spines, thus rendering determination of the tabulation scheme a complex task. However, it is clear that the arrangement of the climactals is somewhat atypical in comparison with published cladopyxiinean schemes. The fourth precingular is planate and the second anterior intercalary plate is linteloid, a situation perhaps more suggestive of a peridiniinean tabulation (Plate II, 3). Migration of the second apical plate along the suture between the third apical and the first anterior intercalary has resulted in contact with the second anterior intercalary; thus the third apical and the first anterior intercalary no longer have a common suture and the second intercalary is therefore seven-sided (Plate II, 3; Fig. IB). The other modification is the reduction in length of the first apical and the concomitant increase in the length of the anterior sulcal (as). Surface sculpture is markedly different from specimen to specimen as stated, although it has not been discounted that the variations in spine length on such small specimens may be caused by selective corrosion by oxidative processes. Despite the small size of the specimens, it has been possible in most instances to deduce the direction of plate overlap or the tegulation pattern (Figs. I B and C), although the information is too incomplete to construct a tegulation formula. The episomal keystone plate can be identified as the third precingular (3"), whereas that on the hyposome is the fourth postcingular plate (4'").
Comparisons: The diminutive size and processes of P. minutum serve immediately to distinguish it from all other described dinocyst taxa with Type E archaeopyles. In a similar manner, the tabulation and archaeopyle style set this species apart from all other described cladopyxiineans. Care must be taken to recognise the cyst as a dinocyst and not as an acanthomorph acritarch with a median-split excystment opening (Strother, 1996), as at high magnifications the tabulation is difficult to discern (indeed few specimens could be located to illustrate this feature). The new taxon is accompanied in the residues by abundant Resticulasphaera medusae Harding, 1990, another small dinocyst with a Type Ea archaeopyle (Plate I, 12). However, the tabulation displayed by this species consists only of a broad cingular area immediately antapically of the archaeopyle suture, it is much thinner-walled, and the surface ornament consists of more densely packed, longer fibrils. The Tethyan Biorbifera johnewingii Habib, 1972 is another species displaying a Type Ea achaeopyle, although being larger and more distinctively elongate about the apico-antapical axis and displaying a slightly depressed cingular area (Plate I, 4 and 9). In the SEM it can be seen that similarities do exist in the wall sculpture of both B. johnewingii and Fallaciodinium minutum (Plate I, 9; Plate II, 12), but the former possesses more densely packed acuminate spines on a cyst surface covered with short fibrils ("very fine luxuriate setules"--translation of Below, 1987, p. 63). Below (1987, p. 63, fig. 21) described an unusual tabulation for B. johnewingii on the basis of his electron microscopic examination (pr, 4', 1a, 5", 6c, 5’’’, 1’’’’,ns), although he specifically stated that "the areation (sic) is clearly discernible only on the hypocyst of certain specimens. The subdivision of the epicyst and cingulum is deduced by comparison with Valvaeodinium." Fensome et al. (1993, p. 145) placed both Valvaeodinium and the monospecific genus Biorbifera within the Family Comparodiniaceae Vozzhennikova, 1979, which they deemed as being of uncertain ordinal status due to the fact that the tabulation "is so unlike any known peridinialean episomal tabulation". On the basis of published electron micrographs (Below, 1987, pl. 27) and new observations (e.g. Plate I, 9), the present author considers Below's tabulation scheme for Biorbifera as uncertain and requiring of further corroboration, in both epi- and hyposomal regions.