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Belodinium nereidis
Belodinium nereidis, Stevens and Helby, 1987
Holotype: Stevens and Helby, 1987, fig.5A-B
Locus typicus: Exmouth Plateau, Australia
Stratum typicum: Early Berriasian
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Original description: [Stevens and Helby, 1987, p. 166-168]:
Description:
Teardrop-shaped, bicavate cysts with an ellipsoidal endocyst and an elongate ellipsoidal pericyst with a prominent, slender apical perihorn and a large antapical pericoel open antapically.
Archeopyle apical, type [tA]; principal suture zigzag with short accessory sutures. Operculum small and free- the endo-operculum small and rounded while the peri-operculum forms a short, tapered, apical perihorn with a closed, truncate, anterior extremity.
Endophragm and periphragm both less than 1µm thick, finely granulate with 1-3µm long hair-like processes prominent at the apex and antapex and to a lesser extent in penitabular or parasutural alignments, particularly in the vicinity of the paracingulum. Periphragm smooth or rarely faintly granulate. Wall layers separated except along the archeopyle suture, thus forming a small apical pericoel over the endo-operculum and a larger pericoel over the rest of the cyst. Apical pericoel relatively small, fully enclosed, and confined by the perihorn. Antapical pericoel extending over the precingular and cingular paraplates as well as over the hypocyst, with separation of the cyst layers gradually increasing posteriorly. Pericoel commonly widest at the periparacingulum and open antapically.
Paratabulation rarely complete, indicated by the archeopyle on both endocyst and pericyst and by the paracingulum and faint parasutural ridges and/or folds on the pericyst. Folding of the periphragm may distort and obscure these parasutural features.
Parasutural ridges on the pericyst low and narrow (about 0.5µm high and wide), commonly surmounted by small, rounded denticles less than 0.5µm in diameter (Fig. 5J). Parasutural alignment of denticles (without ridges) rare.
Paratabulation on the endocyst may be indicated also by penitabular and parasutural lineation of hair-like processes (Fig.5K-O). Paratabulation gonyaulacacean interpreted as 1- ?2pr, 4", 6", 6c, 6""", 1p, 1""", 3s. Truncate tip of the perihorn appearing to be closed by a very small preapical paraplate (Figs 4C, 5N,H). A smaller area (less than l~m), delineated by very low ridges, commonly identified adjacent to, but immediately posterior to the preapical at the tip of the perihorn, at or very close to the boundary of the 3" and 4" paraplates (Fig.5H,N) may represent a second preapical paraplate. The boundary between the small 1""" paraplate and the parasulcus apparently not delineated on the pericyst. The 1"""" paraplate represented by a hole in the pericyst, its shape clearly defined by the posterior margins of the 3""", 4""", 5""", 6""", 1p, and ps paraplates. Arrangement of these paraplate boundaries suggesting that the 1"""" paraplate would incline anterodorsally from the antapex onto the dorsal surface with a typical gonyaulacacean shape (Figs 4B, 5D-E,J). In the postcingular series paraplates 3""", 4""" and 5""" are large and subrectangular (or trapezoidal)- 4""" located mid dorsally; 3""" and 5""" laterally positioned; 6""" wedge-shaped with a very narrow, recessed contact at the antapex. The 1p paraplate with a similar narrow recessed contact at the antapex providing a distinctive symmetry to the ventral antapex (Figs 4A, 5C). Prominent, subdivided periparacingulum 3-5µm wide, offset by one paracingulum width and commonly marked by 1µm high and wide parasutural folds.
Pericingular paraplate series with 4- and 6-sided paraplates, the latter being disposed on the dorsal and lateral surfaces. Endophragm not indented at the paracingulum but commonly marked by parasutural alignment of hair-like processes most prominent laterally (Fig.5L-O). Subdivided parasulcus may be indicated by a smooth, narrow, elongate area on the epipericyst extending from the narrow, shallow, sulcal notch to the paracingulum, and by a smooth, shallow depression on the hypopericyst extending from the paracingulum to the antapex, both areas being commonly folded and distorted and bordered by parasutural folds. Flagellar scar commonly encountered immediately posterior to the junction of the periparasulcus and the periparacingulum (Fig.5F).
Variability:
The teardrop shape of the cyst is essentially constant, although the ratio of width at the periparacingulum to the width at the operculum is variable. In addition, the ratio of endocyst length to pericyst length varies considerably. Most commonly the antapical extremity of the endocyst does not reach the anterior margin of the periparacingulum Fig.5A), however, in some populations the endocyst commonly extends up to (Fig.5D,F) or into the hypopericyst (Fig.5G). Paratabulation is usually clear on the epipericyst, but less clearly expressed on the hypopericyst. Even within a single population there is variation between individuals in the development of denticulate parasutural ridges, most displaying no hypocystal ridges. There is similar variation in the development and parasutural alignment of hair-like processes on the endocyst. The thin-walled cysts are commonly irregularly folded due to compression and many of the parasutural folds may be distorted. The anterior parasulcus is commonly obscured by folding of the epipericyst (Fig . 5A). The prominence of the flagellar scar varies between populations.
Dimensions/Measurements:
Endocyst length (dehisced) 36(43)50 µm, length increased by 6-8 µm when operculum present, endocyst width 24(27)32 µm, pericyst length (dehisced) 50(63)78 µm, length increased by 20-26 µm when operculum present, pericyst width 22(31)38 µm (28 specimens).
Affinities: (p. 169-170):
Belodinium nereidis is distinguished from B. dysculum Cookson & Eisenack 1960 principally by the relationship of the separated wall layers. Stover & Helby (this memoir b) interpret the separation as holocavate in B. dysculum. In the latter species, the ectophragm is indented the paracingulum where it is normally in contact with the autophragm, with the autoparacingulum and the ectoparacingulum aligned. In B. nereidis the pericyst is widest at the paracingulum, where the separation of the periphragm and endophragm is very substantial. The anterior margin of the periparacingulum is almost invariably substantially posterior to the antapical margin of the endoparacingulum. Belodinium obsoletum Dodekova 1975 has an extensive epipericoel; the anterior margins of the precingular paraplates and the posterior margins of the apical paraplates on the endophragm and on the periphragm are substantially separated, unlike B. dysculum and B. nereidis, where these margins are essentially in contact. In B. obsoletum the antapical paraplate is present and is confined to the ventral surface. In B. nereidis the antapical paraplate is not present, and if it were it would slope, in contrast, anteriorly onto the dorsal surface. Belodinium asaphum Drugg 1978 lacks the clear periparacingular expression, the short tapered apical horn and the substantial antapical pericoel that characterize both B. dysculum and B. nereidis.
Holotype: Stevens and Helby, 1987, fig.5A-B
Locus typicus: Exmouth Plateau, Australia
Stratum typicum: Early Berriasian
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Original description: [Stevens and Helby, 1987, p. 166-168]:
Description:
Teardrop-shaped, bicavate cysts with an ellipsoidal endocyst and an elongate ellipsoidal pericyst with a prominent, slender apical perihorn and a large antapical pericoel open antapically.
Archeopyle apical, type [tA]; principal suture zigzag with short accessory sutures. Operculum small and free- the endo-operculum small and rounded while the peri-operculum forms a short, tapered, apical perihorn with a closed, truncate, anterior extremity.
Endophragm and periphragm both less than 1µm thick, finely granulate with 1-3µm long hair-like processes prominent at the apex and antapex and to a lesser extent in penitabular or parasutural alignments, particularly in the vicinity of the paracingulum. Periphragm smooth or rarely faintly granulate. Wall layers separated except along the archeopyle suture, thus forming a small apical pericoel over the endo-operculum and a larger pericoel over the rest of the cyst. Apical pericoel relatively small, fully enclosed, and confined by the perihorn. Antapical pericoel extending over the precingular and cingular paraplates as well as over the hypocyst, with separation of the cyst layers gradually increasing posteriorly. Pericoel commonly widest at the periparacingulum and open antapically.
Paratabulation rarely complete, indicated by the archeopyle on both endocyst and pericyst and by the paracingulum and faint parasutural ridges and/or folds on the pericyst. Folding of the periphragm may distort and obscure these parasutural features.
Parasutural ridges on the pericyst low and narrow (about 0.5µm high and wide), commonly surmounted by small, rounded denticles less than 0.5µm in diameter (Fig. 5J). Parasutural alignment of denticles (without ridges) rare.
Paratabulation on the endocyst may be indicated also by penitabular and parasutural lineation of hair-like processes (Fig.5K-O). Paratabulation gonyaulacacean interpreted as 1- ?2pr, 4", 6", 6c, 6""", 1p, 1""", 3s. Truncate tip of the perihorn appearing to be closed by a very small preapical paraplate (Figs 4C, 5N,H). A smaller area (less than l~m), delineated by very low ridges, commonly identified adjacent to, but immediately posterior to the preapical at the tip of the perihorn, at or very close to the boundary of the 3" and 4" paraplates (Fig.5H,N) may represent a second preapical paraplate. The boundary between the small 1""" paraplate and the parasulcus apparently not delineated on the pericyst. The 1"""" paraplate represented by a hole in the pericyst, its shape clearly defined by the posterior margins of the 3""", 4""", 5""", 6""", 1p, and ps paraplates. Arrangement of these paraplate boundaries suggesting that the 1"""" paraplate would incline anterodorsally from the antapex onto the dorsal surface with a typical gonyaulacacean shape (Figs 4B, 5D-E,J). In the postcingular series paraplates 3""", 4""" and 5""" are large and subrectangular (or trapezoidal)- 4""" located mid dorsally; 3""" and 5""" laterally positioned; 6""" wedge-shaped with a very narrow, recessed contact at the antapex. The 1p paraplate with a similar narrow recessed contact at the antapex providing a distinctive symmetry to the ventral antapex (Figs 4A, 5C). Prominent, subdivided periparacingulum 3-5µm wide, offset by one paracingulum width and commonly marked by 1µm high and wide parasutural folds.
Pericingular paraplate series with 4- and 6-sided paraplates, the latter being disposed on the dorsal and lateral surfaces. Endophragm not indented at the paracingulum but commonly marked by parasutural alignment of hair-like processes most prominent laterally (Fig.5L-O). Subdivided parasulcus may be indicated by a smooth, narrow, elongate area on the epipericyst extending from the narrow, shallow, sulcal notch to the paracingulum, and by a smooth, shallow depression on the hypopericyst extending from the paracingulum to the antapex, both areas being commonly folded and distorted and bordered by parasutural folds. Flagellar scar commonly encountered immediately posterior to the junction of the periparasulcus and the periparacingulum (Fig.5F).
Variability:
The teardrop shape of the cyst is essentially constant, although the ratio of width at the periparacingulum to the width at the operculum is variable. In addition, the ratio of endocyst length to pericyst length varies considerably. Most commonly the antapical extremity of the endocyst does not reach the anterior margin of the periparacingulum Fig.5A), however, in some populations the endocyst commonly extends up to (Fig.5D,F) or into the hypopericyst (Fig.5G). Paratabulation is usually clear on the epipericyst, but less clearly expressed on the hypopericyst. Even within a single population there is variation between individuals in the development of denticulate parasutural ridges, most displaying no hypocystal ridges. There is similar variation in the development and parasutural alignment of hair-like processes on the endocyst. The thin-walled cysts are commonly irregularly folded due to compression and many of the parasutural folds may be distorted. The anterior parasulcus is commonly obscured by folding of the epipericyst (Fig . 5A). The prominence of the flagellar scar varies between populations.
Dimensions/Measurements:
Endocyst length (dehisced) 36(43)50 µm, length increased by 6-8 µm when operculum present, endocyst width 24(27)32 µm, pericyst length (dehisced) 50(63)78 µm, length increased by 20-26 µm when operculum present, pericyst width 22(31)38 µm (28 specimens).
Affinities: (p. 169-170):
Belodinium nereidis is distinguished from B. dysculum Cookson & Eisenack 1960 principally by the relationship of the separated wall layers. Stover & Helby (this memoir b) interpret the separation as holocavate in B. dysculum. In the latter species, the ectophragm is indented the paracingulum where it is normally in contact with the autophragm, with the autoparacingulum and the ectoparacingulum aligned. In B. nereidis the pericyst is widest at the paracingulum, where the separation of the periphragm and endophragm is very substantial. The anterior margin of the periparacingulum is almost invariably substantially posterior to the antapical margin of the endoparacingulum. Belodinium obsoletum Dodekova 1975 has an extensive epipericoel; the anterior margins of the precingular paraplates and the posterior margins of the apical paraplates on the endophragm and on the periphragm are substantially separated, unlike B. dysculum and B. nereidis, where these margins are essentially in contact. In B. obsoletum the antapical paraplate is present and is confined to the ventral surface. In B. nereidis the antapical paraplate is not present, and if it were it would slope, in contrast, anteriorly onto the dorsal surface. Belodinium asaphum Drugg 1978 lacks the clear periparacingular expression, the short tapered apical horn and the substantial antapical pericoel that characterize both B. dysculum and B. nereidis.