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Stelladinium bifurcatum
Stelladinium bifurcatum n. sp., Head, M. J. et al., 2020
Holotype: Head, M. J. et al., 2020, Pl. 1, figs. 1–12, Fig. 2a (d. Sample Paleo IX 38, slide 1, E39/3. ROMIP 65338, from surface sediments of La Paz Bay, Gulf of Californi)
Age: Upper Pleistocene
Strat. range: Upper Pleistocene - Modern
Original description: Head, M. J. et al., 2020:
Diagnosis:
Cysts brown in colour, with long apical and antapical horns of approximately equal length, tapering from relatively narrow bases. Wall appears finely and faintly granulate owing to relief on inside surface of wall. Cingulum approximately level and represented by a broad low fold bearing numerous processes on both dorsal and ventral
surfaces and on lateral margins. Processes can be as long as the horns themselves, and usually at least one or two bifurcate distally. Distal extremities of processes and all horns are thickened or solid. Sulcus represented by a shallow depression restricted to hypocyst; the flagellar scar marked by two prominent invaginations. Archeopyle large, offset to right, and represents the 2a plate; operculum adnate along right lateral margin.
Description:
Cysts are brown in colour, the central body weakly dorsoventrally compressed.
Horns: Long apical and antapical horns have approximately equal length and taper from relatively narrow bases. Antapical horns are weakly to strongly divergent. The flanks of the central body are straight to weakly convex.
Cyst wall: about 0.3 μm or less in thickness and unstratified under light microscopy. It appears finely and faintly granulate , this ornamentation occurring on the inside of the wall, a feature particularly apparent towards the distal ends of processes and horns.
Archeopyle: large, narrows towards the apex, and represents the 2a plate. The archeopyle is offset to the right of the dorsoventral midline, and the
operculum is adnate along its right lateral margin.
The anterior margin is aligned parallel to the cingulum and may be serrated, and the adcingular margin extends near to the cingulum and is often obscured by the cingular fold. The cingulum is approximately level and represented by a broad low fold on the dorsal surface and continuing over much of the ventral surface. This cingular fold bears numerous processes both on the dorsal and ventral surfaces and on the lateral margins.
Processes: are of variable size on a single specimen and may be as long as the horns themselves. On occasional specimens, a few processes may be reduced to a few μm in length. Processes approximately align in two rows presumably reflecting the anterior and posterior margins of the cingulum, although the precise positions of these margins is not otherwise indicated on the cyst. Usually at least one or two processes are bifurcate, with one termination inflected towards the apex and the other towards
the antapex. The degree of bifurcation varies: discrete processes may bifurcate distally, or pairs of adjacent processes may simply fuse at their base. Varying degrees of fusion may occur on the same specimen. The distal extremities of the processes and both apical and antapical horns are solid or internally thickened up to a length of about 10 μm and taper to acuminate or minutely rounded points.
Sulcus: represented by a broad shallow depression. It is restricted to the hypocyst. Two adjoined invaginations, one immediately above the other, and marked also by a slight thickening of the wall, represent the flagellar scar. The anterior invagination has a depth of up to about 3.5 μm, being 2.3 μm on holotype.
There are no other indications of tabulation.
Dimensions:
Maximum length (horn tip to horn tip): holotype 99 μm; range 74 (85.5) 99 μm: based on 17 specimens from Gulf of California surface sample Paleo IX 38
1 specimen measured from equatorial eastern Atlantic sample T89-16, 57 cm: maximum length 101 μm.
Remarks:
It is not always possible to distinguish between cingular processes and horns when specimens are crumpled because their process tip morphologies are identical. Process tips are either solid or darkened due to internal thickening of the process wall. Although the number of processes with bifurcate terminations varies between specimens, almost all specimens observed have at least one bifurcate termination. The flagellar scar is a consistent feature, and was seen on all favourably oriented specimens, including the holotype. Horns and processes may bear an occasional small (one or a few μm in length) solid spine along their length, and occasionally one or a few process tips or horn tips show minute low- or wide-angle bifurcation. The irregular relief on the inner surface of the wall causing cysts to have a granulate appearance has not been reported in other species of the genus, and detailed analysis is needed to determine whether this feature is unique to S. bifurcatum. The operculum was almost always found in place in the specimens examined, being adnate along the right lateral margin of the archeopyle. The primary archeopyle suture is often finely irregular.
A flagellar scar was weakly developed or not seen on the six specimens of Stelladinium cf. reidii examined, although interestingly the finely and faintly granulate appearance of the wall may also be caused by minute relief on its inner surface, as best observed along the distal ends of the horns. This feature, also observed in Stelladinium bifurcatum, may therefore be more prevalent in this genus than hitherto assumed.
Affinities:
Stelladinium bifurcatum n. sp. differs from all other taxa of the genus, and from the cysts of Protoperidinium stellatum, in possessing a cingular fold bearing numerous long processes on the dorsal and ventral surfaces and on the lateral margins. At least one of these processes has a strongly divergent bifurcation. A 2a intercalary archeopyle with its operculum adnate along the right lateral margin, pronounced flagellar scar, and processes and horns all with solid tips, represent an additional unique combination of features.
Stelladinium reidii described from the Arabian Sea (Bradford, 1975) differs in having no cingular processes, or ones that are rarely observed in dorsoventral view. The
archeopyle is free although appears to be adherent in some specimens (Bradford, 1975, and see below).
Stelladinium cf. reidii specimens from Surface sediment samples from Gulf of California differ from Stelladinium bifurcatum in lacking cingular processes, having horn tips that are solid for a shorter distance (up to 4.0 μm but generally 1.5–3.0 μm), a pronounced cingular fold which rises to a crest on both dorsal and ventral surfaces with the exception of a shallow and narrow interruption of the sulcus on the ventral surface, and an operculum that is free rather than adnate, although it may be adherent.
Stelladinium reidii usually does not have the cingulum expressed, although a faint cingular zone was observed in a few specimens (Bradford, 1975).
Holotype: Head, M. J. et al., 2020, Pl. 1, figs. 1–12, Fig. 2a (d. Sample Paleo IX 38, slide 1, E39/3. ROMIP 65338, from surface sediments of La Paz Bay, Gulf of Californi)
Age: Upper Pleistocene
Strat. range: Upper Pleistocene - Modern
Original description: Head, M. J. et al., 2020:
Diagnosis:
Cysts brown in colour, with long apical and antapical horns of approximately equal length, tapering from relatively narrow bases. Wall appears finely and faintly granulate owing to relief on inside surface of wall. Cingulum approximately level and represented by a broad low fold bearing numerous processes on both dorsal and ventral
surfaces and on lateral margins. Processes can be as long as the horns themselves, and usually at least one or two bifurcate distally. Distal extremities of processes and all horns are thickened or solid. Sulcus represented by a shallow depression restricted to hypocyst; the flagellar scar marked by two prominent invaginations. Archeopyle large, offset to right, and represents the 2a plate; operculum adnate along right lateral margin.
Description:
Cysts are brown in colour, the central body weakly dorsoventrally compressed.
Horns: Long apical and antapical horns have approximately equal length and taper from relatively narrow bases. Antapical horns are weakly to strongly divergent. The flanks of the central body are straight to weakly convex.
Cyst wall: about 0.3 μm or less in thickness and unstratified under light microscopy. It appears finely and faintly granulate , this ornamentation occurring on the inside of the wall, a feature particularly apparent towards the distal ends of processes and horns.
Archeopyle: large, narrows towards the apex, and represents the 2a plate. The archeopyle is offset to the right of the dorsoventral midline, and the
operculum is adnate along its right lateral margin.
The anterior margin is aligned parallel to the cingulum and may be serrated, and the adcingular margin extends near to the cingulum and is often obscured by the cingular fold. The cingulum is approximately level and represented by a broad low fold on the dorsal surface and continuing over much of the ventral surface. This cingular fold bears numerous processes both on the dorsal and ventral surfaces and on the lateral margins.
Processes: are of variable size on a single specimen and may be as long as the horns themselves. On occasional specimens, a few processes may be reduced to a few μm in length. Processes approximately align in two rows presumably reflecting the anterior and posterior margins of the cingulum, although the precise positions of these margins is not otherwise indicated on the cyst. Usually at least one or two processes are bifurcate, with one termination inflected towards the apex and the other towards
the antapex. The degree of bifurcation varies: discrete processes may bifurcate distally, or pairs of adjacent processes may simply fuse at their base. Varying degrees of fusion may occur on the same specimen. The distal extremities of the processes and both apical and antapical horns are solid or internally thickened up to a length of about 10 μm and taper to acuminate or minutely rounded points.
Sulcus: represented by a broad shallow depression. It is restricted to the hypocyst. Two adjoined invaginations, one immediately above the other, and marked also by a slight thickening of the wall, represent the flagellar scar. The anterior invagination has a depth of up to about 3.5 μm, being 2.3 μm on holotype.
There are no other indications of tabulation.
Dimensions:
Maximum length (horn tip to horn tip): holotype 99 μm; range 74 (85.5) 99 μm: based on 17 specimens from Gulf of California surface sample Paleo IX 38
1 specimen measured from equatorial eastern Atlantic sample T89-16, 57 cm: maximum length 101 μm.
Remarks:
It is not always possible to distinguish between cingular processes and horns when specimens are crumpled because their process tip morphologies are identical. Process tips are either solid or darkened due to internal thickening of the process wall. Although the number of processes with bifurcate terminations varies between specimens, almost all specimens observed have at least one bifurcate termination. The flagellar scar is a consistent feature, and was seen on all favourably oriented specimens, including the holotype. Horns and processes may bear an occasional small (one or a few μm in length) solid spine along their length, and occasionally one or a few process tips or horn tips show minute low- or wide-angle bifurcation. The irregular relief on the inner surface of the wall causing cysts to have a granulate appearance has not been reported in other species of the genus, and detailed analysis is needed to determine whether this feature is unique to S. bifurcatum. The operculum was almost always found in place in the specimens examined, being adnate along the right lateral margin of the archeopyle. The primary archeopyle suture is often finely irregular.
A flagellar scar was weakly developed or not seen on the six specimens of Stelladinium cf. reidii examined, although interestingly the finely and faintly granulate appearance of the wall may also be caused by minute relief on its inner surface, as best observed along the distal ends of the horns. This feature, also observed in Stelladinium bifurcatum, may therefore be more prevalent in this genus than hitherto assumed.
Affinities:
Stelladinium bifurcatum n. sp. differs from all other taxa of the genus, and from the cysts of Protoperidinium stellatum, in possessing a cingular fold bearing numerous long processes on the dorsal and ventral surfaces and on the lateral margins. At least one of these processes has a strongly divergent bifurcation. A 2a intercalary archeopyle with its operculum adnate along the right lateral margin, pronounced flagellar scar, and processes and horns all with solid tips, represent an additional unique combination of features.
Stelladinium reidii described from the Arabian Sea (Bradford, 1975) differs in having no cingular processes, or ones that are rarely observed in dorsoventral view. The
archeopyle is free although appears to be adherent in some specimens (Bradford, 1975, and see below).
Stelladinium cf. reidii specimens from Surface sediment samples from Gulf of California differ from Stelladinium bifurcatum in lacking cingular processes, having horn tips that are solid for a shorter distance (up to 4.0 μm but generally 1.5–3.0 μm), a pronounced cingular fold which rises to a crest on both dorsal and ventral surfaces with the exception of a shallow and narrow interruption of the sulcus on the ventral surface, and an operculum that is free rather than adnate, although it may be adherent.
Stelladinium reidii usually does not have the cingulum expressed, although a faint cingular zone was observed in a few specimens (Bradford, 1975).