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Nucicla umbiliphora
Nucicla umbiliphora sp. nov., Hartman et al., 2019
Holotype. Plate 1, figure 1–2, Hartman et al., 2019
Type locality: Iceberg Alley, MacRobertson Shelf, East Antarctica
Type stratum: Holocene
Stratigraphical range: Marine Isotope Stage 5.5 to Recent
Original description: Hartman et al., 2019:
Diagnosis:
A species of Nucicla with a scabrate wall structure and with low, undulating or crenulating ridges with no apparent relation to plate boundaries with the exception of the cingulum and sulcus. The sulcus shows a large flagellar scar. Apart from the cingulum and sulcus, tabulation is only indicated by the clear 3I archaeopyle, of which the 2a intercalary is large and latideltaform. In the holotype the operculum is still attached; specimens are usually found without operculum.
Description:
A brown cyst with pentagonal outline, which is dorsoventrally compressed. Apical and antapical ‘horns’ are rounded and broad-based so that the cyst appears more rounded than pentagonal. A few specimens show a small acute apical horn (Plate 1, figures 3, 11). None of the specimens shows acute tips at the antapical ‘horns’. The hypocyst is twice the height of the epicyst.
Wall: The autophragm has folds that form low, undulating or crenulating ridges, which are predominantly longitudinal and can be dendritic. With the exception of the cingulum and sulcus, these ridges have no relation to sutures. The cyst wall is scabrate and the degree of scabration varies between specimens (Plate 2, figures 4–6).
The clearly distinguishable cingulum is level or very slightly descending and outlined by sutural ridges. Within the cingulum, low longitudinal ridges occur, but with no relation to cingular plate boundaries. The sulcus forms a clear depression, and is outlined by sinistral and dextral longitudinal sutural ridges. Some specimens also clearly show a posterior sutural ridge, which outlines the entire posterior sulcal plate (Sp; Plate 1, figures 6 and 8).
Tabulation: The sutural ridge that outlines the Sp extends towards the dorsal side of the cyst, indicating the exceptional antapical position of the Sp. To our knowledge no other protoperidinioid cyst possesses an Sp that is positioned so far posteriorly, thereby pushing the antapical plates to the dorsal side. The sulcus shows a large flagellar scar, with low ridges converging towards it. Applying standard protoperidinioid tabulation to N. umbiliphora, low ridges within the sulcus seem to outline the right sulcal plate (Sd) and the left sulcal plate (Ss) (Plate 1, figures 3 and 9). In some specimens (Plate 1, figures 3 and 9) a low ridge is present at the anterior margin of the left and right sulcal plates, indicating the position of the anterior sulcal plate (Sa) (Figure 2).
Archaeopyle: The 3I archaeopyle is relatively large and spans almost the entire dorsal side of the epicyst. It has a consistent shape with clear angles marking plate junctions, except for the boundaries between the intercalary plates, which are smooth and subtle. Nevertheless, both archaeopyle and operculum outline suggest a 3I archaeopyle, for the following reasons: (1) the upper margin of the archaeopyle is concave; although in dinoflagellate cyst species with a 2a archaeopyle such a concave upper margin could be the result of the inward folding of the apical plates, we are certain that this is not the case for N. umbiliphora, because the outline of the operculum is also concave (Plate 2, figure 6); (2) the upper margin of the archaeopyle is three-sided (particularly visible in Plate 2, figure 3), strongly suggesting that these are the three sides of the third apical plate (3’) that border each of the anterior intercalaries (green, black and blue lines in Plate 2); and (3) in several instances the lower margin of the 2a intercalary appears slightly but nevertheless clearly elevated with respect to the lower margins of the 1a and 3a intercalaries (indicated by the ^ symbols in the figures of Plate 2). The position of the plate junctions in the lower margin of the archaeopyle suggests that the 1a and 3a intercalaries are very narrow, bordering a large latideltaform 2a intercalary.
Tabulation: Although we cannot determine any further tabulation from the cyst of N. umbiliphora, the relatively low height of the epicyst in combination with the large 2a suggests that the height of the 3’’, 4’’, and 5’’ precingular plates is suppressed (Figure 2).
Dimensions:
Holotype: Height = 70 µm; width measured along the cingulum = 62 µm.
Other specimens (n = 10): Height = 57–70 µm, average = 64.5 µm. Width = 52–63 µm, average = 60.0 µm.
Affinities/Differential diagnosis:
This is currently the only species in the genus. Within the Protoperidiniaceae, Nucicla umbiliphora with its pentagonal outline, limited tabulation, rounded antapical horns, scabration and brown colour most closely resembles Lejeunecysta rotunda Clowes et al. 2016. Instead of erecting a new genus one might consider emending Lejeunecysta to include N. umbiliphora. However, Lejeunecysta differs from Nucicla in more than just one aspect. Most importantly, Lejeunecysta has a consistent 2a archaeopyle and closely defined archaeopyle shape. Lejeunecysta also differs in having the epicyst and hypocyst of approximately equal length, symmetrically located horns which are small pointed and solid, a laevigate or chagrinate wall, and tabulation only indicated near the archaeopyle and cingulum, whereas the sulcus is only marked by a shallow depression. Because of the number and clarity of the differences, we found it necessary to erect a new genus. It differs from most other peridinioid genera by its 3I archaeopyle. Other known genera with a 3I archaeopyle are the Cretaceous to Early Palaeocene Trithyrodinium and Palaeocene to Oligocene Vozzhennikovia, but both these genera are cavate. Typically, the apical and antapical horns of these genera are made up of the periphragm, while N. umbiliphora only has an autophragm (Figure 3). Other brown scabrate dinoflagellate cyst species from the present-day Southern Ocean are Selenopemphix antarctica Marret & de Vernal 1997, Brigantedinium pynei Hannah et al. 1998 and Cryodinium meridianum Esper & Zonneveld 2002.
Cryodinium meridianum also has low, sometimes dendritic ridges, but most of these crests reflect tabulation. Furthermore, C. meridianum lacks the pentagonal outline and dorsoventral compression, and has a 2I archaeopyle. Brigantedinium pynei is reminiscent of C. meridianum, but its rugulose surface does not reflect tabulation and it has an I archaeopyle (Clowes et al. 2016). Selenopemphix antarctica does not have ridges, has an I-type archaeopyle, and the width of the cyst is much larger than
its height, so that it typically appears in (ant)apical view on microscope slides. Nucicla umbiliphora was found in Quaternary sediments of the Ross Sea together with the similar-looking Dinocyst sp. B (Wrenn et al. 1998). Because Dinocyst sp. B of Wrenn et al. (1998) has antapical ‘horns’ with acute tips, and the number of intercalary plates that comprise the archaeopyle is uncertain, it is not included in N. umbiliphora.
Discussion:
We placed N. umbiliphora within the family Protoperidiniaceae based on the visible tabulation, the absence of cavation and the brown colour. It lacks plate boundaries between the cingular plates, which hampers definite placement within Protoperidiniaceae. However, several modern cysts with a pentagonal outline and an intercalary archaeopyle but without plate boundaries between the cingular plates do produce Protoperidinium thecae, such as Votadinium, Lejeunecysta, Selenopemphix and Trinovantedinium (Head 1996; Matsuoka & Head 2013; Mertens et al. 2017). In addition, N. umbiliphora does not show green autofluorescence, like many Protoperidinium cysts (Brenner & Biebow 2001; Anderson et al. 2003). Currently, the motile stage of N. umbiliphora is unknown.
Holotype. Plate 1, figure 1–2, Hartman et al., 2019
Type locality: Iceberg Alley, MacRobertson Shelf, East Antarctica
Type stratum: Holocene
Stratigraphical range: Marine Isotope Stage 5.5 to Recent
Original description: Hartman et al., 2019:
Diagnosis:
A species of Nucicla with a scabrate wall structure and with low, undulating or crenulating ridges with no apparent relation to plate boundaries with the exception of the cingulum and sulcus. The sulcus shows a large flagellar scar. Apart from the cingulum and sulcus, tabulation is only indicated by the clear 3I archaeopyle, of which the 2a intercalary is large and latideltaform. In the holotype the operculum is still attached; specimens are usually found without operculum.
Description:
A brown cyst with pentagonal outline, which is dorsoventrally compressed. Apical and antapical ‘horns’ are rounded and broad-based so that the cyst appears more rounded than pentagonal. A few specimens show a small acute apical horn (Plate 1, figures 3, 11). None of the specimens shows acute tips at the antapical ‘horns’. The hypocyst is twice the height of the epicyst.
Wall: The autophragm has folds that form low, undulating or crenulating ridges, which are predominantly longitudinal and can be dendritic. With the exception of the cingulum and sulcus, these ridges have no relation to sutures. The cyst wall is scabrate and the degree of scabration varies between specimens (Plate 2, figures 4–6).
The clearly distinguishable cingulum is level or very slightly descending and outlined by sutural ridges. Within the cingulum, low longitudinal ridges occur, but with no relation to cingular plate boundaries. The sulcus forms a clear depression, and is outlined by sinistral and dextral longitudinal sutural ridges. Some specimens also clearly show a posterior sutural ridge, which outlines the entire posterior sulcal plate (Sp; Plate 1, figures 6 and 8).
Tabulation: The sutural ridge that outlines the Sp extends towards the dorsal side of the cyst, indicating the exceptional antapical position of the Sp. To our knowledge no other protoperidinioid cyst possesses an Sp that is positioned so far posteriorly, thereby pushing the antapical plates to the dorsal side. The sulcus shows a large flagellar scar, with low ridges converging towards it. Applying standard protoperidinioid tabulation to N. umbiliphora, low ridges within the sulcus seem to outline the right sulcal plate (Sd) and the left sulcal plate (Ss) (Plate 1, figures 3 and 9). In some specimens (Plate 1, figures 3 and 9) a low ridge is present at the anterior margin of the left and right sulcal plates, indicating the position of the anterior sulcal plate (Sa) (Figure 2).
Archaeopyle: The 3I archaeopyle is relatively large and spans almost the entire dorsal side of the epicyst. It has a consistent shape with clear angles marking plate junctions, except for the boundaries between the intercalary plates, which are smooth and subtle. Nevertheless, both archaeopyle and operculum outline suggest a 3I archaeopyle, for the following reasons: (1) the upper margin of the archaeopyle is concave; although in dinoflagellate cyst species with a 2a archaeopyle such a concave upper margin could be the result of the inward folding of the apical plates, we are certain that this is not the case for N. umbiliphora, because the outline of the operculum is also concave (Plate 2, figure 6); (2) the upper margin of the archaeopyle is three-sided (particularly visible in Plate 2, figure 3), strongly suggesting that these are the three sides of the third apical plate (3’) that border each of the anterior intercalaries (green, black and blue lines in Plate 2); and (3) in several instances the lower margin of the 2a intercalary appears slightly but nevertheless clearly elevated with respect to the lower margins of the 1a and 3a intercalaries (indicated by the ^ symbols in the figures of Plate 2). The position of the plate junctions in the lower margin of the archaeopyle suggests that the 1a and 3a intercalaries are very narrow, bordering a large latideltaform 2a intercalary.
Tabulation: Although we cannot determine any further tabulation from the cyst of N. umbiliphora, the relatively low height of the epicyst in combination with the large 2a suggests that the height of the 3’’, 4’’, and 5’’ precingular plates is suppressed (Figure 2).
Dimensions:
Holotype: Height = 70 µm; width measured along the cingulum = 62 µm.
Other specimens (n = 10): Height = 57–70 µm, average = 64.5 µm. Width = 52–63 µm, average = 60.0 µm.
Affinities/Differential diagnosis:
This is currently the only species in the genus. Within the Protoperidiniaceae, Nucicla umbiliphora with its pentagonal outline, limited tabulation, rounded antapical horns, scabration and brown colour most closely resembles Lejeunecysta rotunda Clowes et al. 2016. Instead of erecting a new genus one might consider emending Lejeunecysta to include N. umbiliphora. However, Lejeunecysta differs from Nucicla in more than just one aspect. Most importantly, Lejeunecysta has a consistent 2a archaeopyle and closely defined archaeopyle shape. Lejeunecysta also differs in having the epicyst and hypocyst of approximately equal length, symmetrically located horns which are small pointed and solid, a laevigate or chagrinate wall, and tabulation only indicated near the archaeopyle and cingulum, whereas the sulcus is only marked by a shallow depression. Because of the number and clarity of the differences, we found it necessary to erect a new genus. It differs from most other peridinioid genera by its 3I archaeopyle. Other known genera with a 3I archaeopyle are the Cretaceous to Early Palaeocene Trithyrodinium and Palaeocene to Oligocene Vozzhennikovia, but both these genera are cavate. Typically, the apical and antapical horns of these genera are made up of the periphragm, while N. umbiliphora only has an autophragm (Figure 3). Other brown scabrate dinoflagellate cyst species from the present-day Southern Ocean are Selenopemphix antarctica Marret & de Vernal 1997, Brigantedinium pynei Hannah et al. 1998 and Cryodinium meridianum Esper & Zonneveld 2002.
Cryodinium meridianum also has low, sometimes dendritic ridges, but most of these crests reflect tabulation. Furthermore, C. meridianum lacks the pentagonal outline and dorsoventral compression, and has a 2I archaeopyle. Brigantedinium pynei is reminiscent of C. meridianum, but its rugulose surface does not reflect tabulation and it has an I archaeopyle (Clowes et al. 2016). Selenopemphix antarctica does not have ridges, has an I-type archaeopyle, and the width of the cyst is much larger than
its height, so that it typically appears in (ant)apical view on microscope slides. Nucicla umbiliphora was found in Quaternary sediments of the Ross Sea together with the similar-looking Dinocyst sp. B (Wrenn et al. 1998). Because Dinocyst sp. B of Wrenn et al. (1998) has antapical ‘horns’ with acute tips, and the number of intercalary plates that comprise the archaeopyle is uncertain, it is not included in N. umbiliphora.
Discussion:
We placed N. umbiliphora within the family Protoperidiniaceae based on the visible tabulation, the absence of cavation and the brown colour. It lacks plate boundaries between the cingular plates, which hampers definite placement within Protoperidiniaceae. However, several modern cysts with a pentagonal outline and an intercalary archaeopyle but without plate boundaries between the cingular plates do produce Protoperidinium thecae, such as Votadinium, Lejeunecysta, Selenopemphix and Trinovantedinium (Head 1996; Matsuoka & Head 2013; Mertens et al. 2017). In addition, N. umbiliphora does not show green autofluorescence, like many Protoperidinium cysts (Brenner & Biebow 2001; Anderson et al. 2003). Currently, the motile stage of N. umbiliphora is unknown.