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Rhaetogonyaulax nagelii
Rhaetogonyaulax nagelii sp. nov., Mantle et al. 2020, p. 40
Holotype: Mantle et al. 2020, Plate XII, 6
Type locality: Northern Carnarvon Basin
Local stratigraphical range: Middle - Late Triassic (Ladinian to Rhaetian)
Original description: Mantle et al. 2020:
Description:
Small, proximate, acavate, ovoidal to rarely biconical dinoflagellate cysts with roughly equant epicysts and hypocysts. The epicyst is conical, narrowing to a short, triangular apical horn. The hypocyst is hemispherical with a rounded, sub-rounded or lobate antapex; very rare specimens have a conate antapex or a short stubby antapical horn. A rudimentary second hypocystal horn or protuberance, offset from the antapex, is developed in <10% of specimens. The broad cingulum varies from having a flat profile to being weakly concave and is less densely ornamented than the rest of the cyst.
Wall: autophragm ranges from scabrate to comprehensively granulate or verrucate with coarser clusters sometimes forming on the precingular and postcingular plates. Coarser grana or verrucae may also be aligned along the sutural ridges, particularly bordering the cingulum, and along the precingular and postcingular plate boundaries, often imparting a weakly serrated appearance to these plate margins.
Tabulation: precingular and postcingular plates are commonly inflated, forming irregular nodes extending out from each side of the cingulum
Tabulation formula: not known in full, but appears to be ?PR, 4+′, 4–5a, 6′′, nc, 6+′′′, n′′′′, ns
Archaeopyle: commonly formed by the loss of 3–4 anterior intercalary plates, though this disintegrative plate loss may also continue until all the climactal plates are lost. However, the loss of the apical and preapical plates may be solely due to mechanical damage
Operculum: mostly compound via the loss of individual anterior intercalary plates, however the apical and preapical plates may be lost as single piece.
Dimensions:
Overall length (including the attached opercula) 30 μm (38 μm) 51 μm; maximum width 24 μm (31 μm) 41 μm; length of antapical horn (present in <20% of specimens) 1.0 μm (1.6 μm) 2.5 μm
20 measured specimens
Remarks:
Small or poorly preserved specimens of Rhaetogonyaulax nagelii sp. nov. are often not easy to identify as rhaetogonyaulacaceans. The examination of the entire assemblage is frequently required before their identification can be confirmed. Specifically, the lack of an antapical horn, a biconical shape, or an obvious archaeopyle on many specimens makes identification particularly difficult. However, even these specimens usually exhibit some typical rhaetogonyaulacacean traits. These are a prominent and broad cingulum (often less coarsely ornamented than the rest of the cyst), an apical horn, very short precingular plates and sutural grana or verrucae. These smaller forms are commonest in the early–middle Carnian, and probably represent the first moderately common rhaetogonyaulacaceans, which later evolved into larger, more elongate and more clearly tabulate taxa. Sahulidinium ottii, of the Ladinian–early Carnian, is the oldest unequivocal dinoflagellate cyst and also exhibited weak rhaetogonyaulacacean tabulation but is extremely rare.
Affinities/Comparisons:
Rhaetogonyaulax nagelii sp. nov. is very closely related to Rhaetogonyaulax arctica and it was only after careful consideration that this new taxon is erected. This new Australian species is broadly ovoidal with a rounded or subrounded antapex, and thus differs from the predominantly biconical Rhaetogonyaulax arctica as described from Alaska (Wiggins, 1973) and well-illustrated specimens from Indonesia (Below, 1987). Furthermore, Rhaetogonyaulax nagelii sp. nov. is marginally smaller and generally has a much less indented cingulum. Larger specimens of Rhaetogonyaulax nagelii sp. nov. show some gradation towards Rhaetogonyaulax wigginsii, and poorly preserved specimens are best recorded as Rhaetogonyaulax spp. However, the denser surface ornament, frequently inflated hypocystal plates and unclear tabulation of Rhaetogonyaulax nagelii sp. nov. all contribute to a somewhat incoherent overall appearance.
Holotype: Mantle et al. 2020, Plate XII, 6
Type locality: Northern Carnarvon Basin
Local stratigraphical range: Middle - Late Triassic (Ladinian to Rhaetian)
Original description: Mantle et al. 2020:
Description:
Small, proximate, acavate, ovoidal to rarely biconical dinoflagellate cysts with roughly equant epicysts and hypocysts. The epicyst is conical, narrowing to a short, triangular apical horn. The hypocyst is hemispherical with a rounded, sub-rounded or lobate antapex; very rare specimens have a conate antapex or a short stubby antapical horn. A rudimentary second hypocystal horn or protuberance, offset from the antapex, is developed in <10% of specimens. The broad cingulum varies from having a flat profile to being weakly concave and is less densely ornamented than the rest of the cyst.
Wall: autophragm ranges from scabrate to comprehensively granulate or verrucate with coarser clusters sometimes forming on the precingular and postcingular plates. Coarser grana or verrucae may also be aligned along the sutural ridges, particularly bordering the cingulum, and along the precingular and postcingular plate boundaries, often imparting a weakly serrated appearance to these plate margins.
Tabulation: precingular and postcingular plates are commonly inflated, forming irregular nodes extending out from each side of the cingulum
Tabulation formula: not known in full, but appears to be ?PR, 4+′, 4–5a, 6′′, nc, 6+′′′, n′′′′, ns
Archaeopyle: commonly formed by the loss of 3–4 anterior intercalary plates, though this disintegrative plate loss may also continue until all the climactal plates are lost. However, the loss of the apical and preapical plates may be solely due to mechanical damage
Operculum: mostly compound via the loss of individual anterior intercalary plates, however the apical and preapical plates may be lost as single piece.
Dimensions:
Overall length (including the attached opercula) 30 μm (38 μm) 51 μm; maximum width 24 μm (31 μm) 41 μm; length of antapical horn (present in <20% of specimens) 1.0 μm (1.6 μm) 2.5 μm
20 measured specimens
Remarks:
Small or poorly preserved specimens of Rhaetogonyaulax nagelii sp. nov. are often not easy to identify as rhaetogonyaulacaceans. The examination of the entire assemblage is frequently required before their identification can be confirmed. Specifically, the lack of an antapical horn, a biconical shape, or an obvious archaeopyle on many specimens makes identification particularly difficult. However, even these specimens usually exhibit some typical rhaetogonyaulacacean traits. These are a prominent and broad cingulum (often less coarsely ornamented than the rest of the cyst), an apical horn, very short precingular plates and sutural grana or verrucae. These smaller forms are commonest in the early–middle Carnian, and probably represent the first moderately common rhaetogonyaulacaceans, which later evolved into larger, more elongate and more clearly tabulate taxa. Sahulidinium ottii, of the Ladinian–early Carnian, is the oldest unequivocal dinoflagellate cyst and also exhibited weak rhaetogonyaulacacean tabulation but is extremely rare.
Affinities/Comparisons:
Rhaetogonyaulax nagelii sp. nov. is very closely related to Rhaetogonyaulax arctica and it was only after careful consideration that this new taxon is erected. This new Australian species is broadly ovoidal with a rounded or subrounded antapex, and thus differs from the predominantly biconical Rhaetogonyaulax arctica as described from Alaska (Wiggins, 1973) and well-illustrated specimens from Indonesia (Below, 1987). Furthermore, Rhaetogonyaulax nagelii sp. nov. is marginally smaller and generally has a much less indented cingulum. Larger specimens of Rhaetogonyaulax nagelii sp. nov. show some gradation towards Rhaetogonyaulax wigginsii, and poorly preserved specimens are best recorded as Rhaetogonyaulax spp. However, the denser surface ornament, frequently inflated hypocystal plates and unclear tabulation of Rhaetogonyaulax nagelii sp. nov. all contribute to a somewhat incoherent overall appearance.