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Heibergella obscura
Heibergella? obscura sp. nov., Mantle et al. 2020, p. 45
Holotype: Mantle et al. 2020, Plate XVI, 7
Type locality: Northern Carnarvon Basin
Local stratigraphical range: Late Triassic (middle Norian to early Rhaetian)
Original description: Mantle et al. 2020:
Description:
Small, proximate, mostly acavate, ovoidal to subspherical dinoflagellate cysts with a very short conate apex or stubby apical horn (3–5 μm long) and a broadly rounded or hemispherical hypocyst. Wall: autophragm is scabrate to granulate, and typically lacks indications of tabulation; very rarely grana may align along presumed sutures.
The cingulum and sulcus are not discernible. Very rare specimens may show minor cavation at the apical horn.
Archaeopyle: involves the loss of multiple anterior intercalary plates, but the number of plates, or the compound or simple nature of the opercula, are unknown. A dark brown, subcircular accumulation body, 4–7 μm in diameter, is present in most specimens, and is positioned equatorially.
Dimensions:
Overall length 32 μm (36 μm) 40 μm; maximum width 28 μm (32 μm) 41 μm
20 measured specimens
Remarks:
The lack of tabulation makes Heibergella? obscura sp. nov. very difficult to confidently identify, or to even recognise, in rich palynomorph assemblages. It is most commonly preserved in various lateral orientations, but it is also frequently observed in polar orientation due to the short, ovoidal or subspherical shape. These specimens in polar orientation typically appear to be little more than granulate spheres. The darker brown accumulation body is often the only hint of their dinoflagellate affinity, though the anterior intercalary archaeopyle is occasionally evident. Typically, it is only by examining the whole assemblage that the species identification can be confirmed. This species is questionably placed in Heibergella as very rare specimens exhibit minor apical cavation. Heibergella is an acavate genus, but as most dinoflagellate walls are formed from multiple wall layers which are frequently closely appressed, occasional separation of these wall layers, particularly apically, may not be a particularly useful generic distinction. Thus, it is possible that Heibergella (ovoidal, proximate, acavate dinoflagellate cysts with an anterior intercalary archaeopyle) is very closelyrelated, and possibly congeneric with Hebecysta (ovoidal, proximate, cavate dinoflagellate cysts with an anterior intercalary archaeopyle). Hebecysta balmei was originally described as acavate, however Below (1987) noted that it may also be epicavate and transferred it to Hebecysta.
Affinities/Comparison:
Heibergella? obscura sp. nov. is most similar to Heibergella asymmetrica, but is considerably smaller, has a more densely granulate surface and a less pronounced apical horn. It is also slightly smaller than Heibergella? obelixii sp. nov., and lacks the broad cingulum and commonly well-defined sulcus of that species. Heibergella? Obscura sp. nov. is also smaller than most species of Hebecysta and differs in being typically acavate.
Holotype: Mantle et al. 2020, Plate XVI, 7
Type locality: Northern Carnarvon Basin
Local stratigraphical range: Late Triassic (middle Norian to early Rhaetian)
Original description: Mantle et al. 2020:
Description:
Small, proximate, mostly acavate, ovoidal to subspherical dinoflagellate cysts with a very short conate apex or stubby apical horn (3–5 μm long) and a broadly rounded or hemispherical hypocyst. Wall: autophragm is scabrate to granulate, and typically lacks indications of tabulation; very rarely grana may align along presumed sutures.
The cingulum and sulcus are not discernible. Very rare specimens may show minor cavation at the apical horn.
Archaeopyle: involves the loss of multiple anterior intercalary plates, but the number of plates, or the compound or simple nature of the opercula, are unknown. A dark brown, subcircular accumulation body, 4–7 μm in diameter, is present in most specimens, and is positioned equatorially.
Dimensions:
Overall length 32 μm (36 μm) 40 μm; maximum width 28 μm (32 μm) 41 μm
20 measured specimens
Remarks:
The lack of tabulation makes Heibergella? obscura sp. nov. very difficult to confidently identify, or to even recognise, in rich palynomorph assemblages. It is most commonly preserved in various lateral orientations, but it is also frequently observed in polar orientation due to the short, ovoidal or subspherical shape. These specimens in polar orientation typically appear to be little more than granulate spheres. The darker brown accumulation body is often the only hint of their dinoflagellate affinity, though the anterior intercalary archaeopyle is occasionally evident. Typically, it is only by examining the whole assemblage that the species identification can be confirmed. This species is questionably placed in Heibergella as very rare specimens exhibit minor apical cavation. Heibergella is an acavate genus, but as most dinoflagellate walls are formed from multiple wall layers which are frequently closely appressed, occasional separation of these wall layers, particularly apically, may not be a particularly useful generic distinction. Thus, it is possible that Heibergella (ovoidal, proximate, acavate dinoflagellate cysts with an anterior intercalary archaeopyle) is very closelyrelated, and possibly congeneric with Hebecysta (ovoidal, proximate, cavate dinoflagellate cysts with an anterior intercalary archaeopyle). Hebecysta balmei was originally described as acavate, however Below (1987) noted that it may also be epicavate and transferred it to Hebecysta.
Affinities/Comparison:
Heibergella? obscura sp. nov. is most similar to Heibergella asymmetrica, but is considerably smaller, has a more densely granulate surface and a less pronounced apical horn. It is also slightly smaller than Heibergella? obelixii sp. nov., and lacks the broad cingulum and commonly well-defined sulcus of that species. Heibergella? Obscura sp. nov. is also smaller than most species of Hebecysta and differs in being typically acavate.