Back
Alisocysta heilmannii
Alisocysta heilmannii, Casas-Gallego, M. et al., 2021, p. 341
previously recorded as:
1968 Schematophora? sp.; De Coninck, pl. 2, figs 18–19.
1975 cf. Eisenackia sp. A; De Coninck, pl. 17, figs 8–10.
1985 Alisocysta sp. 2.; Heilmann-Clausen, pl. 11, figs 1–6 and 8–11.
Holotype: Casas-Gallego, M. et al., 2021, Plate 1, figures 1–3
Type locality: Gannet Field (UK North Sea)
Local stratigraphical range: Lower Eocene (Ypresian)
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Original description: [Casas-Gallego, M. et al., 2021]:
Diagnosis:
A species of Alisocysta with two closely appressed and markedly thin layers, especially the periphragm, which gives the cyst a distinctive pale aspect. The periphragm forms penitabular crests with an irregular distal margin, usually ranging in height between 5 and 10 µm. The contact between crests and cyst wall is smooth and lacks any distinctive thickening. Wall ornamentation is scabrate or smooth.
Dimensions:
Holotype: polar axis 61 µm long; equatorial axis 55 µm long
11 specimens measured: Length of the cysts ranges between 56 and 67 µm and width is 51–59 µm, including crests and apical plates
Remarks:
Quattrochio and Sarjeant (2003) considered that no features differentiated Alisocysta and Eisenackia, and therefore considered Alisocysta to be a junior synonym of Eisenackia. However, as stressed by Stover and Evitt (1978) and Fensome et al. (2016), the original description of Eisenackia
and subsequent emendations describe it as having depressions (‘fossulae’ or ‘channels’) (Deflandre and Cookson 1955; Davey 1969), while Alisocysta has penitabular ridges or septa. Alisocysta heilmannii clearly displays penitabular septa and, according to the latter criterion, is appropriately allocated within the genus Alisocysta.
In the study area, Alisocysta heilmannii is more abundant in the lower part of the Ypresian. The palaeogeographical distribution of Alisocysta heilmannii extends beyond the North Sea. It has been reported from the Lower Eocene of Kazakhstan and the Thanetian of Crimea (King et al. 2018; Iakovleva and Heilman-Clausen 2020).
Affinities/Comparison:
Alisocysta heilmannii differs from Alisocysta margarita in that it displays a considerably thinner wall that appears delicate under light microscopy, a key feature previously pointed out by Heilmann-Clausen (1985). In Alisocysta margarita the contact between crests and cyst wall
forms distinctive thickenings in the periphragm (Stover and Evitt 1978; Harland 1979), a morphological feature absent in Alisocysta heilmannii.
previously recorded as:
1968 Schematophora? sp.; De Coninck, pl. 2, figs 18–19.
1975 cf. Eisenackia sp. A; De Coninck, pl. 17, figs 8–10.
1985 Alisocysta sp. 2.; Heilmann-Clausen, pl. 11, figs 1–6 and 8–11.
Holotype: Casas-Gallego, M. et al., 2021, Plate 1, figures 1–3
Type locality: Gannet Field (UK North Sea)
Local stratigraphical range: Lower Eocene (Ypresian)
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Original description: [Casas-Gallego, M. et al., 2021]:
Diagnosis:
A species of Alisocysta with two closely appressed and markedly thin layers, especially the periphragm, which gives the cyst a distinctive pale aspect. The periphragm forms penitabular crests with an irregular distal margin, usually ranging in height between 5 and 10 µm. The contact between crests and cyst wall is smooth and lacks any distinctive thickening. Wall ornamentation is scabrate or smooth.
Dimensions:
Holotype: polar axis 61 µm long; equatorial axis 55 µm long
11 specimens measured: Length of the cysts ranges between 56 and 67 µm and width is 51–59 µm, including crests and apical plates
Remarks:
Quattrochio and Sarjeant (2003) considered that no features differentiated Alisocysta and Eisenackia, and therefore considered Alisocysta to be a junior synonym of Eisenackia. However, as stressed by Stover and Evitt (1978) and Fensome et al. (2016), the original description of Eisenackia
and subsequent emendations describe it as having depressions (‘fossulae’ or ‘channels’) (Deflandre and Cookson 1955; Davey 1969), while Alisocysta has penitabular ridges or septa. Alisocysta heilmannii clearly displays penitabular septa and, according to the latter criterion, is appropriately allocated within the genus Alisocysta.
In the study area, Alisocysta heilmannii is more abundant in the lower part of the Ypresian. The palaeogeographical distribution of Alisocysta heilmannii extends beyond the North Sea. It has been reported from the Lower Eocene of Kazakhstan and the Thanetian of Crimea (King et al. 2018; Iakovleva and Heilman-Clausen 2020).
Affinities/Comparison:
Alisocysta heilmannii differs from Alisocysta margarita in that it displays a considerably thinner wall that appears delicate under light microscopy, a key feature previously pointed out by Heilmann-Clausen (1985). In Alisocysta margarita the contact between crests and cyst wall
forms distinctive thickenings in the periphragm (Stover and Evitt 1978; Harland 1979), a morphological feature absent in Alisocysta heilmannii.