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Heterosphaeridium silybum
Derivation of name. From Latin silybum, meaning a kind of thistle - with reference to the spiny appearance of this species.
Holotype. Fig. 5 (1); from a core sample at 2879.7m, well 6305/4-2 S; The central body, including operculum, is 52 μm long and 52 μm wide. Processes are 89 μm long and 79 μm wide; slide co-ordinate EF. V58/4. Repository of type: NHMUK PM FD 1137.
Type locality. Ormen Lange field, Møre Basin (Norway).
Diagnosis. Chorate cysts circular to sub-circular in outline with numerous non-tabular processes which are narrow, expanded at their base, primary solid, although some are membranous with a hollow appearance, and variable in shape and length. The termination of the processes displays considerable variability, including capitate, acuminate, slightly furcate and extensively bi- and trifurcate endings. The processes are never connected distally and rarely connected proximally by low crests (see arrows Fig. 5. 5,6,7). The tabulation pattern is not expressed. The archaeopyle is apical, usually operculum free, although some specimens are preserved with complete operculum attached, revealing the occurrence of a distinctive hollow/membranous apical horn (see arrow in Fig. 5. 1,2), considerably larger than the rest of processes.
Stratigraphic position. Upper Maastrichtian, Springar Formation.
Dimensions. Central body length (without operculum) 42(46)52 μm, width 47(56)68 μm. Processes length 61(68)81 μm, width 66(75)85 μm (20 specimens measured).
Description. The cyst body is composed of two closely appressed layers: a thin periphragm and a thick (2–5 μm) endophragm. The endophragm surface is perforate and granulate. Perforations are less than 1 μm in diameter and are more or less evenly distributed across the entire cyst surface. The cyst develops numerous thin processes, which vary in length from 16 to 21 μm. The processes can be straight or undulated. The archaeopyle is apical, type tA and usually seen detached. Tabulation is not expressed and reflected only by the archaeopyle. The processes are rarely connected proximally by very low crests/ridges that do not exceed 2 μm in height.
Comparison. The occurrence of various types of processes within the same specimen is characteristic of Heterosphaeridium and is the main basis we have used to allocate the new species in this genus, and differentiate it from Cleistosphaeridium (Radmacher et al., 2014). The occurrence of an apical horn arising from the plates that form the archaeopyle is a unique feature that has never been described within the genus Heterosphaeridium. Heterosphaeridium heteracanthum (Deflandre and Cookson, 1955) Eisenack and Kjellström, 1972 is the morphologically closest species to H. silybum. However, H. heteracanthum possesses only solid processes and lacks the apical horn. Heterosphaeridium silybum differs from Heterosphaeridium bellii Radmacher et al., 2014 by the lack of ‘scythe-shape’ termination of processes. Heterosphaeridium conjunctum Cookson and Eisenack, 1968 has processes with a uniform length which display more variability in width than those of H. silybum. Other species of Heterosphaeridium such as H. spinaconjunctum Yun, 1981 and H. verdieri Yun, 1981 clearly differ from the new species proposed here by having distally connected processes. Our specimens have close resemblance to those identified at a generic level as Heterosphaeridium sp. by Gedl (2004) in the Maastrichtian – Danian boundary of the Uzgruň section, Czech Republic.
Remarks. In the study area, the highest abundance of H. silybum occurs in the latest Maastrichtian together with other late Maastrichtian stratigraphic markers, such as Palynodinium grallator. The acme of Heterosphaeridium sp. reported by Gedl (2004) occurs in deposits of the same age (Maastrichtian – Danian boundary in the Czech Republic) supporting the inclusion of such specimens in the synonymy list of H. silybum and suggesting that this increase was a characteristic bioevent beyond the Norwegian Sea.
Holotype. Fig. 5 (1); from a core sample at 2879.7m, well 6305/4-2 S; The central body, including operculum, is 52 μm long and 52 μm wide. Processes are 89 μm long and 79 μm wide; slide co-ordinate EF. V58/4. Repository of type: NHMUK PM FD 1137.
Type locality. Ormen Lange field, Møre Basin (Norway).
Diagnosis. Chorate cysts circular to sub-circular in outline with numerous non-tabular processes which are narrow, expanded at their base, primary solid, although some are membranous with a hollow appearance, and variable in shape and length. The termination of the processes displays considerable variability, including capitate, acuminate, slightly furcate and extensively bi- and trifurcate endings. The processes are never connected distally and rarely connected proximally by low crests (see arrows Fig. 5. 5,6,7). The tabulation pattern is not expressed. The archaeopyle is apical, usually operculum free, although some specimens are preserved with complete operculum attached, revealing the occurrence of a distinctive hollow/membranous apical horn (see arrow in Fig. 5. 1,2), considerably larger than the rest of processes.
Stratigraphic position. Upper Maastrichtian, Springar Formation.
Dimensions. Central body length (without operculum) 42(46)52 μm, width 47(56)68 μm. Processes length 61(68)81 μm, width 66(75)85 μm (20 specimens measured).
Description. The cyst body is composed of two closely appressed layers: a thin periphragm and a thick (2–5 μm) endophragm. The endophragm surface is perforate and granulate. Perforations are less than 1 μm in diameter and are more or less evenly distributed across the entire cyst surface. The cyst develops numerous thin processes, which vary in length from 16 to 21 μm. The processes can be straight or undulated. The archaeopyle is apical, type tA and usually seen detached. Tabulation is not expressed and reflected only by the archaeopyle. The processes are rarely connected proximally by very low crests/ridges that do not exceed 2 μm in height.
Comparison. The occurrence of various types of processes within the same specimen is characteristic of Heterosphaeridium and is the main basis we have used to allocate the new species in this genus, and differentiate it from Cleistosphaeridium (Radmacher et al., 2014). The occurrence of an apical horn arising from the plates that form the archaeopyle is a unique feature that has never been described within the genus Heterosphaeridium. Heterosphaeridium heteracanthum (Deflandre and Cookson, 1955) Eisenack and Kjellström, 1972 is the morphologically closest species to H. silybum. However, H. heteracanthum possesses only solid processes and lacks the apical horn. Heterosphaeridium silybum differs from Heterosphaeridium bellii Radmacher et al., 2014 by the lack of ‘scythe-shape’ termination of processes. Heterosphaeridium conjunctum Cookson and Eisenack, 1968 has processes with a uniform length which display more variability in width than those of H. silybum. Other species of Heterosphaeridium such as H. spinaconjunctum Yun, 1981 and H. verdieri Yun, 1981 clearly differ from the new species proposed here by having distally connected processes. Our specimens have close resemblance to those identified at a generic level as Heterosphaeridium sp. by Gedl (2004) in the Maastrichtian – Danian boundary of the Uzgruň section, Czech Republic.
Remarks. In the study area, the highest abundance of H. silybum occurs in the latest Maastrichtian together with other late Maastrichtian stratigraphic markers, such as Palynodinium grallator. The acme of Heterosphaeridium sp. reported by Gedl (2004) occurs in deposits of the same age (Maastrichtian – Danian boundary in the Czech Republic) supporting the inclusion of such specimens in the synonymy list of H. silybum and suggesting that this increase was a characteristic bioevent beyond the Norwegian Sea.