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Leptodinium sepimentum
Leptodinium sepimentum Stevens and Helby, 1987
Holotype: Stevens and Helby, 1987, fig.12D-F
Locus typicus: Exmouth Plateau, Australia
Stratum typicum: Ealy Berriasian
Original description: Stevens and Helby, 1987, p. 178
Cysts proximochorate and polyhedral to subspherical in shape. Autophragm smooth to scabrate, less than 1Ám thick, commonly folded. Archeopyle precingular, type P (3" only), with an indented and gabled anterior margin indicating the presence of an intercalary paraplate. Operculum free, commonly found inside the cyst. Paratabulation indicated by the archeopyle, the paracingulum, and discontinuous rows of parasutural processes. Solid, laterally flattened, parasutural processes 4-5 Ám in height, with broad elongate bases (1.5-3 Ám in diameter) and expanded tips with denticulate to smooth distal margins. Processes may be isolated (particularly along the paracingulum) or coalesced at the tips and/or bases to form discontinuous perforate septa with denticulate crests. Paratabulation gonyaulacacean, 1pr, 4", 1a, 6"", 6c, 5""", 1p, 1"""", Xs (Fig.llA-C); postcingular paraplates being interpreted as 2""" to 6""". A small pr paraplate delineated close to the apex (on holotype), and a small elongate la paraplate in contact with the 2", 3", 3" and 4" paraplates. Paraplates 1" and 4" extremely long and narrow, with posterior edges of these paraplates only partially delineated by parasutural processes. The 6 " paraplate small as a consequence of the length of the 4" paraplate. The boundary between the parasulcus and the 2""", 6""" and 1p paraplates, and the posterior ends of the 1" and 4" paraplate boundaries only partially delineated by parasutural processes. The 1"""" paraplate extending adcingularly up to one third the hypocyst length on the ventral surface. Paracingulum 3(5)7Ám wide, with 6 paraplates, and offset by 1 to 1.5 paracingulum widths. Paraplates 2c to 5c clearly 4-sided while 1c and 6c may also be 4-sided, without parasutural expression of their margin with the parasulcus.
Short linear parasulcus extending posteriorly from the paracingulum up to two thirds the length of the hypocyst, indicated by an oval flagellar scar (8-10 Ám long) (Fig. 121) and a short groove (6-8 Ám long); ps paraplate delineated by parasutural processes. The anterior margin of the parasulcus not clearly delineated.
Measurements. Cyst length 72(85)100 Ám, cyst width 60(76)92 Ám (18 specimens).
Variability. Cysts are fairly constant in shape and size, however shape and size of the parasutural processes are extremely variable. The processes are aligned as isolated projections or coalesce at their tips and/or bases (Figs IID, 12E,H), to form discontinuous perforate septa (Figs IID, 12C,F).
Affinities:
Stevens and Helby, 1987, p. 179: Leptodinium sepimentum conforms to Leptodinium Klement emend. Stover and Evitt 1978 in all features, except in having discontinuous parasutural septa formed by coalescing processes. Leptodinium sepimentum closely resembles L. eumorphum (Cookson and Eisenack) Sarjeant in Davey et al. 1969 in its unusual epicystal paraplate arrangement, both species having unusually long 1" and 4" paraplates and a small 6" paraplate on the ventral surface and a small la paraplate on the dorsal surface. Apart from having discontinuous parasutural septa, L. sepimentum differs from L. eumorphum in having a short parasulcus which extends posteriorly only up to two thirds the length of the hypocyst due to the unusual adcingular extension of the 1"""" paraplate up to one third the hypocyst length on the ventral surface. Hystrichogonyaulax serrata (Cookson and Eisenack) Stover and Evitt 1978 and H. canadensis (Pocock) Stover and Evitt 1978 resemble L. sepimentum in having parasutural processes or spines, however they differ in having a large 6" paraplate as a consequence of short 1" and 4" paraplates. Diacanthum hollisteri Habib 1972 and Exiguisphaera phragma Duxbury 1979 superficially resemble L. sepimentum in having paratabulation outlined by distally and/or proximally connected processes, but they each differ from L. sepimentum in having a 2P archeopyle and apparently lack an anterior intercalary paraplate.
Holotype: Stevens and Helby, 1987, fig.12D-F
Locus typicus: Exmouth Plateau, Australia
Stratum typicum: Ealy Berriasian
Original description: Stevens and Helby, 1987, p. 178
Cysts proximochorate and polyhedral to subspherical in shape. Autophragm smooth to scabrate, less than 1Ám thick, commonly folded. Archeopyle precingular, type P (3" only), with an indented and gabled anterior margin indicating the presence of an intercalary paraplate. Operculum free, commonly found inside the cyst. Paratabulation indicated by the archeopyle, the paracingulum, and discontinuous rows of parasutural processes. Solid, laterally flattened, parasutural processes 4-5 Ám in height, with broad elongate bases (1.5-3 Ám in diameter) and expanded tips with denticulate to smooth distal margins. Processes may be isolated (particularly along the paracingulum) or coalesced at the tips and/or bases to form discontinuous perforate septa with denticulate crests. Paratabulation gonyaulacacean, 1pr, 4", 1a, 6"", 6c, 5""", 1p, 1"""", Xs (Fig.llA-C); postcingular paraplates being interpreted as 2""" to 6""". A small pr paraplate delineated close to the apex (on holotype), and a small elongate la paraplate in contact with the 2", 3", 3" and 4" paraplates. Paraplates 1" and 4" extremely long and narrow, with posterior edges of these paraplates only partially delineated by parasutural processes. The 6 " paraplate small as a consequence of the length of the 4" paraplate. The boundary between the parasulcus and the 2""", 6""" and 1p paraplates, and the posterior ends of the 1" and 4" paraplate boundaries only partially delineated by parasutural processes. The 1"""" paraplate extending adcingularly up to one third the hypocyst length on the ventral surface. Paracingulum 3(5)7Ám wide, with 6 paraplates, and offset by 1 to 1.5 paracingulum widths. Paraplates 2c to 5c clearly 4-sided while 1c and 6c may also be 4-sided, without parasutural expression of their margin with the parasulcus.
Short linear parasulcus extending posteriorly from the paracingulum up to two thirds the length of the hypocyst, indicated by an oval flagellar scar (8-10 Ám long) (Fig. 121) and a short groove (6-8 Ám long); ps paraplate delineated by parasutural processes. The anterior margin of the parasulcus not clearly delineated.
Measurements. Cyst length 72(85)100 Ám, cyst width 60(76)92 Ám (18 specimens).
Variability. Cysts are fairly constant in shape and size, however shape and size of the parasutural processes are extremely variable. The processes are aligned as isolated projections or coalesce at their tips and/or bases (Figs IID, 12E,H), to form discontinuous perforate septa (Figs IID, 12C,F).
Affinities:
Stevens and Helby, 1987, p. 179: Leptodinium sepimentum conforms to Leptodinium Klement emend. Stover and Evitt 1978 in all features, except in having discontinuous parasutural septa formed by coalescing processes. Leptodinium sepimentum closely resembles L. eumorphum (Cookson and Eisenack) Sarjeant in Davey et al. 1969 in its unusual epicystal paraplate arrangement, both species having unusually long 1" and 4" paraplates and a small 6" paraplate on the ventral surface and a small la paraplate on the dorsal surface. Apart from having discontinuous parasutural septa, L. sepimentum differs from L. eumorphum in having a short parasulcus which extends posteriorly only up to two thirds the length of the hypocyst due to the unusual adcingular extension of the 1"""" paraplate up to one third the hypocyst length on the ventral surface. Hystrichogonyaulax serrata (Cookson and Eisenack) Stover and Evitt 1978 and H. canadensis (Pocock) Stover and Evitt 1978 resemble L. sepimentum in having parasutural processes or spines, however they differ in having a large 6" paraplate as a consequence of short 1" and 4" paraplates. Diacanthum hollisteri Habib 1972 and Exiguisphaera phragma Duxbury 1979 superficially resemble L. sepimentum in having paratabulation outlined by distally and/or proximally connected processes, but they each differ from L. sepimentum in having a 2P archeopyle and apparently lack an anterior intercalary paraplate.