Back
Tetrachacysta keenei
Tetrachacysta? keenei Marshall, 1989
Marshall, 1989 questionably included this species in Tetrachacysta.
Holotype. Marshall 1989: Plate 7, figs. 5-7
Locus typicus: Gippsland Basin, Southeastern Australia
Stratum typicum: Late Cretaceous
Age: Turonian-early Santonian
Description: Marshall 1989, p. 51-52
Cysts compressed dorsoventrally, hour-glass shaped in dorsoventral view, apical margin convex; antapical margin convex or with two convexities separated by a medial groove. Epicystal and hypocystal flanks convex, constricted at equator by a pronounced lateral groove. Autophragm 0.5-2.0 µm thick, surface covered with numerous irregularly spaced spines, up to 2 µm long, with broad bases and pointed, rounded, or less commonly, slightly bulbous tips. Spines appear fairly evenly distributed, but are sparser within the equatorial groove. Surface between spines smooth, finely punctate: punctate less than ca. 0.4 µm in diameter. Dorsal and/or ventral surfaces near apex marked by an angular, lateral rupture lacking consistent outline between specimens. Ruptured autophragm frequently strongly fragmented by numerous splits outlining small polygonal fields of irregular pattern. Rupture usually incomplete, rarely forming a continuous break around the cyst removing part of apex. Paracingulum suggested by an equatorial groove extending around dorsal and edges of ventral surfaces. Ends of groove rarely discernible ventrally and may or may not be offset. Parasulcus suggested by a shallow longitudinal groove extending along middle of hypocyst, and rarely along posterior portion of epicyst. Midventral surface between ends of lateral groove occasionally marked by a small circular depression free from spines, suggesting a flagellar marking.
Size. Autocyst length 46(54)65 µm, width 42(49)58 µm; 22 specimens measured.
Discussion. Marshall 1989, p. 51-52
The rupture across the apical part of the cyst could be identified either on one or both of the ventral and dorsal surfaces. However, only rarely does it extend around the cyst and remove part of the apex. The rupture and the tiny polygonal fields surrounding it cannot be related to any style of dinoflagellate paratabulation, and they lack any regularity between specimens. The lateral groove extending around the equator of the cyst resembles a paracingulum in the way that its ends are offset on either side of the ventral surface on some specimens. The longitudinal groove along the middle of the ventral surface resembles a parasulcus.
Comparison. Marshall 1989, p. 51-52
Tetrachacysta? keenei sp. nov. has a similar shape to that of the dinoflagellates Horologinella lineata Cookson & Eisenack 1962, Tetrachachysta allenii Backhouse 1988, and T. spinosigibberosa (Brideaux & Fisher 1976) Backhouse 1988. However, T.?keenei lacks the apical type (tA) archeopyle and indications of paratabulation that these species possess. The only dinoflagellate-like features on T. ? keenei are its possible paracingulum, parasulcus, and flagellar marking. The first two characters closely resemble equivalent features on H. Iineata, defined by parasutural markings, and on T. spinosigibberosa, indicated by process-free areas. The aperture on T.? keenei resembles the apical archeopyle on these species in occurring near the apex, but it differs in lacking a consistent outline between specimens. The shape and distribution of scultpure on the autocyst of T.? keenei closely resembles that of T.? baculata Backhouse 1988 from the Early Cretaceous of the Perth Basin, Western Australia (Backhouse, 1988,p. 110, 111,pl.42,figs.4,5,pl.51,figs.2,3), but the former has a spinose rather than a predominantly baculate sculpture. Both species are only questionably assigned to the genus because of uncertainty regarding archeopyle type. Tetrachacysta? baculata is also similar to T. ? keenei in that it is only known from a freshwater or brackish water environment. Tetrachacysta? keenei occurs in dredge sample C4/87 21DDa, in association with a mixture of nonmarine to marginal marine cysts and a rare marine form.
Occurrence. Cook Cruise sample C4/87 21DDa.
Marshall, 1989 questionably included this species in Tetrachacysta.
Holotype. Marshall 1989: Plate 7, figs. 5-7
Locus typicus: Gippsland Basin, Southeastern Australia
Stratum typicum: Late Cretaceous
Age: Turonian-early Santonian
Description: Marshall 1989, p. 51-52
Cysts compressed dorsoventrally, hour-glass shaped in dorsoventral view, apical margin convex; antapical margin convex or with two convexities separated by a medial groove. Epicystal and hypocystal flanks convex, constricted at equator by a pronounced lateral groove. Autophragm 0.5-2.0 µm thick, surface covered with numerous irregularly spaced spines, up to 2 µm long, with broad bases and pointed, rounded, or less commonly, slightly bulbous tips. Spines appear fairly evenly distributed, but are sparser within the equatorial groove. Surface between spines smooth, finely punctate: punctate less than ca. 0.4 µm in diameter. Dorsal and/or ventral surfaces near apex marked by an angular, lateral rupture lacking consistent outline between specimens. Ruptured autophragm frequently strongly fragmented by numerous splits outlining small polygonal fields of irregular pattern. Rupture usually incomplete, rarely forming a continuous break around the cyst removing part of apex. Paracingulum suggested by an equatorial groove extending around dorsal and edges of ventral surfaces. Ends of groove rarely discernible ventrally and may or may not be offset. Parasulcus suggested by a shallow longitudinal groove extending along middle of hypocyst, and rarely along posterior portion of epicyst. Midventral surface between ends of lateral groove occasionally marked by a small circular depression free from spines, suggesting a flagellar marking.
Size. Autocyst length 46(54)65 µm, width 42(49)58 µm; 22 specimens measured.
Discussion. Marshall 1989, p. 51-52
The rupture across the apical part of the cyst could be identified either on one or both of the ventral and dorsal surfaces. However, only rarely does it extend around the cyst and remove part of the apex. The rupture and the tiny polygonal fields surrounding it cannot be related to any style of dinoflagellate paratabulation, and they lack any regularity between specimens. The lateral groove extending around the equator of the cyst resembles a paracingulum in the way that its ends are offset on either side of the ventral surface on some specimens. The longitudinal groove along the middle of the ventral surface resembles a parasulcus.
Comparison. Marshall 1989, p. 51-52
Tetrachacysta? keenei sp. nov. has a similar shape to that of the dinoflagellates Horologinella lineata Cookson & Eisenack 1962, Tetrachachysta allenii Backhouse 1988, and T. spinosigibberosa (Brideaux & Fisher 1976) Backhouse 1988. However, T.?keenei lacks the apical type (tA) archeopyle and indications of paratabulation that these species possess. The only dinoflagellate-like features on T. ? keenei are its possible paracingulum, parasulcus, and flagellar marking. The first two characters closely resemble equivalent features on H. Iineata, defined by parasutural markings, and on T. spinosigibberosa, indicated by process-free areas. The aperture on T.? keenei resembles the apical archeopyle on these species in occurring near the apex, but it differs in lacking a consistent outline between specimens. The shape and distribution of scultpure on the autocyst of T.? keenei closely resembles that of T.? baculata Backhouse 1988 from the Early Cretaceous of the Perth Basin, Western Australia (Backhouse, 1988,p. 110, 111,pl.42,figs.4,5,pl.51,figs.2,3), but the former has a spinose rather than a predominantly baculate sculpture. Both species are only questionably assigned to the genus because of uncertainty regarding archeopyle type. Tetrachacysta? baculata is also similar to T. ? keenei in that it is only known from a freshwater or brackish water environment. Tetrachacysta? keenei occurs in dredge sample C4/87 21DDa, in association with a mixture of nonmarine to marginal marine cysts and a rare marine form.
Occurrence. Cook Cruise sample C4/87 21DDa.