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Leptodinium tunellum
Leptodinium tunellum Stover and Helby, 1987
Holotypr: Stover and Helby, 1987, fig.17A-C
Locus typicus: Exmouth Plateau, W Australia
Stratum typicum: Hauterivian
Original description: Stover and Helby, 1987, p. 244-245
Cysts proximate, subspherical to ellipsoidal, bearing an apical horn with a boss-like, truncate tip. Periphragm thin (about 1 µm thick), appressed or very close to the thicker endophragm (about 3 µm thick) except where the periphragm forms parasutural folds, ridges, or bulges. Periphragm smooth or finely ornamented with widely scattered granules and/or tubercles on some precingular and postcingular paraplates. Parasutural features indicate a paratabulation of 4", 0a, 6"", Xc, 5-6""", 1p, 1"""", 1-2s. Archeopyle precingular, type P, 3"" only; operculum free. Parasutural ridges and folds variable in height from 2-5 µm, finely perforate, with smooth or nearly smooth crests. Paracingulum undivided, delimited by transverse, parallel ridges or by an equatorial bulge extending laterally 7-14 µm beyond the endophragm (Fig.17G,H), better developed laterally than dorsally or ventrally, or by a combination thereof. Ends of the paracingulum offset by 1 to 1.5 times the width of the paracingulum. On most specimens parasulcus undivided; however, faint indications of posterior and right sulcal paraplates and of a flagellar scar may be present. In overall length and width specimens 97-125 µm and 75-110 µm, respectively. Endocysts 84-107 µm in length, and 64-88 µm in width. Apical horn 14-17 µm in height; 14 specimens measured.
Paratabulation. Paraplates 1" and 4" are longitudinally elongate, 4" is wider than 1" anteriorly, and 1" extends further adcingularly than 4"; 2" and 3" are similar in size and shape. Of the precingular paraplates 6"" is shorter and narrower than the others, and its contact with 1" is substantial. The remaining precingulars follow the standard gonyaulacacean pattern. In the postcingular series, 1""" is either poorly expressed (the parasutural feature adjacent to the parasulcus is undeveloped) or not indicated. Paraplate 2""" is relatively small and narrows considerably posteriorly; 6""" is likewise relatively small and is only slightly larger than 2""". In contrast, 5""" is quite wide, so much so that the 4"""/5""" parasutural ridge approaches a middorsal position on some specimens. Consequently, 4""" is not middorsal, but instead is offset to the left and 3""" extends well onto the ventral surface (Fig.16A) rather than being mainly lateral. There is some variability in the position of the 4"""/5""" parasutural ridge (Fig.16B). Finally, 1p is narrow and longitudinally elongate and 1""" is 6-sided.
Affinities:
Stover and Helby, 1987, p. 245: The cingular bulge and the offset positioning of the dorsal postcingular paraplates relative to the dorsal precingular ones, together with the relatively large size of the cysts (most specimens are greater than 100µm in length), characterize L. tunellum. Except for L. clathratum (Cookson & Eisenack) Sarjeant in Davey et al. 1969, other species assigned unconditionally to Leptodinium are smaller than L. tunellum. Leptodinium clathratum differs from L. tunellum in having perforate or pillared parasutural ridges and a wider 1p paraplate (Cookson & Eisenack 1960, p.245, text-fig. 2a,b), and in lacking an apical horn and cingular bulge. Relationships of the dorsal paraplates on L. clathratum are unknown.
Holotypr: Stover and Helby, 1987, fig.17A-C
Locus typicus: Exmouth Plateau, W Australia
Stratum typicum: Hauterivian
Original description: Stover and Helby, 1987, p. 244-245
Cysts proximate, subspherical to ellipsoidal, bearing an apical horn with a boss-like, truncate tip. Periphragm thin (about 1 µm thick), appressed or very close to the thicker endophragm (about 3 µm thick) except where the periphragm forms parasutural folds, ridges, or bulges. Periphragm smooth or finely ornamented with widely scattered granules and/or tubercles on some precingular and postcingular paraplates. Parasutural features indicate a paratabulation of 4", 0a, 6"", Xc, 5-6""", 1p, 1"""", 1-2s. Archeopyle precingular, type P, 3"" only; operculum free. Parasutural ridges and folds variable in height from 2-5 µm, finely perforate, with smooth or nearly smooth crests. Paracingulum undivided, delimited by transverse, parallel ridges or by an equatorial bulge extending laterally 7-14 µm beyond the endophragm (Fig.17G,H), better developed laterally than dorsally or ventrally, or by a combination thereof. Ends of the paracingulum offset by 1 to 1.5 times the width of the paracingulum. On most specimens parasulcus undivided; however, faint indications of posterior and right sulcal paraplates and of a flagellar scar may be present. In overall length and width specimens 97-125 µm and 75-110 µm, respectively. Endocysts 84-107 µm in length, and 64-88 µm in width. Apical horn 14-17 µm in height; 14 specimens measured.
Paratabulation. Paraplates 1" and 4" are longitudinally elongate, 4" is wider than 1" anteriorly, and 1" extends further adcingularly than 4"; 2" and 3" are similar in size and shape. Of the precingular paraplates 6"" is shorter and narrower than the others, and its contact with 1" is substantial. The remaining precingulars follow the standard gonyaulacacean pattern. In the postcingular series, 1""" is either poorly expressed (the parasutural feature adjacent to the parasulcus is undeveloped) or not indicated. Paraplate 2""" is relatively small and narrows considerably posteriorly; 6""" is likewise relatively small and is only slightly larger than 2""". In contrast, 5""" is quite wide, so much so that the 4"""/5""" parasutural ridge approaches a middorsal position on some specimens. Consequently, 4""" is not middorsal, but instead is offset to the left and 3""" extends well onto the ventral surface (Fig.16A) rather than being mainly lateral. There is some variability in the position of the 4"""/5""" parasutural ridge (Fig.16B). Finally, 1p is narrow and longitudinally elongate and 1""" is 6-sided.
Affinities:
Stover and Helby, 1987, p. 245: The cingular bulge and the offset positioning of the dorsal postcingular paraplates relative to the dorsal precingular ones, together with the relatively large size of the cysts (most specimens are greater than 100µm in length), characterize L. tunellum. Except for L. clathratum (Cookson & Eisenack) Sarjeant in Davey et al. 1969, other species assigned unconditionally to Leptodinium are smaller than L. tunellum. Leptodinium clathratum differs from L. tunellum in having perforate or pillared parasutural ridges and a wider 1p paraplate (Cookson & Eisenack 1960, p.245, text-fig. 2a,b), and in lacking an apical horn and cingular bulge. Relationships of the dorsal paraplates on L. clathratum are unknown.