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Xenascus asperatus
Xenascus asperatus Stover and Helby, 1987, p. 128
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G.L. Williams short notes on species, Mesozoic-Cenozoic dinocyst course, Urbino, Italy, May 17-22, 1999
Xenascus asperatus Stover and Helby, 1987a. According to Stover and Helby (1987a, p.128-129), Xenascus asperatus has a very prominent apical horn, a long antapical horn and substantial cingular horn jutting laterally from the equatorial region and bending posteriorly from below the extended paracingulum. Has branched parasutural, penitabular or intratabular processes (up to 28 µm long) and tapered spines (up to 7 µm long). X.asperatus differs from Xenascus ceratioides in not having a long, tapered apical horn with parasutural ridges and in the cingulum not being as well defined. Size: operculum length 76-91 µm, width 30-46 µm; loisthocyst, length 71-104 µm, width 45-76 µm, archeopyle width 35-50um; endocyst length 37-56 µm, cingular horn length 36-49 µm, height of endocyst operculum 17-31 µm.
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Holotype: Stover and Helby, 1987, fig.26J-K
Locus typicus: Groote Eylandt, Gulf of Carpentaria, Australia
Stratum typicum: Vraconian (Late Albian)
Original description: Stover and Helby, 1987:
Proximochorate, 2-layered cysts, compressed ceratioid shape with a very prominent apical horn, a long antapical horn and substantial cingular horn jutting laterally from the equatorial region and bending posteriorly from below the extended paracingulum. Outline of the pericyst interrupted by pericoel development at the bases of processes or along the bases of crests. Endocyst ellipsoidal. Periphragm generally less than 1 µm thick, ornamentation variable; smooth, granulate, scabrate or perforate with prominent hollow, distally open and/or closed and commonly branched parasutural, penitabular or intratabular processes (up to 28 µm long and 61 µm wide), and tapered spines (up to 7 µm long) generally parasuturally aligned. Rare parasutural crests may be surmounted by rows of small processes (spines, bacula, etc., up to 2 µm long, 1 µm basal diameter). Endophragm 1-1.5 µm thick, smooth or scabrate to granular. Paratabulation indicated by archeopyle. Parasutural and penitabular lineation of some processes and spines and rare but inconsistent parasutural ridges; mainly along the paracingulum. Hypocystal paratabulation not determined. Epicystal paratabulation formula 4 ', 6". Archeopyle apical, type [tA], principal archeopyle suture zigzag with an offset parasulcal notch. Operculum free. Paracingulum rarely delineated by parasutural ridges, but commonly by parasutural processes, and in other instances may lie between paired gonal processes, particularly on the dorsal and lateral surfaces. Parasulcus poorly defined; anterior limit marked by a deep, offset parasulcal notch.
Variability: There is considerable variation in the shape of the cyst, size and nature of the processes and development of parasutural ridges and crests. The apical horn may be bluntly rounded or distally tapered. The cingular horn varies in extent of lateral displacement from the cyst body and expression of the paracingulum by notching or process location at the lateral edge of the cingular horn. Gonal processes are generally much larger than intergonal processes (Fig.26A,D), while a still smaller group of intergonal processes may surmount some parasutural crests and ridges. Gonal processes range up to 28 µm in length, usually hollow, often open and may be distally flanged, but may be closed and pointed, often branched, with branches up to 15 µm in length. Parasutural ridges occur intermittently. They are most common between apical paraplates where low ridges or crests often extend 30-40 µm from the archeopyle margin. Low ridges may border the paracingulum and are most clearly expressed on the cingular horn and occasionally on the left lateral edge of the cyst. The ridges marginal to the paracingulum may be surmounted by intergonal spines. A spiny crest often marking a periphragm inflation below the paracingulum on the left side of the hypocyst possibly locates the 1'''/2''' parasuture.
Affinities:
Xenascus asperatus resembles X. plotei Below 1981 but the former species has a much longer apical horn, a longer and generally more robust postcingular extension of the cingular horn and a longer, more tapered antapical horn than the latter. Xenascus ceratioides (Deflandre) Lentin & Williams 1973 (as illustrated by Deflandre, 1937, pl. 12, figs 7,8) has a long, tapered apical horn with parasutural ridges but apparently lacks penitabular spines adjacent to the posterior limits of the apical paraplates and also appears to have well defined cingular paraplates. From Deflandre's illustrations we conclude that at least some of the processes on the postcingular and precingular paraplates are gonal. Xenascus australensis Cookson & Eisenack 1969 appears to have a shorter, broader antapical horn than X. asperatus. However, the nature of the illustrations and description and the absence of details of the apical horn prohibit closer comparison.
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G.L. Williams short notes on species, Mesozoic-Cenozoic dinocyst course, Urbino, Italy, May 17-22, 1999
Xenascus asperatus Stover and Helby, 1987a. According to Stover and Helby (1987a, p.128-129), Xenascus asperatus has a very prominent apical horn, a long antapical horn and substantial cingular horn jutting laterally from the equatorial region and bending posteriorly from below the extended paracingulum. Has branched parasutural, penitabular or intratabular processes (up to 28 µm long) and tapered spines (up to 7 µm long). X.asperatus differs from Xenascus ceratioides in not having a long, tapered apical horn with parasutural ridges and in the cingulum not being as well defined. Size: operculum length 76-91 µm, width 30-46 µm; loisthocyst, length 71-104 µm, width 45-76 µm, archeopyle width 35-50um; endocyst length 37-56 µm, cingular horn length 36-49 µm, height of endocyst operculum 17-31 µm.
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Holotype: Stover and Helby, 1987, fig.26J-K
Locus typicus: Groote Eylandt, Gulf of Carpentaria, Australia
Stratum typicum: Vraconian (Late Albian)
Original description: Stover and Helby, 1987:
Proximochorate, 2-layered cysts, compressed ceratioid shape with a very prominent apical horn, a long antapical horn and substantial cingular horn jutting laterally from the equatorial region and bending posteriorly from below the extended paracingulum. Outline of the pericyst interrupted by pericoel development at the bases of processes or along the bases of crests. Endocyst ellipsoidal. Periphragm generally less than 1 µm thick, ornamentation variable; smooth, granulate, scabrate or perforate with prominent hollow, distally open and/or closed and commonly branched parasutural, penitabular or intratabular processes (up to 28 µm long and 61 µm wide), and tapered spines (up to 7 µm long) generally parasuturally aligned. Rare parasutural crests may be surmounted by rows of small processes (spines, bacula, etc., up to 2 µm long, 1 µm basal diameter). Endophragm 1-1.5 µm thick, smooth or scabrate to granular. Paratabulation indicated by archeopyle. Parasutural and penitabular lineation of some processes and spines and rare but inconsistent parasutural ridges; mainly along the paracingulum. Hypocystal paratabulation not determined. Epicystal paratabulation formula 4 ', 6". Archeopyle apical, type [tA], principal archeopyle suture zigzag with an offset parasulcal notch. Operculum free. Paracingulum rarely delineated by parasutural ridges, but commonly by parasutural processes, and in other instances may lie between paired gonal processes, particularly on the dorsal and lateral surfaces. Parasulcus poorly defined; anterior limit marked by a deep, offset parasulcal notch.
Variability: There is considerable variation in the shape of the cyst, size and nature of the processes and development of parasutural ridges and crests. The apical horn may be bluntly rounded or distally tapered. The cingular horn varies in extent of lateral displacement from the cyst body and expression of the paracingulum by notching or process location at the lateral edge of the cingular horn. Gonal processes are generally much larger than intergonal processes (Fig.26A,D), while a still smaller group of intergonal processes may surmount some parasutural crests and ridges. Gonal processes range up to 28 µm in length, usually hollow, often open and may be distally flanged, but may be closed and pointed, often branched, with branches up to 15 µm in length. Parasutural ridges occur intermittently. They are most common between apical paraplates where low ridges or crests often extend 30-40 µm from the archeopyle margin. Low ridges may border the paracingulum and are most clearly expressed on the cingular horn and occasionally on the left lateral edge of the cyst. The ridges marginal to the paracingulum may be surmounted by intergonal spines. A spiny crest often marking a periphragm inflation below the paracingulum on the left side of the hypocyst possibly locates the 1'''/2''' parasuture.
Affinities:
Xenascus asperatus resembles X. plotei Below 1981 but the former species has a much longer apical horn, a longer and generally more robust postcingular extension of the cingular horn and a longer, more tapered antapical horn than the latter. Xenascus ceratioides (Deflandre) Lentin & Williams 1973 (as illustrated by Deflandre, 1937, pl. 12, figs 7,8) has a long, tapered apical horn with parasutural ridges but apparently lacks penitabular spines adjacent to the posterior limits of the apical paraplates and also appears to have well defined cingular paraplates. From Deflandre's illustrations we conclude that at least some of the processes on the postcingular and precingular paraplates are gonal. Xenascus australensis Cookson & Eisenack 1969 appears to have a shorter, broader antapical horn than X. asperatus. However, the nature of the illustrations and description and the absence of details of the apical horn prohibit closer comparison.